[F.4 phy] force calculation

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2012-12-25 13:22:26 補充：
Zhan Yi Xi Wai Tian Se Ran Qi Po Ou Na Mei Li Ke Ju Hu Guo Fang Er Da Chi Bi An
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Use equation of motion: v^2 = u^2 + 2as
with v = 5 m/s, u = 0 m/s, s = 3 m, a =?
hence, 5^2 = 2a.(3)
a = 4.167 m/s

Let Ff be the frictional force, net force acting on the fireman = 70g - Ff
where g is the acceleration due to gravity, taken to be 10 m/s^2
Hence, use net-force = mass x acceleration
(70g - Ff) = 70 x 4.167
Ff = (70g - 70 x 4.167) N = 408.3 N

Yes, the frictional force acting on the fireman is an reation. As the fireman slides downward, he grasps tightly on the pole and tends to pull it downward with him. The action force, being exerted on the pole by the fireman, is thus pointing downward.


2012-12-15 00:21:52 補充：
The acceleration due to gravity g is the "free fall acceleration". That is, the downward acceleration of any object under the pull of gravity only, without the presence of other forces.

2012-12-15 00:25:07 補充：
Because the fireman in this question is NOT under free fall. He is also acted by the upward frictional force. This upward force reduces the downward pull of gravity. Hence, the acceleration is no longer equals to g, but is smaller.

2012-12-15 00:25:46 補充：
It is 4.167 m/s2 as found in this problem, which is the observed acceleration.