2Intuitive Physics We all have an intuition about how objects move. Our beliefs are hard to change since they work well in our day to day lives.But they limit us in developing an understanding of how the world works - we must build on our intuition and move beyond it.
3Galileo vs. AristotleIn our experience, objects must be pushed in order to keep moving. So a force would be needed to have a constant velocity. This is what Aristotle claimed in his in his series of books entitled "Physics", written 2400 years ago.But 400 years ago, another scientist and astronomer, Galileo, proposed the following thought experiment which revealed another perspective.
4Thought ExperimentImagine two perfectly smooth ramps connected together by a perfectly smooth surface. If a ball is let go at the top of the one ramp, what will happen?
5Thought ExperimentImagine two perfectly smooth ramps connected together by a perfectly smooth surface. If a ball is let go at the top of the one ramp, what will happen?
6Thought ExperimentImagine two perfectly smooth ramps connected together by a perfectly smooth surface. If a ball is let go at the top of the one ramp, what will happen?
7Thought ExperimentImagine two perfectly smooth ramps connected together by a perfectly smooth surface. If a ball is let go at the top of the one ramp, what will happen?
8Thought ExperimentIf a ball rolls down one ramp, it keeps rolling up the other side until it reaches the same height.
9Now repeat that experiment, but make the second ramp less steep. Thought ExperimentNow repeat that experiment, but make the second ramp less steep.What Will Happen?
10Now repeat that experiment, but make the second ramp less steep. Thought ExperimentNow repeat that experiment, but make the second ramp less steep.What Will Happen?
11Now repeat that experiment, but make the second ramp less steep. Thought ExperimentNow repeat that experiment, but make the second ramp less steep.What Will Happen?
12Now repeat that experiment, but make the second ramp less steep. Thought ExperimentNow repeat that experiment, but make the second ramp less steep.What Will Happen?
13Thought ExperimentIt will still keep rolling until it reaches the same height, but it has to roll farther!
14Thought ExperimentFinally, make the ramp flat.Now what will happen?
15Thought ExperimentFinally, make the ramp flat.Now what will happen?
16Thought ExperimentFinally, make the ramp flat.Now what will happen?
17Thought ExperimentFinally, make the ramp flat.Now what will happen?
18Thought ExperimentFinally, make the ramp flat.Now what will happen?
19Thought ExperimentFinally, make the ramp flat.Now what will happen?
20Thought ExperimentIt will keep rolling forever, no external force is necessary.
21Galileo vs. Aristotle It's not that Aristotle was wrong. In everyday life, objects do need to keep being pushed in order to keep moving.Push a book across the table. When you stop pushing, it stops moving. Aristotle is right in terms of what we see around us every day.
22Force and MotionIt's just that Galileo, and later Newton, imagined a world where friction could be eliminated.FappliedFfrictionFriction represents an external force acting on the object, just as your push is an external force.In the absence of all external forces, an object's velocity remains constant. Two equal and opposite forces have the same effect, they cancel to create zero net force.
24Sir Isaac NewtonGalileo's observations were more fully formed in 1687 by the 'father of physics, ' Sir Isaac Newton, who called this observation "The First Law of Motion".
25I. Newton's First Law of Motion An object at rest remains at rest, and an object in motion remains in motion, unless acted on by a net external force.In other words, an object maintains its velocity (both speed and direction) unless acted upon by a nonzero net force or an unbalanced fore.Having zero velocity, being at rest, is not special, it is just one possible velocity…a velocity which is no more special than any other.
26Newton's First Law of Motion A. Mass and InertiaMass as a Measure of the Amount of InertiaAll objects resist changes in their state of motion.The tendency of an object to resist changes in its state of motion varies with mass.Mass is that quantity that is solely dependent upon the inertia of an object.The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion.
27Newton's First Law of Motion B. Combining Forces Force is a vector quantity-a push or a pull1. Balanced forces: equal and opposite forcesBalanced forces give no motionNo motion of hands
292. Unbalanced force – more force in one direction (net force)
30Bigger force to the right Smaller force to the leftNet force to the right
31A.K.A. The Law of InertiaThis law is often referred to as the "Law of Inertia." The word inertia comes from the latin word iners which means idle, or lazy. Inertia is the tendency of an object to resist any change in motion.Teacher Instructions: hard boiled egg vs. raw egg. Go to the linkDemo: hard boiled egg vs. raw egg. Click here.
32In the absence of an external force, a moving object will 1In the absence of an external force, a moving object willcAstop immediately.cBslow down and eventually come to a stop.cCgo faster and faster.cDmove with constant velocity.
332When the rocket engines on a spacecraft are suddenly turned off while traveling in empty space, the starship willcAstop immediately.cBslowly slow down, and then stop.cCgo faster and faster.cDmove with constant velocity.
343A rocket moves through empty space in a straight line with constant speed. It is far from the gravitational effect of any star or planet. Under these conditions, the force that must be applied to the rocket in order to sustain its motion iscAequal to its weight.cBequal to its mass.cCdependent on how fast it is moving.cDzero.
354You are standing in a moving bus, facing forward, and you suddenly fall forward. You can infer from this that the bus’scAvelocity decreased.cBvelocity increased.Cspeed remained the same, but it's turning to the right.cDspeed remained the same, but it's turning to the left.c
365You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop. What force caused you to fall forward?cAgravitycBnormal force due to your contact with the floor of the busCforce due to friction between you and the floor of the busccDThere is not a force leading to your fall.
37Inertial Reference Frames Newton's laws are only valid in inertial reference frames:An inertial reference frame is one which is not accelerating or rotating. It is an area in which every body remains in a state of rest unless acted on by an external unbalanced force.
38Inertial Reference Frames When your car accelerates, it is not an inertial reference frame.This is why a drink on the dashboard of a car can suddenly seem to accelerate backwards without any force acting on it.It's not accelerating, it's standing still. The reference frame, the car, is accelerating underneath it.Click here for a very famous videoabout frames of reference.watch the first 2:30 of the video
40II. Nature of Force Definition: A force is a push or pull resulting from the interaction between objects.Whenever there is an interaction between two objects, there is a force upon each of the objects.When the interaction stops, the two objects no longer experience the force.Forces only exist as a result of an interaction.
41II. Nature of Force Type of Forces A force is a push or pull acting upon an object as a result of its interaction with another object. There are two broad categories of forces based on whether the force resulted from the contact or non-contact of the two interacting objects.Contact Forces Non ContactNormal Force Gravitational ForceFrictional Force Magnetic ForceTension Electrical ForceAir Resistance ForceApplied ForceSpring Force
421. Friction - A Resistive Force There are many different types of forces that occur in nature, but perhaps none is more familiar to us than the force of friction (Ffr).Friction is a resistive force that opposes the motion of an object.What does sandpaper have to do with friction?
43Friction - A Resistive Force Friction is the reason objects stop rolling or sliding along a surface. It is the reason it is difficult to start pushing a heavy box along the floor.There are many different types of friction:Friction between solid objects and air is often called air resistance.Friction between two fluids is called viscosity, and so on.
44Friction - A Resistive Force a. Factors affecting friction:Type of surfacesHow hard surfaces are pressed together
45b. Static v. Dynamic Friction Static Friction that acts on something that is not moving; No heat or wear is generatedDynamic Friction, aka Kinetic Friction, results from contact between moving objects (sliding, rolling, fluid)
46– force of attraction between two objects -- pulling force 2. Gravity– force of attraction between two objects-- pulling forcea. Factors Affecting Gravityi. Mass – more mass, more gravityii. Distance – closer, more gravity
49b. Weight vs. Mass Define: weight: force of gravity on an object (Fw); dependent on location of the object (due to masses and distances)mass: amount of matter an object contains; does not change with location1 kilogram mass has a weight of 9.8 N (on earth)
50The case of mass versus weight Mass is the measure of the inertia of an object, the resistance of an object to accelerate. In the SI system, mass is measured in kilograms.Mass is not weight!Mass is a property of an object.It doesn't depend on where the object is located.Weight is the force exerted on that object by gravity. If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same.Click on this link to see a Veritasiumvideo about mass and weight!
51Weight – the Force of Gravity Weight is the force exerted on an object by gravity. Close to the surface of Earth, where the gravitational force is nearly constant, weight can be calculated with:Fg = mgNear the surface of Earth, g is 9.8 m/s2 downwards.
5218Determine the Force of Gravity (weight) of a 6.0 kg bowling ball.
5319Determine the weight of a small car with a mass of 900 kg.
5420Using a spring scale, you find that the weight of a friction block in the lab is around 24 N. What is the mass of the block, in kilograms?
5521A 120 lb woman has a mass of about 54.5 kg.What is her weight?
5622What is the weight of a 25 kg object located near the surface of Earth?
5723Which of the following properties of an object is likely to change on another planet?cAMasscBWeightcCColorcDVolume (size and shape)
5824The acceleration due to gravity is lower on the Moon than on Earth. Which of the following is true about the mass and weight of an astronaut on the Moon's surface, compared to Earth?cAMass is less, weight is samecBMass is same, weight is lesscCBoth mass and weight are lesscDBoth mass and weight are the same
59Weight – the Force of Gravity; and the Normal ForceAn object at rest must have no net force on it. If it is sitting on a table, the force of gravity is still there; what other force is there?FG
60Weight – the Force of Gravity; and the Normal ForceAn object at rest must have no net force on it. If it is sitting on a table, the force of gravity is still there; what other force is there?The force exerted perpendicular to a surface is called the normal force.It is exactly as large as needed to balance the force from the object (if the required force gets too big, something breaks!)The words "normal" and "perpendicular" are synonyms.FNFG
6125A 14 N brick is sitting on a table. What is the normal force supplied by the table?cA14 N upwardsB28 N upwardscC14 N downwardscD28 N downwardsc
6226What normal force is supplied by a desk to a 2.0 kg box sitting on it?
633. ‘Other’In this unit, we will be concerned with weight (force of gravity on an object) and friction (force that opposes motion). We will also conside other contact forces:Normal ForceApplied ForceTensionAir Resistance
65Free Body DiagramsA free body diagram is a drawing physicists use in order to show all the forces acting on an object. Drawing free body diagrams can help when trying to solve for unknown forces or showing the motion of the object.Click here for a Veritasium video onfree body diagrams andreviewing Normal Force!
66Free Body Diagrams1. Draw and label a dot to represent the first object.2. Draw an arrow from the dot pointing in the direction of one of the forces that is acting on that object. Label that arrow with the name of the force.3. Repeat for every force that is acting on the object. Try to draw each of the arrows to roughly the same scale, bigger forces getting bigger arrows.mgmgFT
67Free Body Diagrams4. Once you have finished your free body diagram, recheck it to make sure that you have drawn and labeled an arrow for every force. This is no time to forget a force.5. Draw a separate arrow next to your free body diagram indicating the likely direction of the acceleration of the object. This will help you use your free body diagram effectively.6. Repeat this process for every object in your sketch.mgFTmgFTa
68A hot air balloon hovers in the sky. Draw it's free body diagram: FBa = 0mg
69A boy pulls a toy along (with constant velocity) on a string. Draw the toy's free body diagram:FNa = 0 (const. velocity)FTmg
70An object on a crane descends with constant velocity. Draw the object's free body diagram:FTa = 0mg
71Kinetic Friction Force Friction that acts on an object that is already in motion is called kinetic friction.For kinetic – sliding friction, we write:Kinetic friction is the product of two things: μk is called the coefficient of kinetic friction, and is different for every pair of surfaces. FN is simply the Normal Force, which, on flat surfaces, is equal to the weight of the object.
72Kinetic Friction Force A larger coefficient of friction means a greater frictional force. Notice the friction that occurs between different materialsin the table below:SurfaceCoefficient of Kinetic FrictionWood on Wood0.2Ice on Ice0.03metal on metal (lubricated)0.07Steel on steel (unlubricated)0.6Rubber on dry concrete0.8Rubber on wet concrete0.5Rubber on other solid surface0.04Teflon on Teflon1.0Human Joints in limbs0.01
73A man accelerates a crate along a rough surface. Draw the crate's free body diagram:Determine ΣF in the x and y directionsFNaFfrFappmgΣFy = 0FN = mgThe object has a net force to the right:ΣFx = maxFapp - Ffr = max
7434A 4.0kg brick is sliding on a surface. The coefficient of kinetic friction between the surfaces is What it the size of the force of friction?
7535A brick is sliding to the right on a horizontal surface. What are the directions of the two surface forces: the friction force and the normal force?cAright, downcBright, upcCleft, downcDleft, up
76* Static Friction Force Static friction is the frictional force between two surfaces that are not moving along each other.Static friction keeps objects from moving when a force is first applied.Fappliedv = 0
77* Static Friction Force Fappliedv = 0μs is the coefficient of static friction, and is different for every pair of surfaces.
78Static Friction Force * Note the ≤ symbol in this equation. Imagine pushing on a box until it moves. You can apply a small force... nothing happens. You apply more and more force until the box finally starts moving - this is the maximum amount of static friction.The friction can be LESS than the maximum amount or EQUAL to the maximum amount, but never greater. The force of friction is equal to μsFN at the instant when the object starts to move.Then what happens?
79*Friction ForceThe static frictional force increases as the applied force increases, always equal to the net applied force.Until it reaches its maximum, μsFN.Then the object starts to move, and the kinetic frictional force takes over, μKFN .10302050406070Friction force, fApplied force, FAf = μS FNμk FNno motionsliding
80Coefficient of Static Friction Coefficient of Kinetic Friction *Friction ForceThe table below shows values for both static and kinetic coefficients of friction.SurfaceCoefficient of Static FrictionCoefficient of Kinetic FrictionWood on wood0.40.2Ice on ice0.10.03Metal on metal (lubricated)0.150.07Steel on steel (unlubricated)0.70.6Rubber on dry concrete1.00.8Rubber on wet concrete0.5Rubber on other solid surfaces1-41Teflon on Teflon in air0.04Joints in human limbs0.01Notice that static friction is greater than kinetic friction.Once an object is in motion, it is easier to keep it in motion.
81*36A 4.0 kg brick is sitting on a table. The coefficient of static friction between the surfaces is What is the largest force that can be applied horizontally to the brick before it begins to slide?
82*37A 4.0kg brick is sitting on a table. The coefficient of static friction between the surfaces is If a 10 N horizontal force is applied to the brick, what will be the force of friction?
84Tension ForceFTmgaWhen a cord or rope pulls on an object, it is said to be under tension, and the force it exerts is called a tension force, FT.Any object that is hanging or suspended is considered to have tension acting upward.In some cases, tension can be applied in other directions as well.
85Tension ForceFTmgaThere is no special formula to find the force of tension.We need to use force diagrams and net force equations to solve for it!
8638A 25 kg lamp is hanging from a rope. What is the tension force being supplied by the rope?
8739A crane is lifting a 60 kg load at a constant velocity. Determine the tension force in the cable.
88D. Net ForceSum off all forces acting on an object
90Newton’s Second Law of Motion An object doesn't change its velocity unless a force acts on it.How does an object respond to a force when it is applied?
91A. Calculating with Newton’s Second Law of Motion Fnet = maNewton’s second law identifies the relationship between acceleration and force. Namely, as a net force is applied, an object accelerates.*the word 'net' means overall, or total. We will discuss this in further detail later, but for now just think of the sum of all forces, sometimes written as ΣF as any force on an object
92Units of ForceFnet = maThe unit of force in the SI system is the newton (N).Mass is measured in kilograms (kg). And we already know acceleration is measured in meters/second2 (m/s2).Therefore, the unit of force, the Newton, can be found from the second lawFnet = maN = kg*m/s2
936A 3.5 kg object experiences an acceleration of 0.5 m/s2. What net force does the object feel?
947A 12 N net force acts on a 36 kg object? How much does it accelerate?
958How much net force is required to accelerate a 0.5 kg toy car, initially at rest to a velocity of 2.4 m/s in 6 s?
96Newton’s Second Law of Motion We can use this equation to understand how force, mass, and acceleration are related. When two variables are 'directly proportional', it means that if we increase one, the other will increase as well.Fnet = m aIn Newton's 2nd Law, force and mass are directly proportional. More mass requires more force to create the same acceleration.Similarly, force and acceleration are directly proportional. To increase the acceleration of an object, more force must be applied.
97Newton’s Second Law of Motion We can rearrange this equation to see how mass is related to acceleration:a = FnetmAcceleration is directly proportional to force and inversely proportional to mass.
98Newton’s Second Law of Motion a = FnetmWhen two variables are 'inversely proportional', it means that if we increase one, the other will decrease! So if we increase the mass of an object under a constant force, the acceleration will decrease.Think about what happens when you move a filled wheelbarrow...
999A net force F accelerates a mass m with an acceleration a. If the same net force is applied to mass 2m, then the acceleration will becA4acB2acCa/2cDa/4
100A constant net force acts on an object. The object moves with: 10A constant net force acts on an object. The object moves with:cAconstant accelerationcBconstant speedcCconstant velocitycDincreasing acceleration
10111A net force F acts on a mass m and produces an acceleration a. What acceleration results if a net force 2F acts on mass 4m?Aa/2cB8accC4acD2a
102The acceleration of an object is inversely proportional to: 12The acceleration of an object is inversely proportional to:cAthe net force acting on it.Bits position.ccCits velocity.cDits mass.
104Net ForceΣF = maLet's look at the left side of this equation first.ΣFThe greek letter sigma "Σ" means "the sum of". Sometimes ΣF is written as Fnet, it means the same thing.It means you have to add up all the forces acting on an object.
105Net ForceΣFThe arrow above "F" reminds you that force is a vector. We won't always write the arrow but remember it's there.It means that when you add forces, you have to add them like vectors: forces have direction, and they can cancel out.
106Net ForceΣFExample: A 5.0 kg object is being acted on by a 20N force to the right (F1), and a 30N force, also to the right (F2). What is the net force on the object?First we'll draw a free body diagram. We will discuss these in more detail later on but for now, follow these simple directions. FBDs consists of a dot, representing the object, and arrows representing the forces. The direction of the arrows represents the direction of the forces...their length is roughly proportional to their size.
107Newton’s Second Law of Motion ΣF Example: A 5.0 kg object is being acted on by a 20N force to the right (F1), and a 30N force, also to the right (F2). What is the net force on the object?F1The first force (F1) acts to the right with a magnitude of 20 N
108Newton’s Second Law of Motion Example: A 5.0 kg object is being acted on by a 20N force to the right (F1), and a 30N force, also to the right (F2). What is the net force on the object?F2F1The second force, F2, acts to the right also, with a greater magnitude of 30N. This is drawn slightly larger than F1.
109Newton’s Second Law of Motion Example: A 5.0 kg object is being acted on by a 20N force to the right (F1), and a 30N force, also to the right (F2). What is the net force on the object?To add vectors, move the second vector so it starts where the first one ends.F1F2The sum is a vector which starts where the first vector started, and ends where the last one ends.
110Newton’s Second Law of Motion Example: A 5.0 kg object is being acted on by a 20N force to the right (F1), and a 30N force, also to the right (F2). What is the net force on the object?These free body diagrams are critically important to our work. Once done, the problem can be translated into an algebra problem.F1F2ΣF
111Newton’s Second Law of Motion For example: A 5.0 kg object is being acted on by a 20N force to the right (F1), and a 30N force, also to the right (F2). What is the net force on the object?F1F2ΣFFirst we will define "to the right" as positive.Then we can interpret our diagram to read:ΣF = F1 + F2ΣF = 20N + 30NΣF = 50N to the rightwe get the direction from our diagram and from our positive answer, which we defined as meaning "to the right"
11213Two forces act on an object. One force is 40N to the west and the other force is 40N to the east. What is the net force acting on the object?
113Two forces act on an object. One force is 8.0 14Two forces act on an object. One force is 8.0N to the north and the other force is 6.0N to the south. What is the net force acting on the object?cA14 N to the northcB14 N to the southcC2 N to the northcD2 N to the south
114Newton’s Second Law of Motion maNow let's look at the right side of our equation.Mass is a scalar...it does not have a direction.But acceleration does have a direction...it is a vector.The direction of the acceleration vector is always the same as the direction of the net force, ΣF, vector.
115Newton’s Second Law of Motion For example: A 5.0 kg object is being acted on by a 20N force to the right (F1), and a 30N force, also to the right (F2). We found the net force on the object to be 50N to the right.Now let's find its acceleration.F1F2ΣFAnswerΣF = maa = ΣF / ma = 50N / 5.0 kga = 10 m/s2 to the right s2 to the right
116Newton’s Second Law of Motion Force is a vector, so ΣF = ma is true along each coordinate axis.That means we can add up all the forces in the vertical direction and those will equal "ma" in the vertical direction.And then can do the same thing in the horizontal direction.F1F2F3a = 1 m/s2F1F2F1 +(-F2) = m*aF1 - F2 = 0F3F3 = m*aF3 = (2kg)(1 m/s2)F3 = 2 Na = 1 m/s2
11715A force F1 = 50N acts to the right on a 5.0 kg object. Another force, F2 = 30N, acts to the left.Find the acceleration of the object:
11816A force F1 = 350N pushes upward on 20.0 kg object. Another force, F2 = 450N pulls downward.Find the acceleration of the object:
11917An object accelerates downward at a rate of 4.9 m/s2. If the downward force on the object is 500N and the upward force is 250N, what is the mass of the object?
12040A 90 kg climber rappels from the top of a cliff with an acceleration of 1 m/s2. Determine the Tension in the climber's rope.
12141A crane lifts a 400 kg crate upward with an acceleration of 3 m/s2. Determine the Tension in the crane.
122Newton's 3rd Law of Motion Return toTable ofContents
123You need two objects to have a force. Newton’s Third Law of MotionAny time a force is exerted on an object, that force is caused by another object.You need two objects to have a force.Force exerted on cat by tableNewton’s third law:Whenever one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.Force exerted on table by cat
124Newton’s Third Law of Motion Whenever one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.For every action, there is an equal, opposite reaction. This is another way to state Newton's 3rd Law.It is important to remember that the forces (or actions) are always applied to two different objects.
125Newton’s Third Law of Motion A key to the correct application of the third law is that the forces are exerted on different objects.Make sure you don’t use them as if they were acting on the same object. Then they would add to zero!Force on handsForce on floor
126Newton’s Third Law of Motion Rocket propulsion can also be explained using Newton’s third law. Hot gases from combustion spew out of the tail of the rocket at high speeds. The reaction force is what propels the rocket.Note that the rocket does not need anything to “push” against.
127Newton’s Third Law of Motion Helpful notation: the first subscript is the object that the force is being exerted on; the second is the source.Horizontal force exerted on the ground by person's footFGPHorizontal force exerted on the person's foot by groundFPGSubscripts help keep your ideas and equations clear.FGP = -FPG
128When you sit on a chair, the net external force on you is 27When you sit on a chair, the net external force on you isAzero.ccBdependant on your weight.cCup.cDdown.
12928An object of mass m sits on a flat table. The Earth pulls on this object with force mg, which we will call the action force. What is the reaction force?cAThe table pushing up on the object with force mgcBThe object pushing down on the table with force mgcCThe table pushing down on the floor with force mgThe object pulling upward on the Earth with force mgDc
13029A 20-ton truck collides with a 1500-lb car and causes a lot of damage to the car. Since a lot of damage is done on the carAthe force on the truck is greater then the force on the carcBthe force on the truck is equal to the force on the carcthe force on the truck is smaller than the force on the carCcthe truck did not slow down during the collisioncD
13130As you are sitting in a chair, you feel the chair pushing up on you. The reaction force in this situation is:cAThe chair pushing down on the groundcBGravity pulling down on youcCYou pushing down on the chaircDThe ground pushing up on the chair
13231A student is doing push-ups in gym class. A reaction pair of forces is best described as:cThe student pushes down on the ground -The ground pushes up on the studentAGravity is pulling the student down -The ground is pushing the student upBcGravity is pulling the student down -The student's arms push the student upcCThe student's hands push down on the ground -The students arms push the student upcD
13332Which of Newton's laws best explains why motorists should wear seat belts?Athe first lawccBthe second lawcCthe third lawcDthe law of gravitation
134(Note: there may be more than one answer. Be prepared to explain WHY!) 33If you blow up a balloon, and then release it, the balloon will fly away. This is an illustration of:(Note: there may be more than one answer. Be prepared to explain WHY!)cANewton's first lawcBNewton's second lawcCNewton's third lawcDGalileo's law of inertia