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# Depth of Physics help! watch

1. Hi there. Honestly I'm really sucky at this area of physics and I'm doing revision. Can someone help me with this question? Have I gone the right way about it?

2. (Original post by TobyReichelt)
Hi there. Honestly I'm really sucky at this area of physics and I'm doing revision. Can someone help me with this question? Have I gone the right way about it?

Just wondering, were you given any other data in the first part of the question, such as the mass of the swimmer?
3. (Original post by ombtom)
Just wondering, were you given any other data in the first part of the question, such as the mass of the swimmer?
Yes, mass 65kg, weight 640N, carried vertically upwards from the sea by a cable of negligible mass compared to the swimmer.

The tension T in the cable from the time that she leaves the water at t=0 until t=1.5s is 670N. At t=1.5s T reduces and remains constant at 640N
4. (Original post by TobyReichelt)
Yes, mass 65kg, weight 640N, carried vertically upwards from the sea by a cable of negligible mass compared to the swimmer.

The tension T in the cable from the time that she leaves the water at t=0 until t=1.5s is 670N. At t=1.5s T reduces and remains constant at 640N
You could try using P = Fv, where F is the weight of the swimmer. This will give you the useful power. Divide this by the power provided by the motor to give the efficiency as a decimal. Multiplying by 100 would give you a percentage.

Note that efficiency doesn't have any units, since it is a ratio.

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