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First one.

A pendulum swings back and forth. Does the tension force in the string do work on the pendulum bob?Does the force of gravity do work on the bob?Explain your answers.

I know that the force of gravity does work on the bob but I dont know how to explain it, and I have no idea about the tension force.

Second one.

A karate expert can break a stack of boards in half by hitting the boards with the side of one bare hand. Will the boards break more easily if they are struck swiftly or slowly.

The answer is swiftly. Now .. what I have is that if he strikes swiftly he has more velocity and therefore more kinetic energy. Is that an ok explanation?

Last one. I know I know ... I'm dumb xD.

Two skaters initially at rest push against each other so that they move in opposite directions. what is the total momentum of the two skaters when they begin moving?

Now I'm taking a guess here, but I have that the total momentum would be zero because they are moving in opposite directions and the momentum on one side cancels out the momentum on the other? Don't the masses of the skaters have something to do with it though??? Obviously not becuase it doesnt say that they are equal in mass or what either masses are.. but why doesnt that have anything to do with it?