# Torque and Momentum

Hello,

I have tried to figure out how to do the torque questions on some past NCEA papers but I don’t know how to identify which torque is clockwise and which is anticlockwise.
Along with the torque, I am also confused with momentum questions e.g someone crashed into someone else and went off at a certain speed, what was the velocity before? These kinds of questions don’t make sense to me for how to solve them.

Thank you.

Hi Samar,

For those torque questions, start with a diagram, labelling each force and the point you choose as your pivot. Then imagine each force on it’s own. If that was the only force that was acting, would it make the object rotate clockwise or anticlockwise about the pivot? This example, or this example may help.

For momentum, if asked to find the velocity before the collision, you will need to calculate the momentum of each object after the collision. They may be joined together so it is simply mass x velocity, or you may have to do it for two separate objects and add their individual momentum values together. Since we know momentum is conserved in the absence of external forces, whatever the total momentum was after the collision, would be the same as the total momentum before the colllision. You can then rearrange the p=mv formula to find velocity (v=p/m). Here is an example.

Good questions. I hope this helps.