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Chapter 8 Linear Momentum and Collisions Example: Truck Collision In a head-on collision: Which truck will experience the greatest impulse? Which truck will experience the greatest change in momentum? Which truck will experience the greatest force? Which truck will experience the greatest acceleration? Which truck will experience the greatest change in velocity? Which truck would you rather be in during the collision? • Discussion of the questions Chapter 8.1 and 8.2 • Linear Momentum • Momentum and Newton’s Second Law • Impulse •Graphical analysis and calculation 8-1 Linear Momentum A measure of how hard it is to stop a moving object. Momentum is a vector; its direction is the same as the direction of the velocity. Quantity of motion Momentum and Newton’s Second Law Newton’s second law: is only valid for objects that have constant mass. Here is a more general form, also useful when the mass is changing: 8.2 Impulse The impulse is the change in momentum Impulse is a vector, in the same direction as the average force. 8.1 Impulse The impulse is equal to the change in momentum. Think A 10 g rubber ball and a 10 g clay bear are thrown at a wall with equal speeds. The rubber ball bounces, the clay bear sticks. Which one exerts a larger impulse on the wall? demo Change in Momentum Change in momentum of the clay bear: (a) mv (b) 2mv (c) zero (d) none of these Impulse Therefore, the same change in momentum may be produced by a large force acting for a short time, or by a smaller force acting for a longer time. Let’s talk about traffic law • Why are bicyclists required to ride in the same direction as car traffic? • How does impulse work the safety features for cars (or bicycle helmets)? Example: Truck Collision In a head-on collision: Which truck will experience the greatest change in momentum? Which truck will experience the greatest impulse? Which truck will experience the greatest force? Which truck will experience the greatest acceleration? Which truck will experience the greatest change in velocity? Which truck would you rather be in during the collision? • Discussion of the questions Answer: Truck Collision Comparison of the collision variables for the two trucks: A greater change in velocity implies a greater change in kinetic energy and therefore more work done on the driver. Ride in the bigger truck! There are good physical reasons! Answer Demo Impulse (J) F(N) 3000 2000 area under curve 1000 0 0 1 2 3 4 t (ms)