Introduction

What is friction? Meaning of kinetic and static friction. Plotting the friction graph.

Friction

In this video we will discuss the reason fro the existance of friction between the two surfaces. Factors on which friction depends.

Coeff of Friction

In this video we we will discuss about the meaning of coefficientof friction. variation of static friction with the applied force. formulation of static and kinetic friction.

Q1

A block lies on a floor.
a) What is the magnitude of the frictional force on it from the floor?
b) If a horizontal force of 5 N is now applied to the block, but the block dose not move, what is the magnitude of the frictional force on it?
c) If the maximum value of the static frictional force on the block is 10 N, will the block move if the magnitude of the horizontally applied force is 8 N?
d) If it is 12 N ?

Q2

A block of mass m is placed on a rough surface and the angle of the surface is slowly increased. At an angle q the block just starts to slip. Find the coefficient of static friction between the block and the surface.

Q3

Statement 1: Coefficient of friction can be greater than unity.
Statement 2: Force of friction is dependent on normal reaction and ratio of force of friction and normal reaction cannot exceed unity.
Mark:
(a) If both statement 1 and statement 2 are true and statement 2 is the correct explanation of statement 1.
(b) If both statement 1 and statement 2 are true but statement 2 is not the correct explanation of statement 1.
(c) If statement 1 is true but statement 2 is false.
(d) If statement 1 is false but statement 2 is true.

Q4

Static friction is always greater than the Kinetic friction.
True / False?

Q5

Mark the correct statement(s) regarding friction.
(a) Friction can be zero, even though the contact surface is rough
(b) Even though there is no relative motion between surfaces, frictional force may exist between them
(c) The expressions FS = mS N or FK = mK F are approximate expressions
(d) The expression FS = mS N tell that direction of FS and N are the same

Q6

The contact force exerted by a body A on another body B is equal to the normal force between the bodies.
We conclude that
(a) the surface must be frictionless
(b) the force of friction between the bodies is zero
(c) the magnitude of normal force equals that of friction
(d) the bodies may be rough but they dont slip on each other

Q7

A boy of mass M is applying a horizontal force to slide a box of mass M on a rough horizontal surface. The coefficient of friction between the shoes of the boy and the floor is m and between the box and the floor is m. In which of the following cases it is certainly not possible to slide the box?
(a) m < m, M < M
(b) m > m, M < M
(c) m < m, M > M
(d) m > m, M > M

Features of Friction

In this video we will discuss about the features of the friction acting between the objects like its dependency over the shape, direction of action of force. What do we mean by angle of friction?

Direction of Friction

In this video we will learn how to know the direction of friction acting between the two surface in contact.

Q8

Find the direction of friction on a block sliding down a rough incline plane of angle q with constant velocity.

Q9

A block is being pulled up a rough incline with force F such that the block is stationary. Find the direction of Friction on the block given the incline makes an angle q with the horizontal.

Q10

Find the direction of friction between the two blocks if both are moving with a constant velocity v towards right.

Q11

Find the direction of friction between the surfaces in contact given that the system at rest?

Q12

Identify the role of friction in the process of walking.

Q13

Find unit vector in the direction of friction force acting on block B kept on moving platform P. Given,

1

A 12 N horizontal force F pushes a block weighing 5.0 N against a vertical wall. The coefficient of static friction between the wall and the block is 0.60, and the coefficient of the kinetic friction is 0.40. Assume that the block is not moving initially.
a) Will the block move?
b) What is the force on the block from the wall?
c) What if F = 6N

2

Block B in figure has mass mB.The coefficient of static friction between block B and table is ms. String attached to the ceiling makes an angle q with the horizontal. Assume that the cord between block B and the knot is horizontal. Find the maximum mass of block A for which the system will be stationary.



3

A block m 1 kept on a rough, horizontal surface is connected to m 2 by a string over an ideal pulley as shown in figure. The coefficient of kinetic friction between the block and the surface is m.
Determine the magnitude of the acceleration of the two objects?

4

The two blocks in figure are not attached to each other. The coefficient of static friction between the blocks is ms, but surface beneath the larger block is frictionless.
What is the minimum magnitude of the horizontal force F required to keep the smaller block from slipping down the larger block?

5

When the three blocks in figure are released from rest, they accelerate with a magnitude of 0.500 m/s2 . Block 1 has mass M, block 2 has 2M, and block 3 has 2M.
Find the coefficient of kinetic friction between block 2 and the table?

6

Body A in figure weighs 102 N, and body B weighs 32 N. The coefficients of friction between A and the incline are ms = 0.56 and mk = 0.25. Angle q is 400. Let the positive direction of x-axis be up the incline. In unit-vector notation,
what is the acceleration of A, if A is initially
a) at rest ,
b) moving up the incline, and
c) Moving down the incline?

7

A block of mass m 1 is kept on a fixed inclined plane and attached to a block of mass m 2 by a rope as shown in Figure. The incline makes angle q with horizontal and coefficient of friction is m.
Find range of m 2 for which m 1 remains at rest. (given q > tan-1 m)

8

In figure, two boxes m 1 (5kg) and m 2 (10kg) slide down an inclined plane while attached by a massless rod parallel to the plane. The angle of incline is q = 300. The coefficient of kinetic friction between m1 and the incline is m 1 = 0.2 ; between m 2 and the incline is m 2 = 0.1 Find
a) The tension in the rod
b) The magnitude of the common acceleration of the two boxes.
c) How would the answer to (a) and (b) change if m 2 trailed m 1 ?

9

A slab of mass m 1 = 40kg rests on a frictionless floor, and a block of mass m 2 = 10kg rests on top of the slab. Between block and slab, the coefficient of static friction is 0.60, and the coefficient of kinetic friction is 0.40. The block is pulled by horizontal force F of magnitude 100 N. In unit-vector notation, what are the resulting accelerations of
a) the block and
b) the slab?

10

Consider the situation shown in figure. The friction coefficient between the two blocks is m but the floor is smooth.
What maximum horizontal force F can be applied without disturbing the equilibrium?

11

The friction coefficient between the board and the floor shown is ms.
Find the maximum force that the man can exert on the rope so that the board does not slip on the floor.

12

Two blocks A and B with masses 5 kg and 10 kg respectively are placed on a surface. The coefficient of friction between the blocks is 0.5 and between ground and block B is 0.2. Force F is applied on block A as shown in the figure.
a) Find acceleration of blocks if force F = 40 N.
If the ground is considered to be frictionless,
b) If F = 30 N then find acceleration of blocks.
c) If F = 60 N then find acceleration of blocks

13

In figure, blocks A and B have weights of 44 N and 22 N, respectively.
a) Determine the minimum weight of block C to keep A from sliding if ms between A and the table is 0.20.
b) Block C suddenly is lifted off A. What is the acceleration of block A if mk between A and the table is 0.15?
c) If weight of C is equal to half of that calculated in part (a) Find the acceleration of block A and C when the system is released for
i) m = 0 between A and C
ii) m = 0.2 between A and C?

14

The coefficient of friction between the two blocks is m1 and that between the bigger block and the ground is m 2 .
Find the acceleration of the block of mass m2 in the situation as shown in figure.

15

A crate slides down an inclined right-angled trough .The coefficient of kinetic friction between the crate and the trough is mk .
What is the acceleration of the crate in terms of mk , thetha , and g ?

16

Consider the situation shown in figure. The horizontal surface below the bigger block is smooth. The coefficient of friction between the blocks is m.
Find the minimum and the maximum force F that can be applied in order to keep the smaller blocks at rest with respect to the bigger block.

17

A 1000 kg boat is traveling at 90 km/h when its engine is shut off. The magnitude of the frictional force fk between boat and water is proportional to the speed v of the boat as fk = 70 v, where v is in meters per second and fk is in newtons.
Find the time required for the boat to slow down to 45 km/h.

18

A small body A starts sliding down from the top of wedge whose base is equal to l. The coefficient of friction between the body and the wedge surface is m. At what value of the angle q will the time of sliding be the least ?

19

A block of mass m1 is placed on a block of mass m2 , which rests on a smooth horizontal plane. The coefficient of friction between the blocks is equal to m. The block m2 is subjected to the horizontal force F depending on time t as
F = Kt (K is a constant). Find
a) the moment of time t0 at which the blocks start sliding and
b) the acceleration a1 and a2 of the two blocks respectively.

20

Figure shows a small block of mass m1 kept at the left end of a larger block of mass m2 and length l. The system can slide on a horizontal road. The system started towards right with an initial velocity v. The friction coefficient between the road and the bigger block is m and that between the block is m/2.
Find the time elapsed before the smaller blocks separates from the bigger block.

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