Experiment 8

Collision in 2 Dimensions

Objectives

To test the law of conservation of momentum.

Introduction

The collision of two balls will be examined. One ball will be stationary prior to the collision and the other

will be given a reproducible velocity. The vector sum of momenta before collision should equal the vector

sum of momenta after collision.

Procedure 1

Weigh the glass and steel balls. Secure the apparatus to the edge of the table with a clamp. Place the

setscrew two times the radius of the ball from the end of the track. Locate the ball stop near the upper end

of the track and let a ball roll down the track from the stop. Roll one of the balls down the track to be sure

the ball just misses the setscrew. Note where the ball strikes the floor and tape a piece of paper on the

floor at that spot. Roll a ball down the track several times and record the landing positions by placing

carbon paper over the paper taped to the floor. Find the center of the resulting cluster of points and

measure the distance from the center to the point on the floor below the midpoint between the setscrew and

the end of the track. That distance is proportional to the horizontal velocity of the ball.

Place a glass ball on the setscrew and align the screw so that the steel ball coming down the track will

collide head on with the glass ball. Record, using more paper and carbon paper, the positions where the

two balls land after collision. Repeat to obtain two clusters. Now measure the distance of the centers of

these clusters from the appropriate starting point on the floor.

Procedure 2

Offset the setscrew so that the collisions are not head on and repeat Procedure 1. Now you will need angles

between directions on the floor as well as distance traveled.

Evaluation

Check the conservation of momentum for each procedure by comparing the values of total momentum

before and after the collision. Since the distances on the floor are proportional to the velocity, they may be

used in place of a velocity. Thus your "momentum" unit here will be used in place of "grams cm." Calculate

the percent difference between the total momentum in the forward direction before collision and the same

total after collision. In the second procedure, also calculate a percent difference for the total momentum in

the direction perpendicular to forward.