# Need help with MCAT Physics Question

#### LeKahlua

Mcat Physics Question: A 10 kg. mass is in free fall with no air resistance. In order to slow the mass at a rate equal to the magnitude of g, an upward force must be applied with magnitude:
0 N, 10 N, 100 N, or 200 N.

I solved if F=mg then F=10*10 or 100 N downward force. Therefore an upward force of 100 N resistance would be required. Why is the answer 200 N? Answer book says you want a net force of mg=100 N upwards but I'm not sure why 200 N upward is equal to a magnitude of g, which is only 100 N downward. What am I missing here?

Leigh

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#### Doc Al

Mentor
To "slow the mass at a rate equal to the magnitude of g" means you need to make the acceleration of the mass g upwards. So what must be the net force on the mass to give it such an acceleration?

(If you did nothing, and let gravity do its thing, the mass would have an acceleration of g downward. If you applied an upward force equal to its weight, the net force would be what? And the net acceleration would be what?)

#### LeKahlua

Hmmm...so if I apply 100 N of force upward, then F=0 and if F=ma then acceleration also equals 0. So now...the velocity is constant, but not slowed down. So to slow it down so that a=10 (equal to the magnitude of gravity) I need to apply an additional 100 N of force upward, requiring 200N. If this is the correct line of reasoning then thank you so much for your assistance.

Leigh :)

Last edited:

Mentor
Exactly right.

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