Asprinter springs out of the starting blocks with an acceleration of 11 m/s2 that is essentially horizontal to the right. how much horizontal force did the 51-kg sprinter exert on the starting blocks during a start to produce this acceleration? assume the sprinter is running in the positive direction.

Remark

It's not the number that's the problem. It's the minus sign.

Just use F = m*a to get the numerical answer. F = 51 * 11  = 561 N

Now we come to the hard part. If you learn nothing else in physics, you learn that minus signs are either your best buddy or your worst enemy -- one that you pay a mafia hit man a lot of money to get rid of.

In this case, you better start looking for a guy wearing a white tie.

What you are finding is a force that is acting against the starting block. In order to move forward, you have to push against something to accomplish going forward.  That's  why the block is there: it absorbs the push and you pop forward. You should draw a diagram of the block so you see how many forces act on it.

- 561