Force of Friction

Multiple Choice Questions

Question 1: On the horizontal surface of a truck, a block of mass 1 kg is placed (µ = 0.6) and truck is moving with an acceleration of 5 m/s2, then the frictional force on the block will be—

  1. 5 N
  2. 6 N
  3. 5.88 N
  4. 8 N

Answer: (c) 5.88 N

Explanation: F = µR = µ mg = 0.6 × 1× 9.8 = 5. 88 N

Question 2: A block A of mass 7 kg is placed on a frictionless table. A thread tied to it passes over a frictionless

pulley and carries a body B of mass 3 kg at the other end. The acceleration of the system is (given g = 10 m/s2)—

  1. 100 m/s2
  2. 3 m/s2
  3. 10 m/s2
  4. 30 m/s2

Answer: (b) 3 m/s2

Explanation: For body A, T = M1 a = 7 a

For body B, M2 g _T = M2 a

3 g-7 a = 3 a

10 a = 3 g

Question 3: A light string passing over a smooth light pulley connects two blocks of masses m1 and m2 (vertically). If the acceleration of the system is (g/8), then the ratio of masses is—

  1. 8 : 1
  2. 9 : 7
  3. 4 : 3
  4. 5 : 3

Answer:(b) 9 : 7

Explanation: In the given system,

Question 4: A cylinder rolls up an inclined plane, reaches some height and then rolls down (without slipping throughout these motions). The directions of frictional force acting on the cylinder are—

  1. up the incline while ascending and down the incline while descending
  2. up the incline while ascending as well as descending
  3. down the incline while ascending and up the incline while descending
  4. down the incline while ascending as well as descending

Answer: (b) up the incline while ascending as well as descending

Explanation: As shown in Fig. compo-nent of weight (mg sin q) is always down the inclined plane, whether the cylinder is rolling up or it is rolling down. Therefore, for no slipping, sense of angular acceleration must be the same in both the cases. Therefore, force of friction (f) acts up the inclined plane in both the cases.

Question 5: A lift is moving down with an acceleration a. A man in the lift drops a ball inside the lift. The acceleration of the ball as observed by the man in the lift, and a man standing stationary on the ground are respectively—

  1. g, g
  2. a, a
  3. (g - a), g
  4. a, g

Answer: (c) (g - a), g

Explanation:

When dropped, acceleration of the ball is g, as will be observed by a man standing stationary on the ground. The man inside the lift is having its own downward acc. = a. Therefore, relative acc. of the ball as observed by the man in the
lift = (g _ a).

Question 6: A man weights 80 kg. He stands on a weighing scale in the lift, which is moving upwards with a uniform acceleration of 5 m/s2. What would be the reading on the scale? (g = 10 m/s2)

  1. Zero
  2. 400 N
  3. 800 N
  4. 1200 N

Answer: (d) 1200 N

Question 7: In gravity free space, a man of mass M standing at a height h above the floor throws a ball of mass m straight down with a speed u. When the ball reaches the floor, the distance of the man above the floor will be—

  3. 2h
  4. cannot say

Answer: (a)

Explanation:

Time taken by ball to reach the ground, t = h/u. According to the principle of conservation of linear momentum, velocity of man upwards.

Distance travelled by man upwards

Height of man above the floor

Question 8: A car starts from rest to cover a distance s. The coefficient of friction between the road and tyres is µ. The minimum time in which the car can cover the distance is proportional to—

  1. µ

Answer: (c)

Explanation: From

Question 9: A car starts from rest and moves on a surface in which coefficient of friction between the road and tyres increases linearly with distance (x). The car moves with maximum possible acceleration. The K.E. of the car (E) will depend on x as—

Answer: (c)

Explanation:

K.E. = E =

Question 10: In Fig. block A is released from rest, when spring is at its natural ustretched length. For block B of mass M to leave contact with the ground at some stage, the minimum mass of A must be—

  1. 2 M
  2. M/2
  3. M
  4. cannot say

Answer: (b) M/2

Explanation: The spring will exert maximum force when A is at its lowest position, say at a distance x from the present position.

where k is force constant of the spring. For the block B to leave contact, kx = Mg

or

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