2Have you ever wondered… Why golfers and bowlers “follow through”?Why skydivers bend their knees upon impact?Why falling on a wood floor (or into a swimming pool) hurts less than falling on a cement one?
3Have you ever wondered… What’s the best strategy to win a “water balloon toss” or an “egg toss”?Why we make auto air bags and padded dash boards?How the police can figure out how fast you were going BEFORE an accident by measuring your skid marks?
4Impulse and MomentumTo answer all of these questions--you need to understand: Impulse and MomentumRecall, Newton’s first law…The Law of InertiaThe study of inertia for moving objects is the study of “momentum”
5Consider the following: When a baseball bat hits a baseball, a force is applied OVER A SHORT PERIOD OF TIME.
6ImpulseDefn: A force applied over a period of time is called an IMPULSE.What are the SI units of Impulse?
7Example - ImpulseA batter hits a 500g ball with a force of 200N. If the time of contact was seconds, find the impulse of the ball.
8Example #2A batter hits a 500g ball with a force of 500N. If the time of contact was seconds, find the impulse of the ball.0 N·s102.5 N·s51250 N·sN·sI have no clue…
9Answer: (b)A batter hits a 500g ball with a force of 500N. If the time of contact was seconds, find the impulse of the ball.
10Example #3A batter hits a 500g ball with a force of 500N. If the impulse is 75 N·s, then what is the contact time between the ball & bat?0 sec0.150 sec75 sec37500 secI have no clue…
11Answer: (b)A batter hits a 500g ball with a force of 500N. If the impulse is 75 N·s, then what is the contact time between the ball & bat?
12Example #4A batter hits a 500g ball and the impulse given to the ball is 125 N·s. If the ball & bat are in contact for 200 milli-seconds, then what is the force of the bat?0 N0.625 N2.5 N625 NI have no clue…
13Answer: (d)A batter hits a 500g ball and the impulse given to the ball is 125 N·s. If the ball & bat are in contact for 200 milli-seconds, then what is the force of the bat?
14Back to baseball...Which will have the greater speed afterward? A large or a small ball?A fast or a slow ball?A large or a small bat?A fast or a slow swing?
15MomentumDefn: The MOMENTUM of a body is equal to the product of its mass and its velocity.Is momentum a vector or a scalar?What are the SI units of momentum?
16Determine the following momentums… Ex #1 - A 1200lb car moving at 60mphEx #2 - A 5.0g bullet moving at 900m/sEx #3 - A 3 ton elephant sitting stillEx #4 - A 150g hummingbird flying at 40mphALL answers need to be in kg m/s1 lb = kg; 1 mph = m/s
17Determine the following momentums…Solution Ex #1 - A 1200lb car moving at 60mph= kg m /sEx #2 - A 5.0g bullet moving at 900m/s= 4.5 kg m/sEx #3 - A 3 ton elephant sitting still= 0 kg m/sEx #4 - A 150g hummingbird flying at 40mph= 2.68 kg m/s
18Determine the following momentums… Can the hummingbird and the car ever have the same momentum?Explain…
19Example #5A boy standing at one end of a floating raft that is stationary relative to the shore walks to the opposite end of the raft, away from the shore. As a consequence, the raft.remains stationary,moves away from the shore,moves toward the shoreNot enough information given
20Answer: (c)A boy standing at one end of a floating raft that is stationary relative to the shore walks to the opposite end of the raft, away from the shore. As a consequence, the raft.Answer: (c) moves toward the shore - Newton’s 3 law
21Example #6Which has a greater momentum, a heavy truck at rest or a moving skateboard?Heavy truck,Skateboard,Neither (same momentum)Not enough information given
22Answer: (b)Which has a greater momentum, a heavy truck at rest or a moving skateboard?Answer: (b) The truck at rest has no speed, hence no momentum. So the moving skateboard has greater momentum.
23Making the Connection: Look at the units of Impulse and Momentum…Do you notice anything?Recall, F = m·a…can you rearrange the equation to say something about Impulse or Momentum?
24The Impulse-Momentum Theorem When a net force acts upon a body for a period of time, the Impulse applied by the force is equal to the body’s change in Momentum!
25Impulse-Momentum Theorem #1: If your Force is limited, how can you MAXIMIZE the impulse you apply?By increasing the time of contact!Golfing…Baseball…
26Example – Impulse & Momentum A batter hits a 500g ball with a force of 200N. If the time of contact was seconds, find the speed of the baseball if:(a) it is on a batting tee when hit(b) it is moving toward homeplate with aspeed of 35 m/s
29Impulse-Momentum Theorem #2 If your change in momentum is constant, how can you MINIMIZE the force applied to you?By increasing the time of contact!Bending knees upon impact, rolling with the punches, crumple zones, running shoes, air bags…
30Momentum Changes and Bouncing Is it more dangerous when a flower pot falls on your head and breaks or stays in one piece and bounces back up?When an object bounces, what happens to it’s velocity? How does that effect it’s change of momentum?Since ∆p is greater in bouncing situations, the Impulse applied will be greater!
31The Law of Conservation of Momentum Internal forces are forces that act within a system, external forces are forces that act on a system from outside…If a system is not affected by any net external force, it is said to be “ISOLATED”.In an isolated system, the total linear momentum is always conserved!
32Example - RecoilA hunter shoots a 0.22 rifle and the 3.0 kg gun recoils with a speed of m/s. If the bullet has a mass of 0.5 g, find the muzzle speed of the gun.
34Example #7Jocko, who has a mass of 60kg and stands at rest on ice, catches a 20kg ball that is thrown to him at 10km/h. How fast do Jocko and the ball move across the ice?(a) 0 km/hr (b) 2.5 km/hr (c) 200 km/hr (d) no way to determine
35Answer: (b)(b): The momentum before the catch is all in the ball, 20kg x 10km/h = 200kg·km/h. This is also the momentum after the catch, where the moving mass is 80kg—60kg for Jocko and 20kg for the caught ball.80kg x v = 200kg·km/h v = 200kg·km/h/80kg = 2.5km/h
37Example #8Which would be more damaging:Driving into a massive concrete wallDriving at the same speed into a head-on collision with an identical car traveling toward you at the same speedNeither - the same amount of damageNot enough information given
38Answer: (c)(c): Both cases are equivalent, because either way, your car rapidly decelerates to a dead stop. The dead stop is easy to see when hitting the wall, and identical cars at equal speeds means equal momenta—zero before, zero after collision.
39Example #9Strictly speaking, when a gun is fired, compared with the momentum of the recoiling gun, the opposite momentum of the bullet is(a) less(b) more(c) the same(d) no way to determine(Neglect the effect of the hand.)
40Answer: (a)(a) Why? Because more than just a bullet comes out of the barrel when a gun is fired. The gas, formed when the powder in the cartridge burns, pushes the bullet along the barrel and this gas too has appreciable mass and exits at high speed. So, Momentum of recoiling gun = momentum of bullet + momentum of gases.