# Mometum conservation and torque

hi i had a question based on momentum, which is how to identify if a momentum is conserved would you please be able to give me some examples with it to. another q i had was how do we identify if the disance of an F support force is zero?

Hi @Anne,

Momentum is always conserved. The only question is whether we consider other forces, such as friction, diverting the momentum elsewhere.

For your L2 exam, the statement â€śMomentum is conserved in the absence of external forcesâ€ť would be a common expectation. This means that momentum is conserved in two rollerskaters who collide with each other (sum of momentum before = sum of momentum after) as long as we ignore any frictional forces acting between their skates and the ground. If we consider friction, then some of their momentum will be transferred into the ground, so their combined momentum after the collision could be less. Note, in this example momentum is still conserved, but we may not know how much has been transferred to the ground.

For your second question, in torque or bridge problems you identify a pivot as your reference point. Any forces acting through the pivot are at zero distance from the pivot, therefore contribute zero torque.

This post might also be useful.

Let me know if you would like further clarification.

how would we know the direction of an objects change in momentum. for example the collision of a ball travelling at 16ms^-1 to the right rebounds off a wall with a velocity of 14ms^-1 to the left. the time of interaction with the wall is 0.071s. it says to determine the change in momentum if the ball and the force on the wall with directions.

i got 24.5N but i thought it has a direction to the left. but it seems like it is to the right. would u be able to explain this for me?

Hi @anne,

Well done. The value of your answer is correct, assuming the mass of the ball is 0.058kg

The wording is quite important here:
determine the change in momentum of the ball
and the force on the wall

In the question you describe we can work out the change in momentum of the ball using:

change in momentum = momentum final - momentum initial
change in momentum = mv(final) - mv(initial)
You an also calculate this as:
change in momentum = mass x change in velocity

Let us identify the right as positive, and the left as negative. Therefore:

change in velocity = -14 - 16 = -30 ms^-1

Since the change in velocity of the ball is negative, the change in momentum must also be negative, it must be to the left. This should make sense as the velocity of the ball was to the right, and now it is to the left. So the forcce acting on the ball, and the change in momentum of the ball, must be to the left.

To enable this change in momentum the wall must have provided a force on the ball to the left. However, the question asked us what direction the force on the wall was. We know the ball must have exerted an equal and opposite force on the wall (Newtonâ€™s third law of motion), so the force on the wall was to the right.

Once youâ€™ve read this let me know if you would like any further clarification, or another example.

no that makes sense cause i didnt read the question properly. thank you

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