Would you rather be hit by a tennis ball or a bowling ball?

To answer the question, you would want to consider mass but also velocity. That is momentum.

momentum = mass x velocity Momentum is inertia in motion. Example: What is the momentum of a 10 kg bowling ball moving at 20 m/s ? mom = m x v mom = 10 kg x 20 m/s mom = 200 kg m/s

What would you have to change to change momentum? To change momentum, you can change mass or velocity. These one mass problems will always involve changing velocity. What is the name for a change in velocity? Acceleration.

What does it take to cause an acceleration? A force is needed to cause an acceleration. The longer a force is applied, the greater the change in momentum.

Many sports involve a change in momentum.

Baseball

Golf

Soccer

Impulse is a physics term that relates a force and the amount of time it is applied. force x time = impulse Example: What impulse is applied to a baseball if the batter uses a 50 N force for 0.2 seconds?

F = 50 N t = 0.2 s Impulse = force x time 50 N x 0.2 s = 10 Ns

Try problems 1-12

It is impulse that causes a change in momentum. ∆ is a symbol used to indicate a change in something. Ft = ∆ mom

A change in anything is the final value – the initial value. Example: What is the change in your bank account if you start with $100 and end up with $500? $400

What is the change in momentum if a 1000 kg car accelerates from 20 m/s to 50 m/s? 30,000 kg m/s

m = 1000 kg v i = 20 m/s (initial speed) v f = 50 m/s (final speed) final momentum = mv f = 1000 kg x 50 m/s final momentum = kg m/s inital momentum = mv i = 1000 kg x 20 m/s initial momentum = kg m/s ∆mom = final momentum – initial momentum ∆ mom = kg m/s – kg m/s = 30,000 kg m/s

A 5 kg mouse is moving at 10 m/s. If he comes to a stop, what was his change in momentum? m = 5 kg v i = 10 m/s v f = 0 m/s final momentum = 0 kg m/s initial momentum = 50 kg m/s ∆ mom = 0 kg m/s – 50 kg m/s ∆ mom = - 50 kg m/s The negative indicates a loss of momentum.

Since impulse causes a change in momentum…… Ft = ∆mom Example: A 20 kg object starts at rest. In 5 s, it has a speed of 100 m/s. How much force was used? 400 N

Solution m = 20 kg v i = 0 m/s v f = 100 m/s t = 5 s F = ? Ft = mv f - mv i F(5) = (20)(100) – (20)(0) F(5) = F(5) = 2000 F = 2000/5 F = 400 N

Try Problems 13-16

Give examples of a moving object coming to a rest. How could you minimize the force felt by these objects? Give examples of ways in which the time for an object to stop is increased.

Airbags

A 800 kg car is moving at 30 m/s. The driver slams on the breaks. It takes the car 3 s to stop. What force did the brakes supply? N The negative indicates the force is in the opposite direction of the motion. If the driver of the car above eased on the breaks for 30 s to come to a stop, how much force did the breaks supply? N

Example: A 40 N force is applied to a 5 kg object for 10 s. If the object was initially moving at 20 m/s, what is its new speed? 100 m/s

Try problems 17-22

Conservation of Momentum Physics Definition: In the absence of an external force, the momentum of a system remains unchanged. A system can be one or more objects. Example of internal forces: explosions collisions

The recoil of a gun is a good example of conservation of momentum. Initial speed of both objects is zero.

Gun Recoil

Spud Gun - Recoil

Fire Extinguisher Recoil

Most Collisions are Elastic

Inelastic Collisions In an inelastic collision, the objects involved in the collision stick together after they collide. An Example

Conservation Of Momentum For Collisions The momentum before a collision is equal to the momentum after a collision. momentum before = momentum after

A 5 kg object moving at 10 m/s collides with a 10 kg object at rest. After the collision, the 5 kg object stops moving. What is the speed of the 10 kg object? Momentum before = 5 kg x 10 m/s Momentum before = 50 kg m/s

Momentum after = (10 kg) v mom before = mom after 50 kg m/s = (10kg)v 50 kg m/s (10kg)v = kg (10kg) V = 5 m/s

Inelastic Example A 1000 kg car moving at 20 m/s strikes another 1000 kg car at rest. Afterwards, they stick together. What is their speed? momentum before = 1000 kg x 20 m/s momentum before = 20,000 kg m/s

mom after = (1000 kg+1000 kg)v Since the cars stick together, the mass after the collision is the total of the two masses. mom before = mom after 20,000 kg m/s = (2000kg)v V = 10 m/s