SACE Stage 1 Conceptual Physics Momentum

Momentum A massive truck is harder to stop than a small vehicle when travelling at the same speed. We say the truck has more momentum than the vehicle

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

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

Impulse Force If the momentum of an object changes, either the speed, mass or both will change. If the mass is unchanged, then it must of been the velocity that changed, this would indicate some sort of acceleration

Impulse Force What produces the acceleration? FORCE The greater the force on an object, the greater the acceleration, the greater the change in momentum.

Impulse Force Time is also important, how long is the force applied? Supply a force for a long period of time, greater change in momentum. Supply a force for a short period of time, small change in momentum.

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

Case 1 – Increasing Momentum To increase the momentum of an object, need to exert as larger force possible on that object. A golfer and baseball player do this by “following through” when hitting the ball.

Case 1 – Increasing Momentum The force in an impulse are generally not constant. A golf club strikes a ball exerts zero force on the ball until it strikes it. The force increases rapidly as the golf club and the ball distort. The force then diminishes as it comes up to speed and retains its original shape.

Case 2 – Decreasing Momentum over a Long Time If you were travelling in a car that was out of control, you wouldn’t have to use to much physics to understand that hitting a haystack will better for the occupants than hitting a concrete wall. Why?

Case 2 – Decreasing Momentum over a Long Time Impulse means the product of force and time. When hitting the haystack the force is reduced and the time increased. When hitting the concrete wall the time is reduced and the force increased.

Case 2 – Decreasing Momentum over a Long Time Notice the change in momentum is the same in both cases.

Case 2 – Decreasing Momentum over a Long Time If the boxer is able to make the duration of the impact five times as long by riding with the punch, by how much will the force of impact (impulse) be reduced?

Case 3 – Decreasing Momentum over a Long Period of Time If the change in momentum happens over a short time, the impact force is large. Airbags in cars help to prevent this!!

Bouncing Impulse is greater when bouncing takes place. The Pelton Wheel

Bouncing If a book falls onto your head from a shelf, you may be in trouble. If it bounces from your head, you’re certainly in trouble. The impulse required to make things stop and then “throw back” again is greater than just stopping the object.

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

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

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

Conservation of Momentum Momentum is a vector quantity, this means that direction is important. Although both rifle and bullet gain momentum, it is in opposite directions.

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

Conservation of Momentum The Law of Conservation of Momentum In the absence of an external force, the momentum of a system remains unchanged.

Conservation of Momentum

Conservation of Momentum 1 stationary astronaught catching a non-stationary astronaught.

Conservation of Momentum A big fish catching a little fish.

Conservation of Momentum A 2 Kg brick dropped on a 3 Kg cart.

Conservation of Momentum A 2 Kg brick dropped on a 1 Kg cart.

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

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

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

Concept Summary Momentum of an object is product of mass and velocity. The change in momentum depends on the force that acts and on the length of time it acts. Impulse is force multiplied by the time duration which it acts. The impulse exerted on something is equal to the change in momentum it produces.

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

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