Chapter 8 Momentum, Impulse and Collisions Momentum Impulse Impulse-Momentum theorem Conservation of linear momentum

Collisions Momentum, p The linear momentum p of an object is the product of the objects mass m and velocity v: = Linear momentum is a vector quantity that points in the

same direction as the velocity. SI Unit of Linear Momentum: kilogram meter/second = (kg m/s) Impulse, J The impulse J of a force is the product of the average force and the time interval Dt during which the force acts: tt

Impulse is a vector quantity and has the same direction as the average force. SI Unit of Impulse: newton second = (N s) = kg. m/s Impulse and momentum, both have the same unit. IMPULSEMOMENTUM THEOREM When a net force acts on an object, the impulse of the net

force is equal to the change in momentum of the object: 11. Suppose a child drives a bumper car head on into the side rail, which exerts a force of 4000 N on the car for 0.200 s. (a) What impulse is imparted by this force? (b) Find the final velocity of the bumper car if its initial velocity was 2.80 m/s and the car plus driver have a mass of 200 kg. Neglect the friction between the car and floor.

Impulse and Momentum in Sports Impulse and momentum play important roles in sports. Hitting a baseball Q: How can we determine the impulse?

Method-1: Knowing the average force () and contact time (tt), Impulse = J F Dt Method-2: Impulse = Area under the Force versus Time graph. Hailstones Versus Raindrops Unlike rain, hail usually does not come to rest after striking a

surface. Instead, the hailstones bounce off the roof of the car. If hail fell instead of rain, would the force on the roof be smaller than, equal to, or greater? Answer: Greater Example A baseball (m = 0.14 kg) has an initial velocity, magnitude 38

m/s as it approaches a bat. The bat applies an average force that is much larger than the weight of the ball, and the ball departs from the bat with a final velocity, magnitude 38 m/s, as shown below. Determine the impulse applied to the ball by the bat. Definitions of Terms Internal forces Forces that the objects within the system

exert on each other. External forces Forces exerted on the objects by agents that are external to the system. An isolated system is one for which the vector sum of the external forces acting on the system is zero. 7.2 The Principle of Conservation of Linear

Momentum The total linear momentum of an isolated system remains constant (is conserved). EXAMPLE Assembling a Freight Train A freight train is being assembled in a switching yard, and the Figure below shows two boxcars. Car 1 has a mass of m1 =

65103 kg and moves at a velocity of v01 = +0.80 m/s. Car 2, with a mass of m2 = 92103 kg and a velocity of v02 = +1.3 m/s, overtakes car 1 and couples to it. Neglecting friction, find the common velocity vf of the cars after they become coupled. EXAMPLE: Ice Skaters Starting from rest, two skaters push off against each other on smooth level ice, where friction is negligible. As the

Figure shows, one is a woman (m1 = 54 kg), and one is a man (m2 = 88 kg). Part b of the drawing shows that the woman moves away with a velocity of vf1 = +2.5 m/s. Find the recoil velocity vf2 of the man. Collisions Collisions are often classified according to whether the total kinetic energy changes during the collision:

1.Elastic collisionOne in which the total kinetic energy of the system after the collision is equal to the total kinetic energy before the collision. 2.Inelastic collisionOne in which the total kinetic energy of the system is not the same before and after the collision; if the objects stick together after colliding, the collision is said to be completely inelastic.

Collisions in One Dimension 1. Apply the conservation of momentum. 2. If the collision is elastic, apply the conservation of energy. Problem A car (mass = 1100 kg) is traveling at 32 m/s and collides head-on

with a sport utility vehicle (mass = 2500 kg) traveling in the opposite direction. In the collision, the two vehicles come to a halt. At what speed was the sport utility vehicle traveling? Football: 3rd Down During a 3rd down play with less than a yard to go, a Minnesota Viking player of mass 70-kg moving at 6 m/s was tackled head-on

by a San Francisco 49er of mass 90-kg moving at 5 m/s. Predict the outcome of this play? Car Collision Problem A car with a mass of 850-kg and a speed of 16 m/s approaches an intersection as shown. A 1200-kg minivan traveling at 21 m/s is heading for the same intersection. The car and minivan collide and stick together. Find the speed (vf) and direction () of the wreckage ) of the wreckage

just after the collision, assuming external forces can ignored.