HANDS-ON
ACTIVITY: BOUNCING BALLS
CONTRIBUTED BY: INTEGRATED TEACHING AND
LEARNING PROGRAM AND LABORATORY, UNIVERSITY
OF COLORADO AT BOULDER
KEYWORDS
• energy, momentum, collisio
n, elastic, inelastic, potential
energy, kinetic
energy, conservation of
momentum
LEARNING OBJECTIVES
• Understand that momentum depends on both mass
and velocity.
• Recognize that difference surfaces and materials
promote different types of collisions.
• Collect data to solve equations.
MATERIALS LIST
• 3 different balls: Ping Pong, Golf, Tennis
• 3 different bouncing surfaces: Smooth side of Brick, Table, Cork
Floor
• Electronic Scale
• Meter stick
PRE-ACTIVITY ASSESSMENT
Brainstorming:
• What are sports examples of transfer (and
conservation) of momentum?
INTRODUCTION
• Momentum can be thought of as 'mass in motion' and is
given by the expression:
• Momentum = mass x velocity
• The amount of momentum an object has depends both
on its mass and how fast it is going. For example, a
heavier object going the same speed as a lighter object
would have greater momentum.
• Sometimes when moving objects collide into each
other, momentum can be transferred from one
object to another. There are two types of collisions
that relate to momentum: elastic and inelastic.
ELASTIC COLLISIONS
• An elastic collision follows the Law of Conservation
of Momentum, which states "the total amount of
momentum before a collision is equal to the total
amount of momentum after a collision." In
addition, the total kinetic energy of the system (all
the objects that collide) is conserved during an
elastic collision.
• An elastic collision example might involve a superbouncy ball; if you were to drop it, it would bounce
all the way back up to the original height from
which it was dropped. Another elastic collision
example may be observed in a game of pool. Watch
a moving cue ball hit a resting pool ball. At impact,
the cue ball stops, but transfers all of its
momentum to the other ball, resulting in the hit
ball rolling with the initial speed of the cue ball.
INELASTIC COLLISION
• In an inelastic collision, the total momentum of the
system is conserved, but the total kinetic energy of
the system is not conserved. Instead, the kinetic
energy is transferred to another kind of energy such
as heat or internal energy. A dropped ball of clay
demonstrates an extremely inelastic collision.
• It does not bounce at all and loses its kinetic energy.
Instead, all the energy goes into deforming the ball
into a flat blob.
• In the real world, there are no purely elastic or
inelastic collisions.
• Rubber balls, pool balls (hitting each other), and
ping-pong balls may be assumed extremely elastic,
but there is still some bit of inelasticity in their
collisions. If there were not, rubber balls would
bounce forever. The degree to which something is
elastic or inelastic is dependent on the material of
the object.
PROCEDURE
• Determine the mass in kilograms of each ball and
record it on the data sheet.
• Drop each ball from a distance of 1 meter onto the
surface and record how high it bounces in meters
(Example: .46 meters).
• Note whether the ball and surface showed more of an
elastic or inelastic collision.
• If the ball bounces up more than .5 meter then, it is
more elastic.
• If it bounces up less than .5 meter, then it is
more inelastic.
• Repeat steps 1, 2 and 3 for the two other surfaces
• Calculate the momentum for each ball at the point
that it bounces, and record on the worksheet. Do
one example calculation as a class.
• Note: The momentum calculation is independent of
the bouncing surface, so it only needs to be
calculated once for each ball.
• Equation: Momentum = mass x velocity
• Use the mass determined in step 1. In this example,
use .05 kilograms for the mass. Next, determine the
velocity of the object when it hits the ground.
Velocity of a falling object can be described as:
• where g is gravity (9.81 m/s2) and h is height (1 m).
POST-ACTIVITY ASSESSMENT
• Graphing: Make two separate bar graphs—one for
momentum and one for velocity. Put the surface type on
the x-axis, and the velocity and momentum on the y-axis
(for each respective graph). Then make three different
colored plots on each graph representing the different ball
types. Use these graphs to visually represent which balls
had the most elastic collisions and on which surface they
occurred.
MOMENTUM = MASS X VELOCITY
• Problem Solving: Calculate which case has the
greater momentum.
• Case 1: A big-time slugger hits a 0.14 kilogram (5
ounce) baseball 45 meters/sec (100 mph).
• Case 2: Uncle bill knocks down four pins at the
Bowl-a-Rena by rolling a 7.3 kilogram (16 pound)
ball 4.5 meters/sec (10 mph).