PHYS 220 - THE BALLISTIC PENDULUM or PROJECTILE MOTION
In this lab you will use two different methods to determine the initial velocity of a horizontally launched
steel ball. One method will be to use the classic “ballistic pendulum” which was designed to measure
the initial speed of bullets. The second method will be to use the kinematics equations as we have in
the past for projectiles. You will then compare the results from these two methods and consider the
various factors that might make one a more accurate method than the other.
The Ballistic Pendulum is shown below:
A metal ball of mass m is launched from a gun and is then captured in a barrel. The barrel is part of a
pendulum that can pivot at its top. After the collision, the ball and the pendulum swing up
to some maximum height h.
Hint: Measuring the height of the barrel is not straightforward – check with your instructor!
Step 1: Energy Skater
Play with Energy Skater on a frictionless flat track.
1. If you drop the skater onto the track, is
mechanical energy conserved? What evidence
do you have for this?
2. What type of collision does he have with the
track? Elastic, inelastic or perfectly inelastic?
What evidence do you have for this?
Step 2: Problem Analysis
3. Ballistic Pendulum: Considering what you’ve just seen with Energy Skater, do you expect
mechanical energy to be conserved from the time that the steel ball is fired to top of the swing
for your pendulum/ball combination? In other words will the kinetic energy of the steel ball
equal the total mechanical energy of the ball pendulum combination after the bullet gets
trapped in the barrel? Why?
4. Based on your answer to 3., how do you plan to tackle the problem of solving for the initial
velocity of the steel ball using the final height of the pendulum/ball? What equation(s) do you
plan to apply? Describe specifically how you’ll apply them.
5. Projectile: How can you use kinematics equations to determine the initial velocity of a ball that
has been fired horizontally? Please describe if and how you would break this problem down and
what equations you’d use to solve the needed variables.
Data Collection
Materials and equipment: ballistic pendulum, metal ball, carbon paper/white paper attached to
cardboard, 2 m stick, scale
6. Collect five measurements for the increase in height of the pendulum/ball combination after
firing the ball into the barrel. Hint: Measuring the height of the barrel is not straightforward –
check with your instructor!
Note: Push the ball slowly backwards to the SECOND firing position on the PASCO pendulum (use the
FIRST firing position on the BECK device).
7. Remove the pendulum and fire the ball horizontally from table and measure the range of the
ball. Repeat the measurement five times and find the average range.
Calculations
8. Using your Ballistic Pendulum data, determine the average value of your five measurements. Us
this average value to determine the initial speed of the steel ball.
9. Using your projectile data with kinematics equations in the horizontal and vertical directions
determine the initial velocity of the ball.
10. Do your two experimental values for the initial velocity agree? Determine the percent difference
between these values.
11. In each method, carefully describe all the places that error could have occurred. “human error”
is too vague – be specific. For example, if you had been looking at a graph on logger pro,
“human error” could have been “the person reading the graph choose a poor location not
realizing where the collision occurred”.
Which method of determining the initial velocity do you believe is more accurate? Why?