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Momentum
Unit 3
Momentum is inertia in motion.
Momentum= mass x velocity
A moving object can have a large
momentum if it has a large mass, a
high speed, or both.
A moving truck has more
momentum than a car at the same
speed because the truck has more
mass. But a fast car can have
more momentum than a slow truck,
and a truck at rest has no
momentum at all.
Force causes acceleration.
The greater the force is on an
object, the greater its change in
velocity, & the greater its
momentum.
How long the force acts is also
important. The longer the force is
applied, the greater the change in
momentum.
Impulse= force x time interval
The greater the impulse exerted on
something, the greater will be the change
in momentum
Impulse = change in momentum
Ft= mv
Tiger Woods hits a 0.050kg golf
ball, giving it a speed of 75m/s.
What impulse does he impart to
the ball?
Wayne hits a stationary 0.12kg
hockey puck with a force that
lasts for 0.01s and makes the
puck shoot across the ice with a
speed of 20m/s, scoring a goal
for the team. With what force
did Wayne hit the puck?
A tennis ball traveling at 10m/s is
returned by Venus Williams. It leaves
her racket with a speed of 36m/s in the
opposite direction from which it came.
What is the change in momentum of
the tennis ball?
If the 0.060kg ball is in contact with the
racket for 0.020s, with what average
force has Venus hit the ball?
To demonstrate his new high speed
camera, Flash performs an
experiment in the physics lab in
which he shoots a pellet gun at a
pumpkin to record the moment of
impact on the film. The 1g pellet
travels at 100m/s until it embeds
itself 2cm into the pumpkin. What
average force does the pumpkin
exert to stop the pellet?
Case 1: Increasing momentum
To increase the momentum of an object, it makes
sense to apply the greatest force possible for as
long as possible. The forces involved in
impulses usually vary from instant to instant.
For example, a golf club that strikes a golf ball exerts zero
force on the ball until it comes in contact with it; then the
force increases rapidly as the ball becomes distorted.
The force then diminishes as the ball comes up to speed
and returns to its original shape.
*Remember: impact is force; impulse is impact
force x time!
Case 2: Decreasing momentum
If the change in momentum occurs over a long
time, the force of impact is small.
If the change in momentum occurs over a short
time, the force of impact is large.
Impulses are greater when an
object bounces. The impulse
required to bring an object to a
stop and then to “throw it back
again” is greater than the
impulse required to just bring
the object to a stop.
From Newton’s 2nd law you know that to accelerate an
object, a net force must be applied to it.
In the same way, to change the momentum of an object,
exert an impulse on it.
In either case, the force or impulse must be exerted on the
object by something outside the object.
For example, pushing on the dashboard of a car while
sitting on it won’t cause a change in the car’s momentum.
In every case, the momentum of a system cannot change
unless it is acted on by external forces. When momentum
doesn’t change, it is conserved.
Law of conservation of momentum- in the absence of an
external force, the momentum of a system remains
unchanged
The collision of objects clearly
shows the conservation of
momentum.
Net momentum (before
collision) = Net momentum
(after collision)
Whenever objects collide without
being permanently deformed &
without generating heat, the
collision is elastic.
Whenever colliding objects become
tangled or couple together, they are
inelastic.
Most collisions involve some external
forces.
For example, when two trucks collide,
the moving trucks encounter friction
with the pavement and the air. These
external forces are negligible during the
collision, so the net momentum doesn’t
change during the collision. As the
combined wreck slides along the
pavement, friction provides an impulse
to decrease its momentum.
Tubby and his twin brother Chubby
have a combined mass of 200kg
and are zooming along in a 100kg
amusement park bumper car at
10m/s. They bump Melinda’s car,
which is sitting still. Melinda has a
mass of 25kg. After the elastic
collision, the twins continue ahead
with a speed of 4.12m/s. How fast
is Melinda’s car bumped across the
floor?
Charlotte, a 65kg skin diver,
shoots a 2kg spear with a speed
of 15m/s at a fish who darts
quickly away without getting hit.
How fast does Charlotte move
backwards when the spear is
shot?