Laws of Motion Extra Question for class 9th(lec 6) English

 video lectures for Laws of Motion

Question 1: A bullet of mass `50 g` is fired from a rifle of mass `4 kg` with an initial velocity of  `35ms^{-1}`. Calculate the initial recoil velocity of the rifle.

(a) `-0.44ms^{-1}`

(b) `0.44ms^{-1}`

(c) `-0.44ms`

(d) `0.44ms`

Question 2: Tow objects of masses `100 g` and `200g` are moving along the same line in the same direction with velocities of `2ms^{-1}` and `1ms^{-1}`. Both objects collide. After the collision, the velocity of the first object becomes `1.67ms^{-1}`, then find the velocity of the second object.

(a) `0.165 ms^{-1}`

(b) `1.165ms`

(c) `-1.165 ms^{-1}`

(d) `1.165 ms^{-1}`

Question 3: A truck starts from rest rolling down a hill with constant acceleration. It covers a distance of `400m` in `20 s`. Find its acceleration. If its mass is metric tonnes, calculate the force acting on it. (`1` metric ton = `1000 kg`)

(a) `7000 N`

(b) `14000 N`

(c) `1000 N`

(d) `10,000 N`

Question 4: A stone of mass `1 kg` is thrown with a velocity of `20ms^{-1}` on the frozen surface of a lake. The stone stops after traveling a distance of `50 m`. calculate the force of friction between the stone and the ice.

a) `-4N`

b) `4N`

c) `2N`

d) `-2N`

Question 5: The mass of a car is `1500kg`. If the car is to be brought to rest with a negative acceleration (deceleration) of `1.7ms^{-1}`, what will be the force between the car and the road?

a) `-1500N`

b) `1500N`

c) `2550N`

d) `-2550N`

Question 6: What will be the momentum of an object of mass m whose velocity is v?

a) `(mv)^{2}`

b) `mv^{2}`

c) `\frac{1}{2}mv^{2}`

d) `mv`

Question 7: Two objects, each of mass `1.5 g`, are moving in the same straight line in opposite directions to each other. The velocity of each before the collision is `2.5ms^{-1}`. What will be the combined velocity of both of them if they join each other after collision?

a) `0ms^{-1}`

b) `3ms^{-1}`

c) `2ms^{-1}`

d) `1ms^{-1}`

Question 8:  A hokey ball of mass `200g` moving in a straight line with a velocity of  `10ms^{-1}` collides with a piece of wood of mass `5kg` and is attached to. after that, both move together in the same line. Calculate the total momentum before and after the collision. Calculate the velocity of both object in the connected state.

a) `5ms^{-1}`

b) `13ms^{-1}`

c) `0.38ms^{-1}`

d) `0.5ms^{-1}`

Question 9: A bullet of mass `10g` traveling in a straight line with a velocity of `150ms^{-1}` strikes a wooden block and comes to rest after `0.03s`. How far will the bullet penetrate the wood? Calculate the magnitude of the force exerted by the wooden block on the bullet.

a) `5000 N`

b) `1000 N`

c) `100 N`

d) `50 N`

Question 10: An object of mass `1kg` moving in a straight line with a velocity of `10 ms^{-1}` collides with a wooden block mass `5kg` kept at rest. After that, both move together in the same straight line to calculate the total momentum before and after the collision. Also, calculate the velocity of the interlocked combination.

a) `1.66ms^{-1}`

b) `1.66ms`

c) `2 ms^{-1}`

d) `1 ms^{-1}`

Question 11: The velocity of an object of mass `100kg` moving with uniform acceleration from `5 ms^{-1}` to `8 ms^{-1}` in `6s`. Calculate the momentum of the object before and after. Calculate the magnitude of the force that is exerted on the object.

a) `50 N`

b) `100 N`

c) `25 N`

d) `75 N`

Question 12: A bell of mass `10 kg` is dropped from a height of `80 cm` on the floor. Calculate the value of momentum transferred by the bell to the floor in this position. For simplicity of calculation, take the value of acceleration directed downwards as `10 ms^{-2}`.

a) `20  ms^{-1}`

b) `20 kg ms^{-1}`

c) `40 kg ms^{-1}`

d)  `40 ms^{-1}`

Question 13: A hammer of mass `500 g` strikes a nail with a velocity of `50 ms^{-1}`. the hammer is stopped by the nail for a short time of `0.01 s`. Calculate the force exerted by the nail on the hammer.

a) `5000 N`

b) `2500 N`

c) `1000 N`

d) `100 N`

Question 14: A motor car of mass `1200 kg` is moving in a straight line with a velocity of `90 kmh^{-1}`. Its velocity decreased to `18 kmh^{-1}`  in `4 s` due to an external unbalanced force. Calculated the acceleration and change in momentum. Also, calculate the magnitude of the force applied.

a) `1200 N`

b) `2400 N`

c) `3000 N`

d) `6000 N`

Question 15: Which of the following statement is not correct for an object moving along a straight path in an accelerated motion?

(a) Its speed keeps changing.

(b) Its velocity always changes.

(c) It always goes away from the earth.

(d) A force is always acting on it.

Question 16: According to the third law of motion, action and reaction

(a) always act on the same body

(b) always act on different bodies in opposite directions

(c) have same magnitude and directions

(d) act on either body at normal to each other.

Question 17: A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goal keeper to

(a) exert larger force on the ball

(b) reduce the force exerted by the ball on hands

(c) increase the rate of change of momentum

(d) decrease the rate of change of momentum.

Question 18: The inertia of an object tends to cause the object

(a) to increase its speed

(b) to decrease its speed

(c) to resist any change in its state of motion

(d) to decelerate due to friction.

Question 19: A passenger in a moving train tosses a coin which falls behind him. it means that motion of the train is

(a) accelerated

(b) uniform

(c) retarded

(d) along circular tracks.

Question 20: An object of mass `2 kg` is sliding with a constant velocity of `4ms^{-1}` on a frictionless horizontal table. The force required to keep the object moving with the same velocity is

(a) `32N`

(b) `0N`

(c) `2N`

(d) `8N`

Question 21: Rocket works on the principle of conservation of

(a) mass

(b) energy

(c) momentum

(d) velocity.

Question 22: A water tanker filled up to – of its height is moving with a uniform speed. On sudden application of the brake, the water in the tank would

(a) move backward

(b) move forward

(c) be unaffected

(d) rise upwards.

Question 23: There are three solids made up of aluminium, steel and wood, of the same shape and same volume. Which of them would have highest inertia?

Question 24: Two balls of the same size but of different materials, rubber and iron are jcept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer.

Question 25: Two identical bullets are fired one by a light rifle and another by a heavy rifle with the same force. Which rifle will hurt the shoulder more and why?

Question 26: A horse continues to apply a force in order to move a cart with a constant speed. Explain why?

Question 27: Suppose a ball of mass m is thrown vertically upward with an initial speed u, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed u again before striking the ground. It implies that the magnitude of initial and final momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why?

Question 28: Velocity versus time graph of a ball of mass `50 g` rolling on a concrete floor is shown in Fig. Calculate the acceleration and frictional force of the floor on the ball

a) `-10ms^{-2}`, `0.5 N`

b) `10 ms^{-2}`, `0.5N`

c)  `-5ms^{-2}`, `0.5 N`

d) `-10ms^{-2}`, `1N`

Question 29: A truck of mass `M` is moved under a force `F`. If the truck is then loaded with an object equal to the mass of the truck and the driving force is halved, then how does the acceleration change?

a) `\frac{1^{th}}{4}`

b) `\frac{1^{th}}{3}`

c) `\frac{1^{th}}{2}`

d) `\frac{2^{th}}{3}`

Question 30: Two friends on roller-skates are standing 5 m apart facing each other. One of them throws a ball of 2 kg towards the other, who catches it, How will this activity affect the position of the two? Explain your answer.

Question 31: Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.

Question 32: Using second law of motion, derive the relation between force and acceleration. A bullet of `10 g` strikes a sand-bag at a speed of `103ms^{-1}` and gets embedded after travelling `5 cm`. Calculate

(i) the resistive force exerted by the sand on the bullet

(ii) the time taken by the bullet to come to rest.

a) `10^{5} N`, `10^{-5}s`

b) `10^{4} N`, `10^{-4}s`

c) `10^{5} N`, `10^{4}s`

d) `10^{5} N`, `10^{-4}s`

Question 33: Derive the unit of force using the second law of motion. A force of `5 N` produces an acceleration of `8 ms^{-2}` on a mass `m_{1}`  and an acceleration of `24 ms^{-2}` on a mass `m_{2}`. What acceleration would the same force provide if both the masses are tied together?

a) `2ms^{-1}`

b) `6ms^{-2}`

c) `3ms^{-2}`

d) `6ms^{-1}`

Question 34: What is momentum? Write its SI unit. Interpret force in terms of momentum.

Question 35: Identify the vector physical quantity from the given following 

(a) impulsive force.  

(b) weight. 

(c)  momentum.  

(d) All the above

Question 36: Identify the false statement

(a) It is difficult to run on sand as the force of friction

is small.

(b) Friction is necessary in everyday life

(c) Friction causes wear and tear of the moving

machinery parts.

(d) The coefficient static friction increases on

increasing the area of contact.

Question 37: A block of mass ‘m’ placed on an inclined plane slides

with uniform acceleration. Then

(a) the sum of the forces acting downwards along the plane are equal to the sum of the forces acting upwards along the plane.

(b) the weight of the body acts perpendicular to the inclined plane AB.

(c) the normal reaction of the block is acting perpendicular to the horizontal plane (BC).

(d) the component of weight mg cosθ acts perpendicular to the inclined plane.

Question 38: A tennis player returns, with his racket, a tennis ball of mass `100 g` coming towards him with a speed of `162 kmh^{-1}`. If the ball returns with the same speed and it remains in contact with racket for  `0.1 s`, find the force exerted by him on the ball.

(a) `-90N`

(b) `90 N`

(c) `45 N`

(d) `65 N`

State whether the following statements are true or 

False.

Question 39: Friction depends on the area of contact between two 

surfaces.

Question 40: Friction can be reduced by polishing surfaces.

Question 41: All forces exist in pairs.

Question 42: Impulse and momentum have similar units.

Question 43: Mass of a body is a measure of its inertia.

Direction for questions 44 to 46

Fill in the blanks.

Question 44: The change in momentum of a body has the same 

_____ as that of force applied on it.

Question 45: A car at rest can be moved or a moving car can be 

stopped by applying ______.

Question 46: The total momentum of two bodies before collision 

is equal to their _____ after collision.

Direction for question 47

Match the entries given in Column A with 

appropriate ones from Column B.

Column A                                           Column B

A. Inertia                                             a. variable velocity.

B. Action and Reaction                       b. time period changes with change in length

C. Unit of friction                                 c. dimensionless and 

unitless quantity.

D. Lubricants                                                 d. stretched rubber 

band

E. Unbalanced force                                     e. to reduce force of friction.

 F. Friction                                                      f. inability to change the state of rest on its own.

G. Simple pendulum                                      g. change in kinetic energy 

H. Elastic potential energy                            h. opposition to the relative motion 

I. Coefficient of friction                                 i. acts on two different bodies

J. Work                                                          j. Newton

Answer Key

Q1 (a),  Q2 (d),  Q3 (b),  Q4 (a),  Q5 (d),  Q6 (d),  Q7 (a),  Q8 (c),  Q9 (d),  Q10 (a),  Q11 (a),  Q12 (c),  Q13 (b),  Q14 (d),  

Q15 (c) An object moving along a straight path in an accelerated motion does not always go away from the earth.

Q16 (b)According to the third law of motion, action and reaction always act on different bodies in opposite directions. 

Q17 (b,d): A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goalkeeper to decrease the rate of change of momentum and hence reduce the force exerted by the ball on hands.

Q18 (c): The inertia of an object tends to cause the object to resist any change in its state of motion.

Q19 (a): As the coin tossed falls behind the passenger, so it means that motion of the train is accelerated.

Q20 (b): According to Newton’s first law of motion, an object continues to be in a state of rest or of uniform motion unless acted upon by an unbalanced force. So, there is no requirement of applying force on the object to keep it moving with constant velocity.

Q21 (c): Rocket works on the principle of conservation of momentum.

Q22 (b):Due to inertia of motion, the water in the tank would move forward on sudden application of the brake.

Q23: As the mass is a measure of inertia, the solid of same shape and size having more mass than other solids will have highest inertia. Out of aluminium, steel and wood, density of steel is maximum and hence the mass, therefore, solid made up of steel would have highest inertia.

 Q24: Yes, the balls will start rolling in the direction in which the train was moving. This is because, after applying the brakes, train comes to rest but the balls continue to move due to inertia. Since the masses of the balls are different and hence the inertial forces, the balls will move with different speeds. Iron ball being heavier than the rubber ball will move with lower speed.

Q25: According to conservation of momentum, momentum of each identical bullet fired is equal to the momentum of light rifle as well as momentum of heavy rifle but in opposite direction. So, the light rifle will recoil faster and hence it will hurt the shoulder more.

Q26: A horse continues to apply a force so that frictional force between the cart and the ground is overcome and the cart could move with a constant speed.

Q27: Law of conservation of momentum is applicable to isolated systems, i.e., systems in which no external force is applied. In the mentioned case, change in velocity is taking place due to gravitational pull of earth only, thus, it is not an example of conservation of momentum.

Q28 (a),  Q29 (a),  

Q30: Initially, the momentum of both of them is zero. The one who throws the ball will move backward to conserve the momentum imparted on ball. Now, the other one who catches it will also move backward as he will experience a net force after catching the ball. Hence, separation between them will increase.

Q31: The rotation of water sprinklers used for grass lawns can be explained on the basis of Newton’s third law of motion. As soon as the water comes out of sprinkler, it exerts an equal and opposite force on the sprinkler and it starts rotating.

Q32 (d),  Q33 (b),  

Q34: Momentum is a property of a moving body by the virtue of both its mass and velocity. It is equal to the product of mass and velocity. `p` = `m\timesv`

Its `SI` unit is `kg ms^{-1}`

Q35 (d),  Q36 (d),  Q37 (d),  Q38 (a),

Q39: False  Q40: True  Q41: True  Q42: True  Q43: True,

 Q44: direction  Q45: external force  Q46: total momentum ,

Q47:  A : f  B : I  C : j  D : e  E : a  F : h  G : b  H : d  I : c  J : g