Physics: force and motion

code PLUS

2011-12-23 6:18 pm

1. How to distinguish whether the moment is clockwise or anti-clockwise?

2. Two forces acting on a body in opposite directions have the resultant force of 10 N. If they act at right angles to each other, the resultant is 50 N. Find the two forces.

3. The resultant two forces Fa and Fb acting at a point can have a maximum value of 7 N. When the two forces act at right angles to each other, the magnitude of their resultant is 5 N. What is the minimum resultant of these two forces?

回答 (1)

天同

2011-12-24 2:09 am

✔ 最佳答案

1. Just as the name implies, if the moment causes the objec tot move in a direction similar to the motion of the clock's hands, the moment is termed "clockwise". If the motion of the object is against the motion direction of the clock's hands, the motion is termed "anti-clockwise"

2. Let the two forces be F1 and F2
Hence, F1 - F2 = 10
i.e. F2 = F1 -10

Since 50^2 = (F1)^2 + (F2)^2
2500 = (F1)^2 + (F1 - 10)^2
(F1)^2 - 10(F1) - 1200 = 0
F1 = 40 N or -30 N (rejected)
F2 = (40 - 10) N = 30 N

3. Fa + Fb = 7
hence, Fb = 7 - Fa

Since 5^2 = (Fa)^2 + (Fb)^2
25 = (Fa)^2 + (7 - Fa)^2
simplifying, (Fa)^2 - 7(Fa) + 12 = 0
Fa = 4 N or 3 N
hence, Fb = 3 N or 4 N

Therefore, the two forces are 3 N and 4 N

2011-12-23 19:24:42 補充：
Sorry...I have missed out the last step.
The minimum resultant force occurs when the two forces are opposite in direction.
Hence, min resultant = (4-3) N = 1 N

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