Transcript Momentum
Momentum Webster’s: A property of a moving body that determines the length of time required to bring it to rest when under the action of a constant force or moment. “Moving Inertia” What Quantities determine how hard it is to stop an object? • Mass • Velocity Which of the following has the most momentum? a) A person jogging down the street. b) A Volkswagen Bug traveling down the road. c) A Hummer parked at the mall. d) A freight train at the station. p = mv m = mass (kg) v = velocity (m/s) p = momentum ( kg•m/s = Ns) momentum is a vector quantity, it has magnitude and direction. Pay attention to directions!!!!!! Rewriting Newton’s 2nd Law v F ma a t v f vo mv f mvo p f po p F m t t t t p F t An impulse is a change in momentum, which is caused by applying a force for a period of time. picture impulse J p Ft Graphically: The area of a force vs. t graph is equal to the impulse or the p. F(N) impulse p t(s) The same p can be produced by applying a large force for a short time or a small force for a long time F p = t p = F t Example 1: A net force of 8N acts on a 200g object for a period of 5s. (a) What is the change in momentum? (b) Assuming the object starts from rest what is its final velocity after 5 seconds? a) Ft p 8 N 5s p 40 Ns p b) p mv f mvo kgm m 40 .200kg v f .200kg 0 s s m v f 200 s Example 2: A 300g ball strikes a wall horizontally with a speed of 25m/s and bounces straight back with a speed of 15m/s. The ball is in contact with the wall for 5x10-3s. What average force does the wall apply to the ball? 25m/s 15m/s Ft p Ft mv f mvo .3 15 .325 F 5 10 12 F 5 10 3 3 F 2400 N Example 3: The following graph represents the net force applied to a 5kg object vs. the time the force is applied. Find the final velocity of the object if the original velocity was 12m/s. 100Ns 50Ns 50Ns area = impulse = p 200 Ns p mv f mvo m 200 Ns 5kg v f 5kg 12 s 260 Ns 5kg v f m v f 52 s