# Physics work done

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Thread starter 3 years ago
#1
https://ibb.co/g4jv1L
Hi, I'm a bit confused on this boat question.
Alright so this may seem a bit stupid but how have they calculated the power in this case ? I thought that if something moves at a constant velocity the acceleration is zero so the resultant force is 0 therefore the work done is zero. In simple terms there's no work done on objects moving at a constant speed. Then how is the power not zero if power is defined as the rate of doing work?
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3 years ago
#2
Power is defined as rate of doing work, P = W/t. Work done = F*s, so P = Fs/t. s/t is equal to velocity so P = Fv. As the boat is travelling at a constant speed, the thrust is equal to the drag, hence it is 1800N. It is travelling at 15ms^-1 so P=Fv and 1800 * 15 = 27000 W. The net force is 0, but it t mean the doesn't mean that the power is 0.

I hope it clarifies any concerns and if not, ask about anything you don't understand.
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Thread starter 3 years ago
#3
(Original post by Guarddyyy)
Power is defined as rate of doing work, P = W/t. Work done = F*s, so P = Fs/t. s/t is equal to velocity so P = Fv. As the boat is travelling at a constant speed, the thrust is equal to the drag, hence it is 1800N. It is travelling at 15ms^-1 so P=Fv and 1800 * 15 = 27000 W. The net force is 0, but it t mean the doesn't mean that the power is 0.

I hope it clarifies any concerns and if not, ask about anything you don't understand.
So, as long as there's a force in the direction of the velocity there will be power?
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3 years ago
#4
Yes. If there's a force there's a power. Just imagine that if there's 1800N of force going the other direction, you will need to match that force to travel at a constant velocity of 15ms^-1, and you need power to do so.
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3 years ago
#5
(Original post by Mr.noname)
https://ibb.co/g4jv1L
Hi, I'm a bit confused on this boat question.
Alright so this may seem a bit stupid but how have they calculated the power in this case ? I thought that if something moves at a constant velocity the acceleration is zero so the resultant force is 0 therefore the work done is zero. In simple terms there's no work done on objects moving at a constant speed. Then how is the power not zero if power is defined as the rate of doing work?
Just as the boat is doing work on the water to move forward, the water is doing work on the boat that stops it accelerating. Two separate forces, each causing work to be done in opposite directions. But you're only being asked about the work done by the boat propellor.
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Thread starter 3 years ago
#6
Oh right now I understand thanks !!
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