Physics question

TodayIsGift

2012-06-15 10:59 pm

The following is the question:

圖片參考：http://imgcld.yimg.com/8/n/HA01132001/o/701206150027213873410840.jpg

a) the worker pulls the string such that the box accelerates upwards with
a constant acceleration of 0.5 m/s. Find the force acted by the man on the ground,

Ans:Let the normal force acting on the worker by the ground R and the tension in the string be T.
Consider the motion of the box.
T- mg = ma
T= 20x0.5 + 20x9.81 =206N
Consider the worker,
T+R=Mg
R=Mg -T = 70x9.81 - 206=481N
By the Newton's third law, the force acted by the man on the ground is also 481N.

I don't understand why T+R=Mg, but not R-T=Mg since T should be in the
same direction as Mg.

回答 (1)

天同

2012-06-15 11:34 pm

✔ 最佳答案

The tension T exerted by the string on the man is clearly upward. In lifting up the box, the man needs to pull down the string, this is the action force, and is pointing downward.

The reaction force, the force exerted on the man by the string, is therefore pointing upward.

This agrees with common experience, when you pull the string down, you are somewhow lifting up your body.

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