Momentum and Impulse Ch. 8 in your text book

Transcription

1 Momentum and Ch. 8 in your text book Objectives Students will be able to: 1) Calculate the impulse of an object given a change in mass or velocity 2) Calculate the force felt by and object given an impulse and time duration

2 a change in momentum J = Δp = p f -p i J = mv f -mv i J = m(v f -v i ) Unit kgm/s

3 Check for understanding A 1000 kg car accelerates from 10 m/s to 20 m/s. What was the impulse on the car? J = m(v f -v i ) J = 1000(20-10) J = 10,000 kgm/s

4 Check for understanding A 250 g ball rolls at a speed of 10 m/s. It is then kicked in the opposite direction at a speed of 10 m/s. What was the impulse on the ball? J = m(v f -v i ) J =.25(10 (-10)) J = 5 kgm/s

5 A 180 g egg was dropped from the railing outside. The railing is 5.65 m above the ground. How fast was the egg going right before it hit the ground? How much of an impulse is needed to stop the egg?

6 A 180 g egg was dropped from the railing outside. The railing is 5.65 m above the ground. How fast was the egg going right before it hit the ground?

7 A 180 g egg was dropped from the railing outside. The railing is 5.65 m above the ground. How much of an impulse is needed to stop the egg? J = m(v f -v i ) J =.18(0 (10.63)) J = kgm/s

8 From the 4 th story, the impulse to stop the egg was the same for both guys. What was the difference in why one caught it and the other broke it?

9 If the mass of an object is constant but the velocity changes, the momentum must change (impulse). This velocity change takes some amount of time. A change in velocity over time is acceleration. J = m(v f -v i )

10 Where do you feel the most force? 1. Slamming on the breaks or gently pressing on them 2. Landing on a trampoline or the sidewalk when you jump off the fence Regardless of how long it takes to stop, the change in momentum will be the same. J = Ft J = Ft

11 With your partner, find a couple examples of when you want the impulse on an object to be done over: 1) A short period of time (large force) 2) A long period of time (small force)

12 Example A 1 kg dart is shot at 10 m/s. It hits a 2 kg block of wood that is sitting on an air hockey table (no friction). 1) How fast are the dart and block moving together? Before dart: p = (1)(10) = 10 kgm/s block: p = 0 kgm/s total: p = 10 kgm/s After total: p = 10 kgm/s mass = 3 kg 10 = 3v v = 3.33 m/s

13 Example A 1 kg dart is shot at 10 m/s. It hits a 2 kg block of wood that is on an air hockey table (no friction). 2) What was the impulse on the block and on the dart? J = m(v f -v i ) J dart = m(v f -v i ) J dart = 1( ) J dart = kgm/s J block = m(v f -v i ) J block = 2(3.33-0) J block = 6.67 kgm/s The impulses are equal and opposite.

14 Example A 1 kg dart is shot at 10 m/s. It hits a 2 kg block of wood that is on an air hockey table (no friction). 3) If it took.05 seconds for the dart to stick to the block, how much force did it feel? J = Ft = F(.05) F = N 4) How much force did the block feel?

15 Remember Newton s 3 rd Law of motion? It is still valid. The forces felt are equal and pointed in opposite directions. The accelerations are not equal. The forces are equal and opposite, but the smaller object will have a higher acceleration.

16 Example You get rear ended at a stop light. The 800 kg car that hits your 1100 kg car was going 10 m/s when it hit you. 1) How fast are the cars going moving after the crash if they stick together? Before After them: p = (800)(10) = 8000 kgm/s you: p = 0 kgm/s total: p = 8000 kgm/s total: p = 8000 kgm/s mass = 1900 kg 8000 = 1900v v = 4.21 m/s

17 Example You get rear ended at a stop light. The 800 kg car that hits your 1100 kg car was going 10 m/s when it hit you. 2) If it took.25 seconds for the crash to happen, how much force did you feel? J = Ft m(v f -v i ) = Ft (1100)(4.21 0) = F(.25) F = 18,524 N