Q#1
A body of weight w1 is suspended from the ceiling of a room through a chain of weight w2 . The ceiling pulls the chain by a force(a) w1 (b) w2 (c) w1 + w2 (d) (w1 + w2)/2
Answer: (c)
The ceiling is pulled by a force = w1 + w2 and as per third law of motion the ceiling too pulls the chain by a force = w1 + w2.
Q#2
When a horse pulls a cart, the force that helps the horse to move forward is the force exerted by(a) the cart on the horse (b) the ground on the horse
(c) the ground on the cart (d) the horse on the ground
Answer: (b)
The horse pushes the ground backwards and as per Newton's third law of motion the ground too pushes the horse forward equally.
Q#3
A car accelerates on a horizontal road due to the force exerted by
(a) the engine of the car (b) the driver of the car
(c) the earth (d) the road
Answer: (d)
The wheels of the car push the ground backwards and as per Newton's third law of motion the road pushes the wheels with equal and opposite force forwards that accelerates the car.
Q#4
A block of mass 10 kg is suspended through two light spring balances as shown in figure (5-Q2).
(a) both the scales will read 10 kg (b) both the scales will read 5 kg (c) the upper scale will read 10 kg and the lower zero.
(d) the readings may be anything but their sum will be 10 kg
Answer: (a)
At any point between the load and the ceiling the tension force will be equal to the load as per the third law of motion.
Q#5
A block of mass m is placed on a smooth inclined plane of inclination θ with the horizontal. The force exerted by the plane on the block has a magnitude
(a) mg (b) mg/cosθ (c) mg cosθ (d) mg tanθ
Answer: (c)
See the figure on the right. Weight of the block is mg downwards. We resolve it in two components-along the plane and perpendicular to the plane. Former component does not exert force on the plane while the later component mg.cosθ is the force which the block exerts on the plane. If we take it as action then equal and opposite force mg.cosθ is exerted by the plane on the block as reaction satisfying the "Newton's Third law of Motion".
Q#6
A block of mass m is placed on a smooth wedge of inclination θ. The whole system is accelerated horizontally so that the block does not slip on the wedge. The force exerted by the wedge on the block has a magnitude
(a) mg (b) mg/cosθ (c) mg cosθ (d) mg tanθ
Answer: (b)
When the plane is accelerated to balance t he block, a horizontal force F acts on the block such that its component along the plane Fcosθ is equal and opposite to component of the weight along the plane mg.sinθ, see the figure on the right.
∑Fx = 0
So, Fcosθ – mg.sinθ = 0
F = mgtanθ
The components of F and weight mg perpendicular to plane (F sin θ and mg cosθ) together push the plane. Equal and opposite force is exerted by the plane on the block as per "Newton's Third Law of Motion" and its value is
∑Fy = 0
n – Fsinθ + mg cosθ = 0
n = mg.tanθ.sinθ + mgcosθ = mg(sin2θ +cos2θ)/cosθ
n = mg/cosθ
Q#7
Neglecting the effect of rotation of the earth. Suppose the earth suddenly stops attracting objects placed near its surface. A person standing on the surface of the earth will
(a) fly up (b) slip along the surface
(c) fly along a tangent to the earth's surface (d) remain standing
Answer: (d)
The person will loose weight but remain standing. As no force acts on him, so according to "Newton's First Law of Motion" he will remain in his state of rest as before.
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