I was assigned a worksheet for a biomechanics class that had a simple impulse-momentum problem. My instructor and I disagree about the means to solve the problem.
"A pitched ball with a mass of 150g (.15kg) reaches a catcher's glove traveling at a velocity of 28m/s.
-How much momentum does the ball have?
-How much impulse is required to stop the ball?
-If the ball is in contact with the catcher's glove for 0.5 seconds during the catch, how much average force is applied by the glove?"
The Attempt at a Solution
To me, it seems that the question implies that the ball has a positive momentum (when you solve for p=.15kg*28m/s, it's 4.2N*s), and the second part asks what impulse the glove applies to the ball (since the impulse of the ball on the glove would have nothing to do with stopping the ball). Thus, the answer to the second question in my mind would reflect the need for the force from the glove to be in the opposite direction (-4.2N*s). The final part of the question is simply the impulse divided by the time (-4.2N*s/.5s), and results in a measure of -8.4N.
I feel petty arguing about only losing one point of 26 for this, but my instructor insists that my signs on the second and third parts of the problem are wrong, and that if you solve for those using the velocity only, you would get positive values. I argued with him for twenty minutes about how the impulse could not possibly be positive if the ball was indeed to be stopped. Am I wrong? How else could I approach him about this?