Friction Questions and Answers

1 A block of mass m slips on a rough horizontal table under the action of a horizontal force applied to it The coefficient of friction between the block and the table is The table does not move on the floor Find the total frictional force applied by the floor on the legs of the table Do you need the friction coefficient between the table and the floor or the mass of the table M

9 A block of mass 2 kg is pushed against a rough vertical wall with a force of 40 N coefficient of static friction being 0 5 Another horizontal force of 15 N is applied on the block in a direction parallel to the wall Will the block move If yes in which direction If no find the frictional force exerted by the wall on the block

5 Consider the situation shown in figure 6 E9 Suppose a small electric field E exists in the space in the vertically upward direction and the upper block carries a positive charge Q on its top surface The friction coefficient between the two blocks is u but the floor is smooth What maximum horizontal force F can be applied without disturbing the equilibrium Hint The force on a charge Q by the electric field E is F OF in the direction of Fl

1 Consider the situation shown in figure 8 E2 The system is released from rest and the block of mass 10 kg is found to have a speed 0 3 m s after it has descended through a distance of 1 m Find the coefficient of kinetic friction between the block and the table 4 0 kg 1 0 kg G

A body is projected upwards with a velocity u It passes through a certain point above the ground after t second The time after which the body passes through the same point during the return journey is 1 g 3 t 2 2 t g 4 3 q

a g 57 Block A is placed on a rough wedge B which is placed on a smooth surface The wedge has angle of inclination of 53 and is imparted a horizontal acceleration g towards right Block A is given an initial velocity v with respect to wedge Find the coefficient of friction for which block A moves with constant velocity v with respect to wedge g 10 m s Smooth surface a 2 7 c 4 7 b 3 7 d 1 3 712 B 53

Example 6 5 A 10 g block placed on a rough horizontal floor being pulled by a constant force 50 N Coefficient of kinetic Fiction between the block and the floor is 0 4 Find work done each individual force acting on the block over displacement of 5 m F

4 A block slides down an inclined surface of inclination 30 with the horizontal Starting from rest it covers 8 m in the first two seconds Find the coefficient of kinetic friction between the two

3 In a children s park a heavy rod is pivoted at the centre and is made to rotate about the pivot so that the rod always remains horizontal Two kids hold the rod near the ends and thus rotate with the rod figure 7 E2 Let the mass of each kid be 15 kg the distance between the points of the rod where the two kids hold it be 3 0 m and suppose that the rod rotates at the rate of 20 revolutions per minute Find the force of friction exerted by the rod on one of the kids Figure 7 E2

3 A block of mass m is kept on a horizontal table If the static friction coefficient is u find the frictional force acting on the block

D A boy of mass M is applying a horizontal force to s a box of mass M on a rough horizontal surface coefficient of friction between the shoes of the boy E the floor is and that between the box and the floo u In which of the following cases it is certainly possible to slide the box a u M M c M M b M M d M M

0 A car goes on a horizontal circular road of radius R the du a The friction dt speed increasing at a constant rate coefficient between the road and the tyre is u Find the speed at which the car will skid

A block is placed on a rough floor and a horizontal force F is applied on it The force of friction f by the floor on the block is measured for different values of F and a graph is plotted between them a The graph is a straight line of slope 45 b The graph is a straight line parallel to the F axis c The graph is a straight line of slope 45 for small F and a straight line parallel to the F axis for large F d There is a small kink on the graph

A 40 kg slab rests on a frictionless floor A 10 kg block rests on the top of the slab The static coefficients of friction between the block and the slab is 0 60 while the kinetic coefficient is 0 40 The 10 kg block is acted upon by a horizontal force of 100 N If g 9 8 ms 2 the resultant acceleration of the slab will be a 0 98 ms 2 c 1 52 ms 2 10 kg 40 kg 100 N b 1 47 ms d 6 1 ms

with which it 1 to make the Force F Find it has to be 1987 7M strings of d frictionless 161 26 In the figure the blocks A B and C have masses 3 kg 4 kg nos and 8 kg respectively The coefficient of sliding friction between any two surfaces is 0 25 A is held at rest by a massless rigid rod fixed to the wall while B and Care connected by a light flexible cord passing around a fixed frictionless pulley Find the force F necessary to drag C along the horizontal surface to the left at a constant speed Assume that the arrangement shown in the figure i e B on C and A on B is maintained throughout Take g 10 m s 1978 A

a 11 Two 30 kg blocks rest on a massless belt which passes over a fixed pulley and is attached to 40 kg block If coefficient of friction between the belt and the table as well as between the belt and the blocks B and C is and the system is released from rest from the position shown the speed with which the block B falls off the belt is 2m 40 kg A 30 kg B 2 m 130 kg A 2 2 m s if u 0 2 B 2 m s if u 0 2 C 2m sifu 0 5 D 25m sifu 05

A parabolic bowl with its bottom at origin has the shape y x 20 Here x and y are in mete The maximum height at which a small mass m can be placed on the bowl without slipping coefficient of static friction is 0 5 is A 2 5 m B 1 25 m Y Vertical X Horizontal C 10m D 40m

F 20N 4kg 0 6 0 4 From the fig the acceleration of the block is

Statement 1 A car moves along a road with uniform speed The path of car lies in vertical plane and is shown in fig The radius of curvature R of the path is same everywhere If the car does not loose contact with road at the high point It can travel the shown path without loosing contact with road anywhere else Statement 2 For car to loose contact with road the normal reaction between car and road should be zero

One Correct Options block A of mass M moving with speed u collides stically with block B of mass m which is connected to ck C of mass m with a spring AU B M m oooooooo C m When the compression in spring is maximum the veloci of block C with respect to block A is Neglect the friction everywhere a zero c m M m u b d M M m m U u

A labourer lifts a stack of several identical bricks each of amss 1 kg by pressing hard with his hands The corfficient of static friction for all surfaces is u 0 5 The minimum normal force applied by each hand to just hold the bricks hold the bricks is 50 N Which of the following statement is correct a The number of bricks hold by labourer is 5 if normal force by hand is 50 N b The number of bricks hold by labourer is 10 if normal contact force by hand is 50 N c If normal contact force applied by labourer is slightly less than 50 N number of bricks hold by labourer is 10 d If labourer just hold the stack the contact force applied by each hand on the brick is 55 9 N

Three blocks A B and C of equal mass m are placed one over the other on a smooth horizontal ground as The maximum value of shown in figure Coefficient of friction between any two blocks of A B and C is mass of block D so that the blocks A B and C move without slipping over each other is

A plank of mass m 8 kg with a bar of mass m 2 kg placed on its rough surface lie on a smooth floor of elevator ascending with an acceleration g 4 The coefficient of friction is 1 5 between m and m A horizontal force F 30 N is applied to the plank Then the acceleration of bar and the plank in the reference frame of elevator are A 3 5 m s 5 m s 25 8 C 2 5 m s m s g 4 3 3 F m B 5 m s 50 m s D 4 5 m s 4 5m s

Fatos furtam dinin The force which results when there is a direct physical contact between two interacting objects is known as a contact force 7 14 pm The force which occurs without any physical contact between two objects is known as a force at a distance or field force ASSESSMENT Answer the following questions

The centre of mass of a system cannot change its state of motion unless there is an external force acting on it Yet the internal force of the brakes can bring a car to rest Then a the brakes stop the wheels b the friction between the brake pads and the wheel stops the car c the car is stopped by the road d the car is stopped by the driver pressing the pedal

A 0 5 kg block slides from the point A on a horizontal track with an initial speed 3 m s towards a weightless horizontal spring of length 1 m and force constant 2 N m The part AB of the track is frictionless and the part BC has the coefficient of static and kinetic friction as 0 22 and 0 20 respectively If the distance AB and BD are 2 m and 2 14 m respectively find the total distance through which the block moves before it comes to rest completely g 10 m s

A block of mass 4 kg is placed on another block B of mass 5 kg and the block B rests on a smooth horizontal table If the maximum force that can be applied on A so that both the blocks move together is 12 N the maximum force that can be applied on B for the blocks to move together will be 1 15 N 2014 3 27 N 2 25 N 4 48 N

Block A of mass 1 kg is kept on block B of mass 2kg which is kept on smooth surface as shown n figure At t 0 both blocks are at rest a force of 6N is applied on block B Friction coefficient between A B is 0 5 Which of the following is are CORRECT A B H 0 5 Z 0 A Acceleration of both the blocks is 2m s B Work done by friction in ground frame on A is zero from t 0 to t 2 sec C Friction force on A is 2N D Kinetic energy of A at t 2 sec is 8 1

Two masses m 5 kg and m 10 kg connected by an inextensible string over a frictionless pulley are moving as shown in the figure The coefficient of friction of horizontal surface is 0 15 The minimum weight m that should be put on top of m to stop the motion is JEE Mains 2018 0 103 ka m m B 18 3 kg T T m C 27 3 kg m g D 43 3 kg

R 2005 A smooth block is released at rest on a 45 incline and then slides a distance d The time taken to slide is n times as much to slide on rough incline than on a smooth incline The coefficient of friction is 1 Hk 1 3 H 1 1 n n 2 Mk 4 s 1

A body of mass 2 kg moving on a horizontal surface with an initial velocity of 4 m sec comes to rest after 2 sec If one wants to keep this body moving on the same surface with a velocity of 4 m sec the force required is 1 8 newton 3 zero newton 2 4 newton 4 2 newton

The friction coefficient between the table and the block shown in figure 6 E4 is 0 2 Find the tensions in the two strings 15 kg 5 kg Figure 6 E4 5 kg

2 The breaking strength of the cable used to pull a body is 40 N A body of mass 8 kg is resting on a table of coefficient of friction 0 20 The maximum acceleration which can be produced by the cable is take g 10ms 2 2 a 6ms b 3ms c 8ms 2 d 8ms 2

A car is moving on a circular path and takes a turn If R and R be the reactions on the inner and outer wheels respectively then A R R B R R C R 2 R D R R

3 A constant force is being applied on a rod of length l kept at rest on two parallel conducting rails connected at ends by resistance R in uniform magnetic field B as shown R OB F a the power delivered by force will remain constant with time b the power delivered by force will be increasing first and then it will decrease c the power delivered by force will be increasing continuously d the power delivered by force will be decreasing

25 A block of mass m is in contact with the cart C as shown in the figure The coefficient of static friction between the block and the cart is u The acceleration a of the cart that will prevent the block from falling statisfies A a C a g g um B a D a mg LL g SEEMANTY C

A car has two horns having a difference in frequency of 180 Hz The car is approaching a stationary obser with a speed of 60 ms Calculate the difference in frequencies of the notes as heard by the obserw velocity of sound in air is 330 ms When both source and observer approach each other with a speed equal to the half the speed of s cted by the listener

7 A body kept on an inclined plane has an angle of repose 30 If the inclination is increased to 60 with wha acceleration will the body slide down the plane 566 cm

35 A uniform rod AB of weight W rests with the end A on rough horizontal surface and the other end B against a smooth vertical wall If the rod is in equilibrium in the position shown in figure then the normal reaction at B is B W 1 3 3 W 0 30 TTTTTTT 2 A 3 2 W W

15 A car is moving round a curve of radius 20 m without slipping on a banked road with 0 45 Assuming the coefficient of friction between the road and tyres to be 0 2 What is the maximum speed with which the car can move Take g 10 m s NCERT Pg 104 1 10 m s 3 15 m s 2 5 m s 4 10 3 m s

For what minimum value of the applied force P there will be no relative motion between 4 kg block and 2 kg block in the arrangement shown in figure Take g 10 m s P P 0 1 50 N 4 kg 2 210 N NAM Hs 2 kg 0 4 3 120 N 4 150 N

2 ideal The block of mass m is placed on a rough horizontal floor and it is pulled by a string as shown by a constant force F As the block moves towards right the friction force on block 55 T mg Aremains constant C decreases fid m F fen Y 27 Ams 2 B increases D can t be said 2

Two blocks of mass 2 kg and 3 kg are kept on level ground having coefficient of static friction and 0 4 respectively from ground The blocks are connected by a spring of constant 100 N m What can be a possible extension in the spring so that both the blocks are in equilibrium g 10 ms 1 3 cm 3 9 cm 2 6 cm 4 15 cm

A tram is moving with an acceleration of 49 cm s 2 If 50 of the engine power is used up to overcome fric tion and the remaining 50 is spent for increasing velocity coefficient of friction between the wheel and the track should be 0 03 B 0 02 0 05 0 10

A car is accelerating on a horizontal road with acceleration 20 m s A box that is placed inside the car of mass m 10 kg is put in contact with the vertical wall as shown The friction coefficient between the box and the wall is 0 6 A The acceleration with respect to ground of the box will be 20 m sec 2 0 6 10kg B The friction force acting on the box will be 100 N C The contact force between the vertical wall and the box will be 100 N D The net contact force between the vertical wall and the box is vertical 20 m

A block of mass M is kept on a frictionless horizontal table A body of mass m is kept on the block If a force F parallel to the table top is applied on the block the body on the block tends to slide backwards What is the coefficient of static friction between the block and the body CM F M m g on Inclined Surface

Use your brain power the As a civil engineer you are given contract to construct a curved road in a ghat In order to obtain the banking angle you need to decide the speed limit How will you decide the values of speed y and radius r wht ma acting vertically downwards

23 An engine of mass 105 kg is moving up an inclined plane of gradient 5 at a speed of 100 m h The engine has an effciency of 4 for converting heat into work claculate the amount of coal the engine has to burn up in one hour if the coefficient of friction between engine and rail is 0 1 Burning of 1 kg coal yields 50 000 joule Ans 9 51 x 10 kg

box starts to slip and slides 4 0 m down the plank in 4 0 s The coefficients of static and kinetic friction between the box and the plank will be respectively a 0 5 and 0 6 c 0 6 and 0 6 2015 Three blocks A B and C of masses 4 kg 2 kg and 1 kg respectively are in contact on a frictionless surface as shown If a force of 14 N is applied on the 4 kg block then the contact force between A and B is a 8 N c 2 N c b 0 4 and 0 3 d 0 6 and 0 5 2015 Cancelled 10 A block A of mass m rests on a horizontal table A light string connected to it passes over a frictionless pully at the edge of table and from its other end another block B of mass m is suspended The coefficient of kinetic friction between the block and the table is u When the block A is sliding on the table the tension in the string is a m m 1 8 m m m um 8 m m H A system consists of three masses m m and m3 connected by a string passing over a pulley P The mass m hangs freely and m and m are on a rough horizontal table the b 18 N d 6 N b d m m 1 H 8 m m m m 8 m m 2015 Cancelled P m m M3 a 9 c g 1 2 2014 N The force F acting on a particle of mass m is indicated by the force time graph shown below The change in momentum of the particle over the time interval from zero to 8 s is coefficient of friction The pulley is frictionless and of negligible mass The downward acceleration of mass m is Assume a 24 Ns c 12 Ns b 2gu d g 1 2 2ma g a ma g a 6 r s 2014 13 A balloon with mass m is descending down with an acceleration a where a g How much mass should be removed from it so that it starts moving up with an acceleration a a b 20 N s d 6 Ns b d 2ma g a ma 2014 g a M Three blocks with masses m 2m and 3m are connected by strings as shown in the figure After an upward force F is applied on block m the masses move upward at constant speed w What is the net force on the block of mass 2m g is the acceleration due to gravity a 3mg b 6mg c zero d 2mg m 2m 3m NEET 20 19 An explosion breaks a rock into three parts horizontal plane Two of them go off at r angles to each other The first part of mass moves with a speed of 12 m s and the sec part of mass 2 kg moves with 8 m s spee the third part flies off with 4 m s speed its mass is a 7 kg c 3 kg b 17 kg d 5 kg NEET 16 The upper half of an inclined plan inclination is perfectly smooth while

Two blocks of masses m and m are connected by a spring of stiffness k The coefficient of friction between th blocks and the surface is u 1 2 The the minimum constant force F to be applied to m in order to slide th mass m is p m g qm g Find p q m ooooo my F