1-Two men want to support a uniform board measuring 6 meters in length and weigh

Question

1-Two men want to support a uniform board measuring 6 meters in length and weighing 200 N. If one man exterts 50 N of force on one end of the board, with how much force and how far from the other end must the other man hold the board to keep it horizontal and motion- less?

2- A 13.5-kg box slides over a rough patch 1.75 m long on a horizontal floor. Just before entering the rough patch, the speed of the box was 2.25 m/s, and just after leaving it, the speed of the box was 1.20 m/s. The magnitude of the average force that friction on the rough patch exerts on the box is closest to:

3-When you blow some air above a paper strip, the paper rises. This is because

(a)  the air above the paper moves faster and the pressure is higher.

(b)  the air above the paper moves slower and the pressure is higher.

(c)  the air above the paper moves faster and the pressure remains constant.

(d)  the air above the paper moves faster and the pressure is lower.

(e)  the air above the paper moves slower and the pressure is lower.

Explanation / Answer

a) to balance it F= 150N

distance will be found by moment M=0 (at the centre)

50*3+150*d=0

d= 1 m from centre

b) time in rough surface = (2.25-1.20)/1.75=0.6 sec

so deaceleration a=1.75

friction force F=Ma

=13.5*1.75

=23.62 N

C) d) the air above the paper moves faster and the pressure is lower.

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