a 0.065kg tennis ball moving to the right with a speed of 15 m/s is struck by a

Question

a 0.065kg tennis ball moving to the right with a speed of 15 m/s is struck by a tennis racket, causing it to move to the left with a speed of 15m/s. if the ball remains in contact with the racquet for .020s, what is the magnitude of the average force experienced by the ball?

I can get the correct magnitude but I don't understand why it is positve and not negitive.

From definitions of momentum andimpulse, we have:

{Change In Momentum} = {Average Force}*{Time Duration}

----> (0.065 kg)*(-15.0 m/sec) -(0.065 kg)*(15.0 m/sec) = F*(0.020sec)
^^^ Ball Moving Left^^^ ^^^ Ball Moving Right^^^

----> (-1.95) = (0.02)*F (<---now solve for Force F)

F = -97.5

Explanation / Answer

P = 0.065* (15 -(-15))

F =P/t =0.065*30/0.02 = 97.5 N

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