1. What causes acceleration? straight line (line of site) path of bullet path of monkey the monkey begins to fall ad the precise moment when the ball leaves the barrel of the gun The ball and the monkey arrive at the point marked by the red dot at the same time

2. What causes acceleration? Analysis First, imagine what would happen in the absence of gravity ObjectExternal InfluenceMotion monkeynone“floats” in place ballnonestraight line with constant speed Result: ball hits monkey ObjectExternal InfluenceMotion monkeygravityuniform acceleration toward ground ballgravitysideways at constant speed + downward with uniform acceleration Result: ball hits monkey It’s very useful to consider motion without the effects of gravity!

3. What causes acceleration? Aristotle: forces cause velocities Galileo: “external influences” Newton: “unbalanced force” For now we define force according to its effect on the motions of things -the operational definition we find is concisely expressed by three (3) simple rules First we examine how forces act – then we look at a restatement of the rules Preliminary Issue: mass, weight and what they mean to freely falling monkeys Mass 1. a measure of how much matter is in an object.. literally proportional to the number and size of atoms in the object 2. a measure of the internal resistance of an object to a change in its motion…perhaps you could think of it as a measure of the objects “desire” to remain in motion at a constant speed in a straight line.

4. What causes acceleration? Mass * mass is an intrinsic property of an object – to change the mass of an one must make a change (physical, chemical, or nuclear) in the object * in order to make two objects accelerate at the same rate one must pull or push harder on the more massive object In the monkey demonstration (and in Galileo’s law of falling) we observe that objects under the influence of gravity alone fall with equivalent rates of acceleration objectExternal InfluenceAcceleration monkeyforce of gravity (weight of monkey)10 m/s 2 ballforce of gravity (weight of ball)10 m/s 2 grain of sandweight of grain of sand10 m/s 2 dump truckweight of dump truck10 m/s 2 let’s look at this…………….

5. What causes acceleration? object resistance to accelerationExternal InfluenceAcceleration monkeymass of monkeyweight of monkey10 m/s 2 ballmass of ballweight of ball10 m/s 2 grain of sandmass of grain of sandweight of grain of sand10 m/s 2 dump truckmass of dump truckweight of dump truck10 m/s 2 amount of stuff external influence on the stuff acceleration of the stuff (the motional reaction) Conclusion: There must be a direct proportionality between weight and mass more mass greater influence of weight force but, same acceleration!!

6. What causes acceleration? By analyzing free-fall we conclude that: AND that the weight force is proportional to the mass: mass weight twice as much mass twice as much weight same ratio

7. What causes acceleration? The genius of Newton and the power of explanation Newton: gravity is only one type of force (what about friction, pushing pulling, electric attraction or repulsion…) gravity is “special” in the sense that gravitational force (a.k.a. weight) is proportional to mass Nevertheless, whenever any net force acts upon any mass to cause any acceleration, the rule is: NET FORCE = MASS X ACCELERATION F = m a the meaning of “net” force: an object a force exerted on the object F1F1 F2F2 F3F3 4 another force exerted on the object IF: acceleration (F 3 = F 4 ) F 2 > F 1 to the right (F 3 = F 4 ) F 1 > F 2 to the left (F 3 = F 4 ) F 1 = F 2 none (i.e. straight line at constant speed) NOW YOU TRY!!

8. Net Force When forces balance, there is equilibrium. F 1 = Force felt by Bo because of Diddley. F 2 = Force felt by Diddley because of Bo. F 1 = F 2 F1F1 F2F2 BoDiddley

9. What causes acceleration? Force is a vector quantity! TOTAL NET FORCE ON AN OBJECT MASS OF THE OBJECT ACCELERATION OF OBJECT CAUSED BY NET FORCE = X expressed in “Newton's” [N] expressed in “kilograms” [kg] expressed in “meters per second, per second” m/s 2 F1F1 F2F2 mass say F 1 > F 2 then (F 1 – F 2 ) = mass x acceleration = ma acceleration for F 1 > F 2

10. What causes acceleration? The “Newton” is the standard measure of force in the system we use. Push on a 1 kilogram MASS with a FORCE of 1 Newton then the object will ACELLERATE at a rate of 1 m/s 2 in the direction of the force….provided that the force is not “balanced” by another force on the object. If your body contains 75 kg of mass then… on earth you weigh (force) about 75kg X 10 m/s 2 = 750 Newtons mass acceleration Some examples 1.Mass and weight (a) in free-fall on Earth 10 kg weight acceleration = 10 m/s 2 net force = mass X acceleration a 10 kilogram block has a weight on Earth of 100 N!

11. 1 kg weight acceleration = 10 m/s 2 1 kg weighs 10 Newtons on Earth Convert to lbs= 1 kg weighs about 2.2 lbs 2. sitting on a table on Earth What forces are acting on the apple? W = weight force exerted by the Earth on the apple F = contact force exerted by table on the block Net force upward massacceleration =X F – W = 1 kg X 0 m/s 2 F – W = 0 Newtons for the acceleration to be equal to zero the forces must be balanced F = W = 1 Newton 1 kg If two forces act in opposite directions, the net force is in the direction of the larger force and is equal to the difference between the larger and smaller force

12. 3. skydiving 10 kg W = weight force exerted by Earth ON block F air = force of air resistance exerted BY air on block acceleration If F air = 60 Newtons at what rate does the block accelerate? 1.First find the “NET FORCE” : W-F air What’s the weight? W = mass X g = 10 kg X 10 m/s 2 = 100 N NET FORCE = 100 N – 60 N = 40 N 2.NET FORCE = MASS X ACCELERATION 40 N = 10 kg X ACCELERATION 3. Solve for acceleration:

13. Terminal velocity – The speed at which air resistance (aka drag) matches the pull of gravity,resulting in a constant fall rate. What is the net force on a skydiver as she falls at terminal speed? NET FORCE = MASS x ACCELERATION F ait – W = M x A F ait – W = M x 0 = 0 F ait – W = 0 What is the force of air resistance on a skydiver whose mass is 40 kilograms when she falls at terminal speed? NET FORCE = ZERO F ait – W = 0 F air = W = 40kg X 10 m/s 2 40 kg W = weight force exerted by Earth ON block F air = force of air resistance exerted BY air on block acceleration = 0

14. Laws of Motion 1. Newton’s 1 st Law: An object at rest, remains at rest, OR if in motion, travels in a straight line at constant velocity, UNLESS acted on by a net force.

15. Laws of Motion 2. Newton’s 2 nd Law: For a body having constant mass, M, the NET FORCE APPLIED to the body, the Mass of the body, and the observed Acceleration of the body are related by: FORCE = MASS X ACCELERATION F = MA A 2 kg mass is acted on by a 2 N force. What is its acceleration? a = 2 N / 2 kg = 1 m/s 2 What if a 0.5 N frictional force was also in place? a = (2 - 0.5 N)/2 kg = 0.75 m/s 2 2 kg F = 2N 2 kg F = 2N f = 0.5 N

16. Laws of Motion 3.Newton’s 3 rd Law: For every action there is an equal and opposite reaction. – rule of force pairs Objects can not act on one another without being acted upon. When you strike a wall, does it hurt your hand? You might say the wall struck you. Newton would say the force you applied to the wall was the same as that which the wall applied to you. The wall is bigger and more massive, therefore has more inertia and was not harmed as much as you. Weight Applied Force “Pushing” frictional force from wall Reaction force from wall If you push on a wall, the wall pushes back on you. It does so with a force equal in strength to the one you exert on it!

17. The “rule of force pears” For every force exerted ON an object, the object exerts an equal and oppositely directed force ON its environment Force on Rock from Earth = Force on Earth from Ball a = F/m = g Rock acceleration a = F/m Earth’s acceleration

19. Gravity

20. contact force exerted by table on the apple contact force exerted by the apple on the table force of Earth on the apple force of apple on the Earth