Momentum

[Teen]

Can some go through the two rules of momentum . If u  have time , please do some examples for each of the following rules.

[Mr. Study]

If you mean conservation of momentum that I'll do a quick example.

Say we had two blocks. Block 1 has a mass of 10 kg and a speed of 50 ms^-1 and it collided with a another block, Block 2, which was initially stationary and weighed 5kg. After the collision, both blocks are 'stuck together'. Now we want to know the speed of this entire block.

x. This is the initial momentum of the system.

Now we can use the Conversation of Momentum law, which is basically . (Momentum initial equal momentum final).

Now we can do this:. The LHS is the Initial Momentum and the RHS is the final momentum. We already know what the initial momentum is.

Therefore:. However, in the question, it states that after the collision, both blocks are stuck together. That would mean we have a common mass.

Hence: . Now just solve for v and that would be .

Now I assume is the 2nd rule is that Momentum is independent of whether or not the collision is elastic or inelastic. (Meaning if the collision is elastic or inelastic, this doesn't matter as momentum is constant throughout the collision).

Hopefully that helped.

(IF you want another example, Just ask! )

EDIT: Seeing how I am in such a lovely mood. I'll do another example and then a graph of a collision.

Okay, Say we had two cars travelling in opposite directions. Car A was of mass 500 kg and travelling at a speed of 25 ms^-1, to the right (east). Now we had a Car B, which was of mass 1000kg, and travelling at a speed of 30 ms^-1, to the left (west). Both cars collide and travel as a complete object. Find the direction and speed of this tangled wreck.

Firstly, we must assign directions as positive or negative. I like to let Left be negative and Right be positive. (If you do it the other way around, Nothing really happens).

Okay, Let us find the intial Momentum.

. Notice how the answer produces a positive answer? Think about this logically, we have a massive Car B colliding with a 'tiny' Car A. It would not be possible for Car A to push Car B to the east, as it has a lower momentum.

Okay. Now we can do

.

Notice how much the answer 'makes sense'? The Car B was travelling at 30 ms^-1. But due to this collision, it will be travelling at a reduced speed, with the Car A.

Think about it like that.

(If you want Kinetic Energy - Elastic/Inelastic Collision just ask!)

Now. Lets hypothetically think about this. IF we were to say this collision were inelastic, then Kinetic Energy was lost during the collision. What would happen to the momentum? It is conserved throughout this process, meaning it is always constant.

So if we drew a graph:


Total Momentum
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It is constant throughout. Let me just double check everything.                                       

[Teen]

Thanks mr study.