3Why does the Earth Spin? Second clip? Second clip?
4Force - A push or pull on an object. VocabularyForce - A push or pull on an object.Net Force – vector sum of all forcesEquilibrium:the state in which the net force on an object is zero or balancedno change in velocity
61st Law of Motion (Law of Inertia) 2.2.4 State Newton’s first law of motion Describe examples of Newton’s first law.1st Law of Motion (Law of Inertia)An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.
71st Law of Motion (Law of Inertia) Translation: Objects keep doing what they are doing, in a straight line.
8Newton’s 1st Law – Inertia Inertia is a measurement of ______________ TO CHANGEIt is measured in terms of ______. The greater the _____, the greater the inertia.RESISTANCEmassmass
9What is Mass? Mass is the amount of material a body/object contains You are what you eatBigger does not equal more mass
10FBD’s are a neat way to show the forces acting on an object. Identify the forces acting on an object and draw free-body diagrams representing the forces actingFree Body DiagramsFBD’s are a neat way to show the forces acting on an object.The object is drawn as a dot or a box, and then vectors are drawn to show the forces acting on it.Ex:10 N6 N5 N
11FBD’s Can you draw the FBD for you sitting on your chair? Draw the FBD for your friend pushing you in a rolling chair.
12FBD’sSolve the net force for the following FBD.10 N6 N5 N
13FBDThe picture at the bottom shows a snowman pulling a sled. Draw a free-body diagram for this sled. The magnitudes of the forces acting on the sled are 60N by the string, 130N from gravity, and 90N upward by the ground.
14FBD Now try doing some of the free-body diagrams on pg 124 Practice A Pg 124 Section Review # 2-5
152. 2. 6 - State the condition for translational equilibrium. 2. 2 State the condition for translational equilibrium Solve problems involving translational equilibrium.Equilibrium in a straight line.Fnet = 0 so there is constant velocity
162. 2. 6 - State the condition for translational equilibrium. 2. 2 State the condition for translational equilibrium Solve problems involving translational equilibrium.Derek leaves his physics book on top of a drafting table that is inclined at 35º angle. The FBD to the right shows the forces acting on the book. Find the net force acting on the book.
17Vocabulary Force - A push or pull on an object. Net Force – vector sum of all forcesEquilibrium:the state in which the net force on an object is zero or balancedno change in velocity
182.2.8 State Newton’s second law of motion. The net force of an object is equal to the product of its mass and acceleration, or F=ma.Rolling chairs
19Newton’s 2nd Law – Simplified So, a little easier to read would be: a = ΣF OR ΣF = ma mSo… what are the units for forceForces are measured in newtons N = (kg)(m/s2).
20What does F = ma mean?Force is directly proportional to mass and acceleration.Imagine a ball of a certain massmoving at a certainacceleration. Thisball has a certainforce.
21What does F = ma mean?Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball has twice the force of the old ball.
22What does F = ma mean?Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.
23More about F = maIf you double the mass, you double the force. If you double the acceleration, you double the force.What if you double the mass and the acceleration?(2m)(2a) = 4FDoubling the mass and the acceleration quadruples the force.So what if you decrease the mass by half? How much force would the object have now?
24What does F = ma say?F = ma basically means that the force of an object comes from its mass and its acceleration.Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force.
25Newton’s 2nd Law – Vectors! Remember, acceleration is a vector quantity, so Force will be too Some equations to remember:Pythagorean TheoremSin θCos θTan θ
262. 2. 1 Calculate the weight of a body using the expression W = mg 2.2.1 Calculate the weight of a body using the expression W = mg. Gravity causes weightThe weight of an object is caused by the force of attraction between the earth and objects on the surface of the earth.Gravitation force is equal to weight, therefore…
27How much does a person with a mass of 70.0 kg weigh on the earth? Weight = mass X 9.8 m/s2Weight = 70.0 kg X 9.8 m/s2Weight = 686 NRemember that 1 N = (1 kg) (1m/s2)
28Mass vs. Weight Your MASS is the amount of material in your body. Your MASS doesn’t depend on where you are.Your WEIGHT is how much your body pushes down on a scale.Your WEIGHT depends on how much MASS you have and where you are.Weight is caused by gravity.
29What does Marty weigh? 18,700 N 44 N Marty m = 68 kg G(sun)= 275 m/s2 G(pluto)= m/s244 N
302.2.9 Solve problems involving Newton’s second law. Determine the accelerations that result when a 12-N net force is applied to a 3-kg object.A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. What is the mass of the encyclopedia?
312.2.9 Solve problems involving Newton’s second law. Now try answering some problems on pg 132 practice C # 1-4
32There is a net force of 12N acting on a desk which has a gravitational force of 40N. How much is the desk accelerating?
33A 50N crate experiences a horizontally applied force of 10N from a student and a frictional force of 4N. What is the acceleration of the crate?
34Newton’s 2nd Law – Let’s practice You have run out of gas and have to push your car. Your car weighs 560 N and you push with a force of 395 N, and your friend helps with 275 N. What is the acceleration of the car?You push a rolling chair (m=20kg) with a force of 17 N. A friend pushes with 15 N at 30o. What is the net force? What is the acceleration of the chair?
35Quick QuizConsider the following free-body diagram. Ignoring air resistance and friction, can this object be moving in a horizontal direction?We’ll come back to this question.
362. 2. 14 State Newton’s third law of motion. 2. 2 State Newton’s third law of motion Discuss examples of Newton’s third law.
37For every action, there is an equal and opposite reaction. 3rd Lawball
38What does this mean?For every force acting on an object, there is an equal force acting in the opposite direction.Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving.There is a balanced force acting on you– gravity pulling down, your seat pushing up.Put really simply: F = F
39Think about it . . .What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).
40Newton’s Third Law It states: It requires two forces: Action Reaction Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first objectIt requires two forces:_____________ Force______________ ForceActionReaction
41Newton’s Third LawDoes it matter which force we call the ________ and which is the _________?Forces always occur in ________. Therefore, it ________ matter which is the action and the reaction, as long as you can identify both.actionreactionpairsdoesn’tBlanks: pairs, doesn’t
42Review Newton’s First Law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.Newton’s Second Law:Force equals mass times acceleration (F = ma).Newton’s Third Law:For every action there is an equal and opposite reaction.
43Quick QuizConsider the following free-body diagram. Ignoring air resistance and friction, can this object be moving in a horizontal direction?YES!!!! IT CAN!!!
44Quick Quiz Think about this situation. What are the forces on the boy on the sled between points B and C?I hope you got something that looked a lot like this.
45Quick QuizThis means a force is not required to keep a moving object in motion.
50The Normal ForceThe normal force (FN) is one component of the force that a surface exerts on an object with which it is in contactNamely, the component that is ____________ to the surface.Perpendicular
51The Normal Force How do I find the normal force? Easy, it is equal and opposite to the weight of an object if the object is on a horizontal surface.The Fn is not always horizontal.Lets look at Fig 12 (pg136)The Fn can be found byFn=mg cosΘ (where Θ is the angle between the contact surface and horizontal)
53What is Friction? Types Static Kinetic Fluid is a force resisting the relative motion of two surfaces in contact with each other.is the parallel component of the force on an object that is in contact with a surface.is a force that prevents motion from occurring.TypesStaticKineticFluid
54What causes friction?There are two factors which affect friction between two surfaces:Kind of surfaces in contact (rough or smooth)Amount of force pressing the surfaces together.The rougher the surface and the stronger the force between the surfaces, the greater the amount of friction.
56Static and Kinetic Friction Static Friction- Friction force that acts on objects that are not movingKinetic Friction - Friction force that opposes the direction of motion of an object as it slides over a surface.I better be safe Ump!!
57Static FrictionStatic Friction is the force that prevents the start of motion.With no movement, Fs = FappliedHas a range of 0 – Fs,max
58Kinetic FrictionOnce motion has begun, the static friction force is no longer a concern.Instead, kinetic friction is important.Kinetic friction is resistance to movement of an already moving object.
59Fluid Friction- Friction force that opposes the motion of an object through a fluid.balloon
60Frictional ForcesThe frictional force will vary depending on the surface interactions.Ex. Carpet vs tileFriction can be calculated by using the coefficient of friction.Use the basic formula of:Ff = μFnIt can be rearranged to:μk = (Fk/Fn) and μs = (Fs/Fn)
61Frictional Forces Use the formula of: Ff = μFnFk = μkFn and Fs = μsFnA 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.
62Force of TensionTension commonly means the tendency of a rope to be pulled apart.Tension is typically the same everywhere in a rope because the rope is treated as massless. If it isn’t (and is vertical) you must account for the mass of the rope.
642.2.6 State the condition for translational equilibrium Translational equilibrium is equilibrium in a straight line.The net force acting on a free falling object at terminal velocity is zero, so it falls to the ground at constant speed.
65Terminal VelocityThe velocity at which a free falling object ceases to accelerate.Force of Gravity120 mi/hrAir ResistanceT.V. is the constant velocity of a falling object when the force of gravity equals the force of air resistance
66Spring ForceWe will dive deep into spring force when we get to elastic energy.Until then, play safely!!
67- forces that act between charged objects or particles such as protons and electrons Balloon2 tape stripsElectric Forces
76Use the formula of:Fk = μkFn and Fs = μsFnA hockey puck has a coefficient of kinetic friction of μk = If the puck feels a normal force (FN) of 5 N, what is the frictional force that acts on the puck? What is the mass of the puck?A desk has a mass of kilograms. If the coefficient of static friction between the desk and the floor is 1.14, what force must be used to move the desk from rest?
77Quick Review (Net Force) There is a 15kg book sliding down a desk that has an incline of 25º. If there is a frictional force of 20N, what is the acceleration of the book?Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. There is a frictional force of 11N, a normal force of 18N and gravitational force of 22N. What is the net force?
78On your own!! (Net Force)A student is designing a support to keep a tree upright. Two wires have been attached to the tree and placed at right angles to each other. One wire exerts a force of 30N on the tree; the other wire exerts a 40N force. Determine where the third wire should be placed and how much force is should have so that the net force acting on the tree is zero.A stationary, flying kite is acted on by a force of 9.8N downward. The wind exerts a force of 45N at an angle of 50° above the horizontal. Find the force that the string exerts on the kite.
79Putting it all together!! A student attaches a rope to a 20kg box of books. He pulls with a force of 90N at an angle of 30° with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is Find the acceleration of the box.Slow down, take a deep breath and lets take this one step at a time.
80Putting it all together!! Now lets try pg 141 Practice F, # 1-4Don’t get bogged down with #2
81What is momentum?The momentum of an object is equal to the product of its mass and its velocity. The unit for momentum is kg m/s