Ballistic Pendulum
Equipment
โข Safety Glasses (not necessarily chemical goggles)
โข Calculator, Computer
โข Ballistic Pendulum Apparatus (Stand, Launcher, Pendulum, Ball)
โข Ruler
Objectives
To determine the initial velocity of a projectile through proper application of the principles of
conservation of momentum and energy.
Introduction
The ballistic pendulum is a classic method of determining the velocity of a projectile. It is also a
good demonstration of many of the basic principles of physics. The ball is fired into the ballistic
pendulum, which then swings up a measured amount. From the height reached by the
pendulum, you can calculate its gravitational potential energy. The gravitational potential
energy is equal to the kinetic energy of the pendulum at the bottom of the swing, just after the
collision with the ball. You cannot equate the kinetic energy of the pendulum after the collision
with the kinetic energy of the ball before the swing since the collision between ball and
pendulum is inelastic, and kinetic energy is not conversed in inelastic collisions. Momentum is
conserved in all forms of collisions, so you know that the momentum of the ball before the
collision is equal to the momentum of the pendulum after the collision. Once you know the
momentum of the ball and the ballโs mass, you can determine the initial velocity.
Momentum
As discussed in the Collisions lab, the momentum of an object depends on its mass and velocity,
and the units of momentum are kgยทm/s. Remember: As a vector, momentum in a direction can
be positive or negative!
Definition:
๐๎ฌฆ๐= ๐๐ฃ๎ฌฆ๐
(1)
In a collision, the total momentum of the two objects is conserved.
๐๎ฌฆ๐= ๐๎ฌฆ๐
๐1๐ฃ1๐ + ๐2๐ฃ2๐ = ๐1๐ฃ1๐ + ๐2๐ฃ2๐
(2)
Kinetic and Potential Energy
Again, as discussed in Lab 11, the kinetic energy of an object is a positive, scalar (not vector) quantity
that also depends on its mass and velocity. Energy is important because it can be transformed back into
