SACE Stage 1 Conceptual Physics Momentum. Momentum A massive truck is harder to stop than a small...

SACE Stage 1 Conceptual SACE Stage 1 Conceptual PhysicsPhysics

MomentumMomentum

MomentumMomentum

A massive truck is harder to stop than a small A massive truck is harder to stop than a small vehicle when travelling at the same speed.vehicle when travelling at the same speed.

We say the truck has more We say the truck has more momentummomentum than the than the vehiclevehicle

vmP

velocity massmomentum

MomentumMomentum

By the definition of momentum, we can By the definition of momentum, we can see that an object can have a large see that an object can have a large momentum if it either has large mass or momentum if it either has large mass or large velocity.large velocity.

MomentumMomentum

Question – Can you think of a case when Question – Can you think of a case when the roller skate will have more momentum the roller skate will have more momentum than the truck?than the truck?

Impulse ForceImpulse Force

If the momentum of an object changes, If the momentum of an object changes, either the speed, mass or both will either the speed, mass or both will change.change.

If the mass is unchanged, then it must of If the mass is unchanged, then it must of been the velocity that changed, this would been the velocity that changed, this would indicate some sort of accelerationindicate some sort of acceleration


What produces the acceleration? FORCEWhat produces the acceleration? FORCE

The greater the force on an object, the The greater the force on an object, the greater the acceleration, the greater the greater the acceleration, the greater the change in momentum.change in momentum.


Time is also important, how long is the Time is also important, how long is the force applied?force applied?

Supply a force for a long period of time, Supply a force for a long period of time, greater change in momentum.greater change in momentum.

Supply a force for a short period of time, Supply a force for a short period of time, small change in momentum.small change in momentum.


momentuminchangeimpulse

oror

PtF

““force multiplied by the time during force multiplied by the time during which it acts equals change in which it acts equals change in momentum.”momentum.”

Case 1 – Increasing MomentumCase 1 – Increasing Momentum

To increase the momentum of an object, To increase the momentum of an object, need to exert as larger force possible on need to exert as larger force possible on that object.that object.

A golfer and baseball player do this by A golfer and baseball player do this by “following through” when hitting the ball.“following through” when hitting the ball.

Case 1 – Increasing MomentumCase 1 – Increasing Momentum

The force in an impulse are generally not The force in an impulse are generally not constant. constant.

A golf club strikes a ball exerts zero force A golf club strikes a ball exerts zero force on the ball until it strikes it. The force on the ball until it strikes it. The force increases rapidly as the golf club and the increases rapidly as the golf club and the ball distort. The force then diminishes as it ball distort. The force then diminishes as it comes up to speed and retains its original comes up to speed and retains its original shape.shape.

Case 2 – Decreasing Momentum Case 2 – Decreasing Momentum over a Long Timeover a Long Time

If you were travelling in a car that was out If you were travelling in a car that was out of control, you wouldn’t have to use to of control, you wouldn’t have to use to much physics to understand that hitting a much physics to understand that hitting a haystack will better for the occupants than haystack will better for the occupants than hitting a concrete wall.hitting a concrete wall.

Why?Why?


Impulse means the product of force and Impulse means the product of force and time. When hitting the haystack the force time. When hitting the haystack the force is reduced and the time increased.is reduced and the time increased.

When hitting the concrete wall the time is When hitting the concrete wall the time is reduced and the force increased.reduced and the force increased.


Notice the change in momentum is the Notice the change in momentum is the same in both cases.same in both cases.


If the boxer is able to make the duration of If the boxer is able to make the duration of the impact five times as long by riding with the impact five times as long by riding with the punch, by how much will the force of the punch, by how much will the force of impact (impulse) be reduced?impact (impulse) be reduced?

Case 3 – Decreasing Momentum Case 3 – Decreasing Momentum over a Long Period of Timeover a Long Period of Time

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

Airbags in cars help to prevent this!!Airbags in cars help to prevent this!!

BouncingBouncing

Impulse is greater when bouncing takes Impulse is greater when bouncing takes place.place.

The Pelton Wheel

BouncingBouncing

If a book falls onto your head If a book falls onto your head from a shelf, you may be in from a shelf, you may be in trouble. If it bounces from your trouble. If it bounces from your head, you’re certainly in head, you’re certainly in trouble.trouble.

The impulse required to make The impulse required to make things stop and then “throw things stop and then “throw back” again is greater than just back” again is greater than just stopping the object.stopping the object.

Conservation of MomentumConservation of Momentum

Consider a rifle firing a bullet. The force Consider a rifle firing a bullet. The force that pushes on the bullet is the equal and that pushes on the bullet is the equal and opposite to the force that recoils the rifle opposite to the force that recoils the rifle (Newton’s 3(Newton’s 3rdrd Law). Law).


The total momentum before firing the The total momentum before firing the bullet is zero as both the rifle and bullet bullet is zero as both the rifle and bullet are stationary.are stationary.


After the rifle is fired, the Net Momentum is After the rifle is fired, the Net Momentum is still ZERO. No net momentum is gained still ZERO. No net momentum is gained and no net momentum is lost.and no net momentum is lost.


Momentum is a vector quantity, this Momentum is a vector quantity, this means that direction is important.means that direction is important.

Although both rifle and bullet gain Although both rifle and bullet gain momentum, it is in opposite directions.momentum, it is in opposite directions.


The addition of these momentums still The addition of these momentums still result to ZERO as the initial momentum is result to ZERO as the initial momentum is zero.zero.


The Law of Conservation of MomentumThe Law of Conservation of Momentum

In the absence of an external force, the In the absence of an external force, the momentum of a system remains momentum of a system remains unchanged.unchanged.


1 stationary astronaught catching a non-1 stationary astronaught catching a non-stationary astronaught.stationary astronaught.


A big fish catching a little fish.A big fish catching a little fish.


A 2 Kg brick dropped on a 3 Kg cart.A 2 Kg brick dropped on a 3 Kg cart.


A 2 Kg brick dropped on a 1 Kg cart.A 2 Kg brick dropped on a 1 Kg cart.

Momentum VectorsMomentum Vectors

Momentum is conserved even when Momentum is conserved even when colliding objects move at an angle to each colliding objects move at an angle to each other.other.


A falling fire cracker exploding into 2 A falling fire cracker exploding into 2 pieces. The momenta of the fragments pieces. The momenta of the fragments combine by vector rules to equal the combine by vector rules to equal the original momentum of the falling fire original momentum of the falling fire cracker.cracker.


A 2-D collision between 2 cars at right A 2-D collision between 2 cars at right angles.angles.

Concept SummaryConcept Summary

Momentum of an object is product of Momentum of an object is product of massmass and and velocityvelocity..The change in momentum depends on the The change in momentum depends on the

force that acts and on the length of time it force that acts and on the length of time it acts.acts.

Impulse is force multiplied by the time Impulse is force multiplied by the time duration which it acts.duration which it acts.

The impulse exerted on something is equal to The impulse exerted on something is equal to the change in momentum it produces.the change in momentum it produces.


According to the law of conservation of According to the law of conservation of momentum, momentum is conserved momentum, momentum is conserved when there is no external net force.when there is no external net force.When objects collide in the absence of When objects collide in the absence of

external forces momentum is conserved.external forces momentum is conserved.


Momentum is a vector quantity.Momentum is a vector quantity.Momenta combine by vector rules.Momenta combine by vector rules.

SACE Stage 1 Conceptual Physics Momentum. Momentum A massive truck is harder to stop than a small...

Documents

Transcript of SACE Stage 1 Conceptual Physics Momentum. Momentum A massive truck is harder to stop than a small...