# Aim: How do we explain momentum and impulse? Aim: How do we explain momentum and impulse? Definition of impulse-impulse momentum theorem The total impulse of the net force F on a F on a particle equals the change in momentum of the

particle. I=Fdt = pF on a Fdt = pp It also applies to a system of particles in which we consider the net force external to the system I=Fdt = pF on a Fextdt = pptot

Impulse approximation I = pp=F on a Fpt The impulse approximation: We assume that one of the forces exerted on a particle acts for a short time but is much greater than any other force present.

Thought Question 1 Two objects are at rest on a frictionless surface. Object 1 has a larger mass than object 2. When a force is applied to object 1, it accelerates through a displacement px. The force is removed from object

1 and is applied to object 2. At the moment when object 2 accelerates through the same displacement px, which statements are true? a) p1 p2 d) K1 K2

Thought Question 2 In boxing matches of the 19th century, bare fists were used. In modern boxing, fighters wear padded gloves. How does this better protect the brain of the boxer from injury? The time over which the blow is delivered is

lengthened so the force of the blow is not as great. Thought Question 3 A race car travels rapidly around a circular race track at constant speed. For a given portion of the track, what is the direction of the

impulse vector. Once the car returns to the starting point, there is no net change in momentum for the entire trip around the track. Thus according to the impulse momentum theorem, there must be no impulse. Yet a nonzero force acted over the time interval for one rotation around the track. How can the impulse be zero? The impulse is zero because the average net force over one

rotation is zero. The direction of the impulse can be found by adding the final momentum vector to the negative of the initial momentum vector. Problem 1 In a crash test, an automobile of mass 1,500 kg collides

with a wall. The initial and final velocities of the automobile are vi = - 15.0i m/s and vf = 2.6i m/s. If the collision lasts for 0.150s, find the impulse due to the collision and the average force exerted on the automobile.

Problem 2 A 0.150 kg baseball is thrown with a speed of 40.0 m/s. It is hit straight back at the pitcher with a speed of 50.0 m/s. a) What impulse is delivered to the baseball? b) Find the average force exerted by the bat on

the ball if the two are in contact for 2.00 x 10-3 s. Problem 3 An estimated force-time curve for a baseball struck by a bat is shown. a) Determine the impulse delivered to the ball.

b) Determine the average force exerted on the ball. c) Determine the peak force exerted on the ball. Problem 4 A 3.00 kg steel ball strikes a wall with a speed of 10.0 m/s at an angle of 60 degrees. It bounces off

with the same speed an angle. If the ball is contact with the wall for 0.200s, what is the average force exerted on the ball by the wall?