2 Forces What is a Force? Anything that changes the state of rest or motionof an objectIt’s what causesACCELERATIONhas magnitude and directionTherefore force is a vector
3 Force cont.A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. Units = Newtons (N)
4 Forces can be COMBINED: Net Force: the combination of all of the forces acting on the objectBalanced Forces: When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion.Warning: Doesn’t always meanthe object is at rest;Example: Tug of war,Pushing piano, cartraveling at constantvelocity
5 Results when net force acting on an object is NOT equal to zero Unbalanced ForcesResults when net force acting on an object is NOT equal to zeroWhen an unbalanced force acts on an object, the object accelerates in the direction of the net forceFig 14, p.332*(moves in direction of net force)
6 Force A push or pull on an object Units: Newton (N) = 1 kg x m/s2 Net Force (Fnet) : sum of all of the forces that are acting on an objectBalanced Forces: Fnet = 0Object is at rest or moving with constant velocityUnbalanced Forces: Fnet is not equal to 0,Object is accelerating or decelerating
7 Types of ForcesApplied – contact force in the direction the object is movingTension- caused by a rope, cable, etc., directed away from the objectFriction- opposes the motion of objects, must have contactNormal- caused by a surfaceForce of gravity- force at a distance, caused by attraction between two objects
8 Two Types of Friction Static Friction of an object at rest Greater than kinetic friction (harder to get an object moving than to keep it moving)Friction is greatest when surfaces are roughex. the force that is keeping this block fromsliding downhillKineticFriction of a moving object
9 Types of Kinetic Friction SlidingTwo surfaces moving past each otherRollingMovement of a circular object on a surfaceEx. Cart on wheelsFluid FrictionMovement of an object through a liquid or gasEx. Air resistance
10 FRICTION STATIC SLIDING ROLLING FLUID is defined as& an example isThe friction between surfaces that are stationaryforce that exists when objects slide past each otherforce that exists when a round object rolls over a flat surface (usually less than sliding friction)force that exists when an object moves through a fluid (air, water)a book sitting on a table-hockey puck on ice-child going down a slide-a sled down hill-a roller blade on a sidewalk-bowling ball on bowling alley-a car driving down the road-swimmer swimming through pool
11 How can we decrease friction? Watch this demo and see…Can you think of a situation in which you would want to increase friction?
12 Gravity is a force: natural phenomenon in which objects that have mass are attracted to one another Gravity is an attractive force pulls objects togetherEarth’s gravity acts downward toward the center of the Earth.There is an upward force that balances gravityAIR RESISTANCE
13 Falling Objects have two forces acting on them:Gravity causes objects to accelerate downwardair resistance acts in the direction opposite to the motion & reduces acceleration.
14 Free Fall: the motion of a body when only the force of gravity is acting on the body Free fall acceleration of an object is directed toward the center of EarthBecause free fall acceleration results from gravity, its symbol is gAcceleration due to gravity on Earth = 9.8 m/s2Formula for objects in free fall: d = ½ gt2
15 In a vacuum, two objects would accelerate at the same rate because both are in free fall (Fig 8, p.354)Question: What other force is not present in vacuum that would affect acceleration?Answer = air resistance
16 Drawing Free Body Diagrams Identify the types of forces acting on the objectUse a dot or a box to represent the objectUse arrows to represent the direction and size of the force-change the size of the arrow if more force isbeing applied in one direction versus the otherLabel the arrows with the type of force
17 Free Body DiagramsHow will the object move in the diagram to the right?Answer: Object may be at rest and not move or it could be moving at a constant velocity since all the forces are balanced and the net force is zero
19 II.) Newton’s First Law of Motion A) Historical Development1) Aristotle (384 BC- 322 BC): Incorrectly proposed that force is required to keep an object moving
20 2) Galileo (1564 – 1642): Concluded that moving objects not subjected to friction or other force, would continue to move indefinitely; Disproved Aristotle
21 3) Newton (1643 – 1727): Defined mass and force; Introduced 3 Laws of Motion
22 C) Newton’s 1st Law of Motion 1) According to Newton’s 1st Law, the state of motion of an object does not change as long as the net force is zero.a) Basically saying that an object at rest will stay at rest unless a force acts on it1) Example: Soccer ball will remain (at rest) on the grass unless a force is acted on it
23 “Science and the Consumer” 2) Sometimes called the “Law of Inertia”a) Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object1) Car crash: You continueforward because of inertia“Science and the Consumer”p.348
24 How is inertia related to mass? P 347 Mass is a measure of inertia.Who would you rather be tackled by…a toddler or a defensive lineman?What is easier to move? An empty garbage can or a garbage can full of lead? Why?The empty garbage can has less mass= less inertia= less resistance to being accelerated.