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 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 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 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 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 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 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 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?