Q # 1: Consider a box that is placed on different surfaces. Marks = 10
a) In which situation (s) is there no force acting on the box?
b) In which situation (s) is there a static friction force acting on the box?
c) In which situation (s) is there a kinetic friction force acting on the box?
Various possible situations are as under:
I. The box is at rest on a rough horizontal surface.
II. The box is at rest on a rough tilted surface.
III. The box is on the rough-surfaced flat bed of a truck- the truck is moving at a constant velocity on a straight, level road and the box remains in the same place in the middle of the truck bed.
IV. The box is on the rough-surfaced flat bed of a truck- the truck is speeding up on a straight, level road, and box remains in the same place in the middle of the truck bed.
V. The box is on the rough-surfaced flat bed of a truck- the truck is climbing a hill, and the box is sliding towards the back of the truck.
Solution: Answer Q1.
Answer (a) (I), (iii)
Answer (b) (ii), (IV)
Answer (c) (v)
In situation (I) and (ii) the box is not accelerating (so the net force on it must be zero) and there is no other force acting parallel to the horizontal surface; hence no friction force is needed to prevent sliding. In situation (ii) and (IV) the box would start to slide over the surface if no friction were present, so a static fraction force must act to prevent this.
In situation (V) the box is sliding over a rough surface, so a kinetic friction force acts on it.
Solution:-
In this situation there are no forces acting on the spaceship, so according to Newton’s first law, it will not stop. It continues to move in a straight line with constant speed, some science fiction movies have made use of very accurate science; this was not one of them.
Q # 3: A traffic light of weight W hangs from two light cables; one on each side of the light. Each cable hangs at a 450 angle from the horizontal. What I the tension in each cable? Marks = 5
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