unit 6 honors physics

On a Force (y axis) vs. Time (x axis) graph, how do you determine the total impulse?

Area "under" the graph. (this is also called the integral.) Remember the area "under" the graph really means from the graph back to the x axis.

When we talk about inertia, why is it not enough to just talk about mass?

Because how fast (velocity) an object is moving also plays a role in how hard it is to change the motion of the object.

Under what conditions do you have conservation of momentum?

In a closed (no change in mass) and isolated (no external forces) system.

define Law of Conservation of Momentum

In a closed and isolated system, the total momentum of the system does not change.

If you fall and hit the ground, does it hurt more if you bounce or if you thud? Why?

In general, it will hurt more to bounce. Bouncing requires a bigger change in momentum and therefore usually a bigger force (more hurt.) The exception would be something like a trampoline that drastically increases the time to cause a change in momentum.

What quantities make-up momentum?

Mass and velocity

Explain how the Impulse-Momentum theorem relates to Newton's Second Law (Acceleration)

Recall that reworking the quantities in the Impulse-Momentum Theorem will yield the equation for the Second Law. Therefore, I-M is basically just a different way to say the Second Law.

Right after a bullet is fired out of a gun, what can you say about the momentums of the bullet and the gun? What about the velocities of each? How do you account for the difference in the velocity of each?

Right after firing, the bullet and the gun both have the same value on momentum but with opposite signs. The bullet has a big velocity and the gun a small velocity because the masses are the opposite; the gun has a large mass and the bullet a small mass.

Thinking of the Impulse-Momentum theorem, how does padding help protect a person when hitting a hard object?

The amount of change in momentum is determined at the time of impact. Therefore there is a trade-off between force and time. Padding will increase the stopping time and therefore lower the force need to stop an object.

define mass

The amount of matter contained in an object. OR The amount of inertia an object has.

define impulse

The change in momentum of an object.

define Impulse - Momentum Theorem

The concept that the change in momentum is equal to the impulse.

In the Law of Conservation of Momentum, what quantity(s) are conserved and are not conserved?

The only quantity that is conserved is TOTAL momentum of the system. Individual momentum and velocity are not conserved.

If you have conservation of momentum in a crash between two cars and one car loses momentum, what can you say about the momentum of the second car?

The other car must gain the momentum lost by the first car.

Define momentum

The quantity of motion. OR The product of the mass and velocity of an object.

Define inertia

The resistance an object has to a change in its motion (speeding up, slowing down or changing direction)

When an object bounces (like a ball off of a wall) what do you have to do with the velocities?

The sign on the velocity going into the wall and the velocity going away from the wall must be opposite to each other.

When an object has a set amount of momentum that will change, like a car stopping, what can you say about the relationship between the force and time needed to change the momentum?

There is a trade-off between force and time. If you use a relatively small force, it will require a relatively large time. If you want to stop fast and use a small time, you would have to provide a large force.

How does conservation of momentum explain the propulsion of a rocket in outer space?

When fuel is burn in a rocket and then expelled at high speed out the nozzle of rocket, it is like a bullet and a gun problem. Relative to each other, the rocket and fuel are initially at rest. When ignited, the molecules of the hot gas have small mass but high velocity and the rocket has a large mass and the relatively small velocity. After the expulsion of billions and billions and billions of molecules the rocket could reach a high speed by the cumulated effect of gaining just a little velocity from each molecule.

Can a marble rolling on a floor have more momentum then a bowling ball rolling across a floor? Under what conditions?

Yes. First if the marble is rolling and the bowling ball is at rest the marble has more momentum. Second if both are moving and the marble is very fast compared to the bowling ball it could have more momentum.

What quantities make-up impulse?

force and time

what's the equation for impulse-momentum theorem?

m•(Dv) = F•t

How do you calculate change in velocity - always - every time - without exception?

velocity FINAL minus velocity INITIAL

What is the momentum of a 1500 kg car moving at 0 m/s?

0 kg•m/s

What is the momentum of a 1500 kg car moving at 5 m/s?

7500 kg•m/s

Define isolated system

A collection of objects in which there are no external forces exerted on the objects. (There may be internal forces)

define closed system

A collection of objects where the total mass of the objects does not change.

define internal force

A force between two objects, both of which are contained in a defined system.

define external force

A force between two objects, one of which is not part of a defined system

define force

A push or pull on an object. Also can be defined as that thing which could cause an acceleration of an object.