PHY ICS - The Institute of Physics in Ireland

forces 19
Impulse and momentum
UK
A change in momentum is greater when there is a rebound
You will need...
✔ two balls of similar size
and mass (one should
be elastic and rebound
well and the other
inelastic with little or no
rebound)
✔ a retort stand
✔ a block of wood
Background
Momentum = mass × velocity
and is a vector quantity.
Impulse = change of
momentum.
This experiment is used to
show that, since momentum
is a vector quantity, the
change of momentum is
greater when there is a
rebound (i.e. a change in
direction).
Follow these steps:
1 Cut the two balls in half
and, using one half of each,
glue two halves together to
make a composite ball,
embedding one end of a
length of string in the middle.
2 Suspend the composite
ball from the clamp of the
retort stand.
3 Set up the block of wood a
short distance away from the
ball so that the top of the
wood is level with the ball.
4 Using one side of the ball,
raise it through a fixed height
from the top of the wood and
then let it drop to see if it has
enough force to knock over
the wood.
5 Repeat this process using
the other side of the ball.
So what happened?
The wood should be knocked
over much more easily by
one side of the ball than by
the other. This is because the
elastic ball rebounds on
impact with the wood, thus
causing a bigger change in
momentum due to the
direction change, while the
other ball does not rebound
and therefore has a smaller
change in momentum.
What next?
This can be shown to have
many applications in sports
like tennis and squash.
PHYSICS ON STAGE 3