MomentumMomentum

Unit 3Unit 3

Momentum is inertia in motion.Momentum is inertia in motion.

Momentum= mass x velocityMomentum= mass x velocity

A moving object can have a large A moving object can have a large momentum if it has a large mass, a momentum if it has a large mass, a

high speed, or both. high speed, or both.

A moving truck has more A moving truck has more momentum than a car at the same momentum than a car at the same speed because the truck has more speed because the truck has more

mass. But a fast car can have mass. But a fast car can have more momentum than a slow truck, more momentum than a slow truck,

and a truck at rest has no and a truck at rest has no momentum at all.momentum at all.

Force causes acceleration.Force causes acceleration.

The greater the force is on an The greater the force is on an object, the greater its change in object, the greater its change in

velocity, & the greater its velocity, & the greater its momentum.momentum.

How long the force acts is also How long the force acts is also important. The longer the force is important. The longer the force is applied, the greater the change in applied, the greater the change in

momentum.momentum.

Impulse= force x time intervalImpulse= force x time interval

The greater the impulse exerted on The greater the impulse exerted on something, the greater will be the change something, the greater will be the change in momentumin momentum

Impulse = change in momentumImpulse = change in momentum

Ft= mvFt= mv

Tiger Woods hits a 0.050kg golf Tiger Woods hits a 0.050kg golf ball, giving it a speed of 75m/s. ball, giving it a speed of 75m/s. What impulse does he impart to What impulse does he impart to

the ball?the ball?

Wayne hits a stationary 0.12kg Wayne hits a stationary 0.12kg hockey puck with a force that hockey puck with a force that lasts for 0.01s and makes the lasts for 0.01s and makes the

puck shoot across the ice with a puck shoot across the ice with a speed of 20m/s, scoring a goal speed of 20m/s, scoring a goal for the team. With what force for the team. With what force

did Wayne hit the puck?did Wayne hit the puck?

A tennis ball traveling at 10m/s is A tennis ball traveling at 10m/s is returned by Venus Williams. It leaves returned by Venus Williams. It leaves

her racket with a speed of 36m/s in the her racket with a speed of 36m/s in the opposite direction from which it came. opposite direction from which it came. What is the change in momentum of What is the change in momentum of

the tennis ball?the tennis ball?

If the 0.060kg ball is in contact with the If the 0.060kg ball is in contact with the racket for 0.020s, with what average racket for 0.020s, with what average

force has Venus hit the ball?force has Venus hit the ball?

To demonstrate his new high speed To demonstrate his new high speed camera, Flash performs an camera, Flash performs an

experiment in the physics lab in experiment in the physics lab in which he shoots a pellet gun at a which he shoots a pellet gun at a pumpkin to record the moment of pumpkin to record the moment of impact on the film. The 1g pellet impact on the film. The 1g pellet travels at 100m/s until it embeds travels at 100m/s until it embeds itself 2cm into the pumpkin. What itself 2cm into the pumpkin. What average force does the pumpkin average force does the pumpkin

exert to stop the pellet?exert to stop the pellet?

Case 1: Increasing momentumCase 1: Increasing momentum

To increase the momentum of an object, it makes To increase the momentum of an object, it makes sense to apply the greatest force possible for as sense to apply the greatest force possible for as long as possible. The forces involved in long as possible. The forces involved in impulses usually vary from instant to instant. impulses usually vary from instant to instant.

For example, a golf club that strikes a golf ball exerts zero 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 on the ball until it comes in contact with it; then the force increases rapidly as the ball becomes distorted. force increases rapidly as the ball becomes distorted. The force then diminishes as the ball comes up to speed The force then diminishes as the ball comes up to speed and returns to its original shape.and returns to its original shape.

*Remember: impact is force; impulse is impact *Remember: impact is force; impulse is impact force x time!force x time!

Case 2: Decreasing momentumCase 2: Decreasing momentumIf the change in momentum occurs over a long If the change in momentum occurs over a long

time, the force of impact is small.time, the force of impact is small.

If the change in momentum occurs over a short If the change in momentum occurs over a short time, the force of impact is large.time, the force of impact is large.

Impulses are greater when an Impulses are greater when an object bounces. The impulse object bounces. The impulse

required to bring an object to a required to bring an object to a stop and then to “throw it back stop and then to “throw it back

again” is greater than the again” is greater than the impulse required to just bring impulse required to just bring

the object to a stop.the object to a stop.

From Newton’s 2From Newton’s 2ndnd law you know that to accelerate an law you know that to accelerate an object, a net force must be applied to it.object, a net force must be applied to it.

In the same way, to change the momentum of an object, In the same way, to change the momentum of an object, exert an impulse on it.exert an impulse on it.

In either case, the force or impulse must be exerted on the In either case, the force or impulse must be exerted on the object by something outside the object.object by something outside the object.

For example, pushing on the dashboard of a car while For example, pushing on the dashboard of a car while sitting on it won’t cause a change in the car’s momentum.sitting on it won’t cause a change in the car’s momentum.

In every case, the momentum of a system cannot change In every case, the momentum of a system cannot change unless it is acted on by external forces. When momentum unless it is acted on by external forces. When momentum

doesn’t change, it is conserved.doesn’t change, it is conserved.

Law of conservation of momentum- in the absence of an Law of conservation of momentum- in the absence of an external force, the momentum of a system remains external force, the momentum of a system remains

unchangedunchanged

The collision of objects clearly The collision of objects clearly shows the conservation of shows the conservation of

momentum.momentum.

Net momentum (before Net momentum (before collision) = Net momentum collision) = Net momentum

(after collision)(after collision)

Whenever objects collide without Whenever objects collide without being permanently deformed & being permanently deformed &

without generating heat, the without generating heat, the collision is elastic.collision is elastic.

Whenever colliding objects become Whenever colliding objects become tangled or couple together, they are tangled or couple together, they are

inelastic.inelastic.

Most collisions involve some external Most collisions involve some external forces.forces.

For example, when two trucks collide, For example, when two trucks collide, the moving trucks encounter friction the moving trucks encounter friction

with the pavement and the air. These with the pavement and the air. These external forces are negligible during the external forces are negligible during the collision, so the net momentum doesn’t collision, so the net momentum doesn’t

change during the collision. As the change during the collision. As the combined wreck slides along the combined wreck slides along the

pavement, friction provides an impulse pavement, friction provides an impulse to decrease its momentum.to decrease its momentum.

Tubby and his twin brother Chubby Tubby and his twin brother Chubby have a combined mass of 200kg have a combined mass of 200kg

and are zooming along in a 100kg and are zooming along in a 100kg amusement park bumper car at amusement park bumper car at

10m/s. They bump Melinda’s car, 10m/s. They bump Melinda’s car, which is sitting still. Melinda has a which is sitting still. Melinda has a

mass of 25kg. After the elastic mass of 25kg. After the elastic collision, the twins continue ahead collision, the twins continue ahead with a speed of 4.12m/s. How fast with a speed of 4.12m/s. How fast

is Melinda’s car bumped across the is Melinda’s car bumped across the floor?floor?

Charlotte, a 65kg skin diver, Charlotte, a 65kg skin diver, shoots a 2kg spear with a speed shoots a 2kg spear with a speed

of 15m/s at a fish who darts of 15m/s at a fish who darts quickly away without getting hit. quickly away without getting hit. How fast does Charlotte move How fast does Charlotte move backwards when the spear is backwards when the spear is

shot?shot?