AP Physics C Lab
Conservation of Momentum
Objective
Using conservation of momentum and a colliding pendulum, determine the mass of a projectile
ball
Procedure
1.
Set up the CPO Colliding Pendulum as shown in the demonstration apparatus. Pay careful
attention to the leveling of the stand and alignment of the projectile and target balls.
2.
For the first four trials, use the small ball as the projectile ball and one of the medium balls
as the target ball. Turn on the timer and set it in interval mode. Turn on light A and turn off
light B. Release the projectile ball from the 50° mark and allow the balls to collide. Catch the
target ball after is passes through the photogate B. The projectile ball will swing through
photogate A, then bounce back for a second pass through photogate A. Catch the projectile
ball before it swings through photogate A a third time.
3.
The time displayed on the timer is the time for the projectile ball to pass through photogate
A the second time (“bounce back” time). Record this value as time tp, the projectile time
after the collision. Press memory. The time displayed on the timer is the time for the
projectile ball to pass through photogate A the first time. Record this value as time top, the
projectile time before the collision.
4.
Turn off light A and turn on light B. The time displayed is the time for the target ball to pass
through photogate B. Record this value as time tt, the target time after the collision.
5.
Repeat Procedures 2-4 for the initial positions shown in Data Table 1.
6.
Repeat Procedures 5-8, for the initial positions shown in Data Table 1. Use the large ball as
the target ball.
1
Calculations
1.
Calculate the velocity for each ball in each trial and record each speed in Data Table 1. Note,
vp for the projectile ball is in the opposite direction of its initial velocity. The diameter of the
small ball is: 0.0190 m; the diameter of the medium ball is 0.0254 m; the diameter of the
large ball is 0.0318 m. Record each velocity in Data Table 1.
2.
Using an equation for conservation of momentum, to find the mass of the projectile ball for
each trial. The mass of the medium target ball 0.0676 kg and the mass of the large target
ball is 0.1313 kg. Record the mass as Experimental Mass in Data Table 2. Find the average
mass of the projectile ball for the eight trials and record this value in Data Table 2.
3.
Using your average experimental mass, find the initial kinetic energy K0 and final kinetic
energy K for each trial.
Questions
1.
The actual mass of the projectile ball is 0.02871 kg. How closely does your experimental
mass of the projectile mass compare with the actual mass?
2.
What fraction of the energy is not conserved for each trial? How closely do your collisions
approximate an elastic collision?
3.
Suppose the projectile ball were larger than the target ball. Describe the motion of the
projectile ball after the collision. Explain your answer.
4.
Describe a way use the same apparatus and commonly available materials to produce an
inelastic collision.
2
AP Physics C Lab
Conservation of Momentum
Data Table 1
Projectile before collision
Trial
1
2
3
4
5
6
7
8
tOp (s)
Projectile after
collision
tp
vp
v0P (m/s)
Small
60°
50°
40°
30°
Small
60°
50°
40°
30°
Target after collision
tt
Medium
Large
Data Table 2
Experimental Mass
Kinetic Energy
K0
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Trial 6
Trial 7
Trial 8
Average
Mass
3
K
ΔK/K0
vt