Physics Chapter 6

Collisions

For all collisions in the absence of external forces, net momentum before collision equals net momentum after collision. in equation form: (net mv)before = (net mv)after

Half. Explanation:After the collision, the mass of the moving freight cars has doubled. Can you see that their speed is half the initial velocity of freight car A?

Freight car A is moving toward identical freight car B that is at rest. When they collide, both freight cars couple together. Compared with the initial speed of freight car A, the speed of the coupled freight cars is...

Conservation of Momentum

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

Momentum: Equation

Momentum = mass × velocity

Momentum: Example

A moving boulder has more momentum than a stone rolling at the same speed. A fast boulder has more momentum than a slow boulder. A boulder at rest has no momentum.

momentum, energy, speed

A moving object has..

Impulse: Example

A brief force applied over a short time interval produces a smaller change in momentum than the same force applied over a longer time interval. or If you push with the same force for twice the time, you impart twice the impulse and produce twice the change in momentum.

impulse. Explanation: The average force on the cannonball will be the same for a short- or long-barreled cannon. The longer barrel provides for a longer time for the force to act, and therefore, a greater impulse.

A cannonball shot from a cannon with a long barrel will emerge with greater speed because the cannonball receives a greater..

Yes for 1 and 2 Explanation: Although stopping the momentum is the same whether done slowly or quickly, the force is vastly different. Be sure to distinguish among momentum, impulse, and forceA fast-moving car hitting a haystack or hitting a cement wall produces vastly different results. 1.Do both experience the same change in momentum? 2.Do both experience the same impulse? 3.Do both experience the same force?

A fast-moving car hitting a haystack or hitting a cement wall produces vastly different results. 1.Do both experience the same change in momentum? 2.Do both experience the same impulse? 3.Do both experience the same force?

More Complicated Collisions

A firecracker exploding; the total momentum of the pieces after the explosion can be added vectorially to get the initial momentum of the firecracker before it exploded.

Impulse Changes Momentum

Apply the greatest force for as long as possible and you extend the time of contact. Force can vary throughout the duration of contact. Examples: Golfer swings a club and follows through. Baseball player hits a ball and follows through.

Impulse Changes Momentum

Case 3: decreasing momentum over a short time short time interval produces large force. Example: Karate expert splits a stack of bricks by bringing her arm and hand swiftly against the bricks with considerable momentum. Time of contact is brief and force of impact is huge

Elastic collision occurs when colliding objects rebound without lasting deformation or any generation of heat.

Elastic collision

Impulse Changes Momentum

Example (continued): In jumping, bend your knees when your feet make contact with the ground because the extension of time during your momentum decrease reduces the force on you.

Example of elastic collision:

Example of elastic collision: single car moving at 10 m/s collides with another car of the same mass, m, at rest From the conservation of momentum, (net mv)before = (net mv)after (m × 10)before = (2m × V)after V = 5 m/s

Conservation of Momentum

Examples: When a cannon is fired, the force on the cannonball inside the cannon barrel is equal and opposite to the force of the cannonball on the cannon. The cannonball gains momentum, while the cannon gains an equal amount of momentum in the opposite direction—the cannon recoils.

Impulse Changes Momentum

Examples: When a car is out of control, it is better to hit a haystack than a concrete wall. Physics reason: Same impulse either way, but extension of hitting time reduces the force.

Bouncing

Impulses are generally greater when objects bounce. Example: Catching a falling flower pot from a shelf with your hands. You provide the impulse to reduce its momentum to zero. If you throw the flower pot up again, you provide an additional impulse. This "double impulse" occurs when something bounces.

occurs when colliding objects result in deformation and/or the generation of heat.

Inelastic collision

Impulse

Product of force and time (force × time) In equation form: Impulse = Ft

More Complicated Collisions

Sometimes the colliding objects are not moving in the same straight line. In this case you create a parallelogram of the vectors describing each initial momentum to find the combined momentum. Example: collision of two cars at a corner

Impulse Changes Momentum

The greater the impulse exerted on something, the greater the change in momentum. In equation form: Ft = Δ(mv)

No, both are the same. Explanation:The momentum becomes zero in both cases, so both change by the same amount. Although the momentum change and impulse are the same, the force is less when the time of momentum change is extended.

When a dish falls, will the change in momentum be less if it lands on a carpet than if it lands on a hard floor? (Careful!)

Conservation of Momentum

When no external force is present, no external impulse is present, and no change in momentum is possible.

doubled

When the force that produces an impulse acts for twice as much time, the impulse is...doubled

.doubles

When the speed of an object is doubled, its momentum ...doubles

Momentum

a property of moving things means inertia in motion more specifically, mass of an object multiplied by its velocity