Apowered winch is used to pull a sailboat to shore. the winch uses a 900 w motor. if the motor is used for 30 s, how much work does it do? (power: p = w/t) 0.03 j 30 j 960 j 27,000 j

Well first of all, if a motor is marked "900 W", that doesn't mean it works at the rate of 900 watts whenever you turn it on.  It means that's its maximum ability.  You can use the motor for whatever amount of power you need, as long as you don't try to use it for more than 900 watts, because that's the most it can deliver without overheating and falling apart.

So the way this question is built, we have to assume that you've loaded the motor in such a way that it's delivering exactly 900 watts.

1 watt = 1 Joule per second

(900 Joules/sec) x (30 sec)  =  27,000 Joules

c. newton's third law of motion