laura.cp897117 wrote:For Q6:

6) If you lift a 10 kg box 1 meter in 2 seconds, then your friend does the same thing but it takes 6 seconds, which of the following is true about the work done on the box?

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Explanation: Work is force times distance. In both cases, the force is about 100 newtons (i.e. mg) and the distance is 1 meter so there is no change. The fact that the box is moved faster in one case compared to the other is a function of the power (i.e. the rate of work or W/t).

I used F = ma instead of F=mg, giving me for the first object:

W = kg.m/s^2 = 10x1/(2)^2 = 10/4

For the second object:

W = kg.m/s^2 = 10x1/(6)^2 = 10/36

This is clearly different than using F = mg. Why do we use F = mg rather than F = ma in this case?

Well, let's take a closer look at the statement in the Explanation "the force is about 100 newtons ".

Because the truth is, your calculation was partly, very reasonable. However, think of raising a box. To move it upwards, you absolutely must act against gravity, or more specifically, you must counteract the weight mg of the objective. If you don't act against mg, then the object accelerates downwards. So in fact you are doing 2 things, just by holding it, you counteract mg AND by accelerating it, you are causing ma.

So the sum of forces F = mg + ma

But mg is about 100 and ma in both cases is less than 3! So given the answer choices, the results are approximately the same.