The comprehensive approach in Kerala Syllabus 9th Standard Physics Notes Pdf Chapter 3 Laws of Motion Notes Extra Questions and Answers ensures conceptual clarity.
Kerala Syllabus Std 9 Physics Chapter 3 Laws of Motion Extra Questions and Answers
Question 1.
Using Newton’s First Law, describe the motion of a skateboarder who strikes a rough area on the sidewalk.
Answer:
The skateboard slows down suddenly when the skater strikes a rough surface because the increased friction behaves like an unbalanced force. Skater may fall forward because of inertia of motion
Question 2.
State true or false
a) Due to inertia, when someone jumps off a moving swing, they will likely keep going in the same direction.
b) A book is placed on a flat table. It starts moving without applying any external force.
Answer:
a) True
b) False
Question 3.
Calculate the momentum of a body of mass 1000 kg at rest.
Answer:
m = 1000 kg; v = 0 m/s
p = mv = 1000 kg × 0 m/s = 0 kg m/s
Question 4.
“You’re at the grocery store pushing a cart.”
a) What happens to the cart’s acceleration when you push it harder?
b) How does the acceleration differ when the cart is empty, and you apply the same force as when it is full?
Answer:
a) The acceleration of the cart increases because force and acceleration are directly proportional.
b) The empty cart will accelerate more than the full cart because it has less mass.
Question 5.
Match the following.
| A horse pulls a cart. | The rocket moves upward. |
| A hammer strikes a nail. | The air pushes the bird upward. |
| A bird flaps its wings downward. | The cart pulls back on the horse. |
| A rocket engine expels hot gases downward. | The nail pushes back on the hammer. |
Answer:
| A horse pulls a cart. | The cart pulls back on the horse. |
| A hammer strikes a nail. | The nail pushes back on the hammer. |
| A bird flaps its wings downward. | The air pushes the bird upward. |
| A rocket engine expels hot gases downward. | The rocket moves upward. |
Question 6.
In each case, state which is action and reaction.
a)A ball dropped upon the ground bounces back.
b) A boat that is motionless is pushed away from the dock by someone who is standing on it.
Answer:
a) Action – The ball pushing on the ground downwards when it hits.
Reaction – The ground pushing up on the ball.
b) Action – The person pushing the boat away from the dock.
Reaction – The boat pushing the person back towards the dock.
Question 7.
As mass increases, inertia _____
Answer:
increases.
Question 8.
____ forces can neither move an object at rest nor change the direction or speed of an object in motion. (balanced/unbalanced)
Answer:
Balanced
Question 9.
Observe the figure. Find the resultant force acting on the object of mass 50 kg.
Answer:
0 (Balanced force)
Question 10.
Classify the following instances into inertia of motion and inertia of rest
a) When a running bus stops suddenly, passengers tend to fall forward.
b) A fan continues to rotate for a while even after it is switched off.
c) The mango falls down when branches are shaken.
d) It is possible to strike out a single carrom coin without disturbing others, from a set of coins stacked one over the other.
Answer:
Inertia of motion – a, b
Inertia of rest – c, d
Question 11.
A girl sitting on a chair is trying to lift the same chair by herself. Will it be possible to lift a chair while seated on it. Justify your answer.
Answer:
No. Internal force cannot move an object, but only external unbalanced force can cause its motion.
Question 12.
When two boys are pushing the table from opposite sides with equal force, the table continues in its state of rest.
a) Which law is associated with this?
b) State the law.
Answer:
a) First law of motion.
b) Every object continues in its state of rest or of uniform motion along a straight line unless an unbalanced external force acts on it. This is Newton’s First Law of Motion.
Question 13.
Write down an activity to prove inertia of rest.
Answer:
Make a pile of coins on a table. Strikes off the lowest coin by a knife quickly. Only that particular coin is thrown off and the others remains in the previous manner. This is due to the tendency of coins to remain in its state of rest.
Question 14.
What could be happened if we travel in a car without wearing seat belt? Why does it happen so?
Answer:
When we travel in a car we are also moving with the car. That is we are in a state of motion. If we don’t wear seat belt, during a crash, we could be thrown off due to our inertia of motion. Travelling in a car without wearing seat belt is dangerous.
Question 15.
Statements related to inertia of motion and inertia of rest are given below. Classify them suitably.
a) When the branch of a mango tree is shaken, mangoes fall just as when the branch starts moving.
b) When a carpet is suspended and hit with a stick, dust particles come out.
c) An athlete runs some distance before taking a long jump.
d) When a moving bus is suddenly stopped, the standing passengers tend to fall forward.
Answer:
| Inertia of rest | Inertia of motion |
|
|
Question 16.
Observe the figure.
a) When the card is suddenly struck, what happens to the coin? Why?
b) To which law of motion the observed phenomenon is related?
Answer:
a. The coin falls into the glass. It is due to the inertia of rest of the coin.
b. First law of motion
Question 17.
Give reasons
a) When a bullet is fired from a gun, the gun recoils.
b) When a bus at rest suddenly moves forward, the passengers standing in the bus fall backward.
c) We slip on the mossy surface.
Answer:
a) The gun recoils due to the reaction force applied by the shot to the gun. Forward movement of the shot is action and the backward movement of the gun is reaction.
b) Inertia of rest is the reason. The passengers tends continue in state of rest.
c) Absence of reaction force is the cause for this.
Question 18.
State true or false.
a) All internal forces are unbalanced forces.
b) All internal forces are balanced forces.
c) All balanced forces are internal forces.
Answer:
a) False
b) True
c) False
Question 19.
Give two examples for inertia of rest and two examples of inertia of motion.
Answer:
Inertia of rest,
- When the branch of a mango tree is shaken mangoes fall.
- When the carpet is trapped, dust particle scatters.
Inertia of motion
- A running athlete cannot stop himself abruptly at the finishing point.
- A man stepping down from a slowly moving bus, stops after few steps of running.
Question 20.
A body of mass 3 kg is moving with a velocity of 4 m/s. What is its momentum?
Answer:
12 Kg m/s
Question 21.
Find out the relation from the first pair and complete the second pair.
Impulse = Force × Time
Momentum = _______ × _______
Answer:
mass × velocity
Question 22.
The force required to produce an acceleration of 1 m/s2 on a body of mass 1 kg is ______
Answer:
1 N or 1 kg m/s2
Question 23.
Identify the relation between the first pair and complete the second.
Momentum: m × V; Impulse: _______
Answer:
F × t
Question 24.
Find the odd one.
F, \(\frac{mv-mu}{t}\), mv-mu, ma
Answer:
mv – mu.
Question 25.
Write down the unit of impulse.
Answer:
Ns
Question 26.
Identify the relation between the terms in the first pair and complete the second pair
Force : N
Momentum: ______
Answer:
Kg m/s
Question 27.
A stone of mass 0.5 kg is thrown with a velocity of 20 m/s. Find the momentum of the stone.
Answer:
m = 0.5kg
y =20 m/s
Momentum, p = m × v = 0.5 × 20 = 10 kg m/s
Question 28.
A car of 1000 kg moves with a velocity 20m/s. On applying brakes, it comes to rest in 5s.
a) What is its initial momentum?
b) What is its final momentum?
Answer:
a) Initial momentum = mu
= 1000 × 20 = 20000 kg m/s
b) Final momentum = mv = 100 × 0 = 0 m
Question 29.
An object of mass 10 kg is moving with a velocity of 15 m/s. Its velocity increases to 25 m/s on applying a constant force continuously for 5 s.
a) Calculate the acceleration of the object.
b) Calculate the force applied.
Answer:
m 10kg
u = 15 m/s
v = 25m/s
t = 5 s
a) a = \(\frac{v-u}{t}\) = \(\frac{25-15}{5}\) = 2 m/s2
b) F = ma = 10 × 2 = 20 N
Question 30.
What is impulse – momentum principle? Prove it.
Answer:
According to Newton’s second law of motion,
F = \(\frac{mv-mu}{t}\)
F × t = m (v-u)
F × t = mv – mu
i.e., impulse = change in momentum
This is known as impulse-momentum principle. It states that a change in momentum of an object is equal to the impulse experienced by it.
Question 31.
Velocity – time graph of a sphere of mass 3 kg rolling along a surface is given.
a) What is the initial momentum?
b) What is the change in momentum?
c) Calculate the force exerted by the floor to make the sphere stop.
Answer:
a) Initial momentum = mu = 3 × 10 = 30 kg m/s
b) Change in momentum = mv – mu
= 3 × 0 – 3 × 10
= -30 kg m/s
c) Force,
F = ma = \(\frac{mv-mu}{t}\)
= \(\frac{-30}{5}\) = -6 N
Question 32.
State the reason
a) Karate experts move their hands with great speed to break strong bricks.
b) While catching a cricket ball the player draws his hands backward along with the ball.
c) A running athlete cannot stop himself abruptly at the finishing line in a race.
d) It is easy to stop a rolling empty drum. But it is difficult to stop a rolling drum filled with tar.
Answer:
a) The force acting on a body will be inversely proportional to the time taken. As time decreases, force increases.
b) The force acting on a body will be inversely proportional to the time taken. As time increases force decreases.
c) Due to inertia of motion
d) As mass increases, inertia decreases.
Question 33.
A body of mass 2 kg is falling down freely from rest
a) What is its velocity at the end of 2 s ?
b) Calculate the change of momentum during this time.
c) Find the impulse of force acting on this body at this time interval.
d) Name the principle relating impulse and change of momentum.
Answer:
m = 2 kg, u = 0m/s, g = 10m/s2
a) t = 2s
v = u + gt
= 0 + 10 × 2
= 20 m/s
b) mu = 2 × 0 = 0 kg m/s
mv =2 × 20 = 40 kg m/s
Change in momentum = mv – mu = 40 – 0 = 40 kg m/s
c) Impulse = F × t
= Change in momentum = 40 Ns
d) Impulse – momentum principle
Question 34.
A car moving with a speed of 72 km/h comes to rest after 4s on applying brake. If the mass of the car including the passengers ¡s 1000kg. What will be the force applied when brake is applied?
Answer:
u = 72 km/h = \(\frac{72 \times 5}{18}\) = 20 m/s
v = 0
m = 1000 kg
t = 4s
Force, F = ma = \(\frac{mv-mu}{t}\)
= \(\frac{1000 \times 0-1000 \times 20}{4}\)
= – 5000 N
Question 35.
A loaded lorry of mass 1500 kg moves with a velocity of 12 m/s. Within a small interval of time the velocity becomes 10 m/s.
a) What is the initial momentum of the lorry?
b) What is its final momentum?
c) What is the change in momentum?
Answer:
m = 1500 kg
u = 12 m/s
v = 10 m/s
a) Initial momentum = mu = 1500 × 12 = 18000 kg m/s
b) Final momentum = mv = 1500 × 10 = 15000 kg m/s
c) Change in momentum = mv – mu = 15000 – 18000 = -3000 kg m/s
Question 36.
Can an auto rickshaw move if pushed from inside? Why?
Answer:
No, it is an internal force. Internal force is balanced force. It can riot move a body.
Question 35.
According to Newton’s Third Law, if object A exerts a force on object B, then object B exerts a force on object A that is ___________ in magnitude and ___________in direction.
Answer:
equal, opposite
Question 37.
Find out the appropriate law for the given situations.
a) When the tip of an inflated balloon is released, the balloon moves in a direction opposite to the direction of motion of air.
b) When a bus moves forward suddenly from rest, the standing passengers tend to fall back.
c) During a pole wait jump the impact is reduced by falling on the foam bed.
d) Gas at high pressure pushed out from the chambers of rocket causes rocket propulsion.
Answer:
a) Newton’s third law of motion
b) Newton’s first law of motion
c) Newton’s second law of motion
d) Newton’s third law of motion
Question 38.
All are aware of Newton’s third law of motion
a) When a man jumps from a boat to the shore even though action and reaction are equal and opposite, they do not cancel each other, he can jump forward. Explain the reason.
b) If we push a cart on mud, it won’t move. What may be the reason?
Answer:
a) Because action and reaction are acting on different bodies.
b) This is because no reaction is obtained from mud.
Question 39.
Match the items in columns A, B and C suitably.
| A | B | C |
| Inertia | Change of motion | N |
| Impulse | Newton’s third law of motion | Kg m/s |
| Force | Newton’s first law of motion | Ns |
| – | Newton’s second law of motion | mass |
Answer:
| A | B | C |
| Inertia | Newton’s first law of motion | mass |
| impulse | Change of momentum | Ns |
| Force | Newton’s second law of motion | N |
Question 40.
Give reasons for the following.
a) Accidents that happen to passengers who do not wear seat belts are more fatal.
b) At the time of catching a cricket ball, the time of catching is extended by moving the hand backward with the ball.
c) Action and reaction do not cancel each other.
Answer:
a) It is due to inertia of motion. In moving vehicle, the passengers are also in the state of motion. When the vehicle is suddenly stopped the passengers have the tendency to move forward and hit on the body of vehicle that causes accident more fatal if they do not wear seat belt.
b) it is to reduce impulsive force thereby reduces the momentum and impact on our hand.
c) Action and reaction are equal and opposite but they never cancel each other because they act on different objects.
Question 41.
In the figure given below C is a frictionless toy car of low mass. Threads are taken from either end of the car through pulleys and weights are hung at the ends as shown.
a) What will you observe on placing 1oo g weight each in pans A and B?
b) What will be your observation on piacing 100g weight in A and 200 g weight in B? Justify your answer.
Answer:
a) Toy car does not move.
b) Toy car moves to the right side (towards 13) in the first case, both forces are equal and acting in the opposite directions. So the resultant force is zero. In the second case, the weight on the side B is more than that in A. So resultant force acts along B.
Question 42.
“A man jumps from a boat to the shore”
a) Identify the action and reaction in the above situation.
b) State the law related to the given situation.
c) Write another example related to this law.
Answer:
a) Action – The boat moves backward. Reaction – The man lands on the shore.
b) Newton’s third law ofmotion2
e) Recoil of a gun,when bullet moves forward.
Question 43.
Rockets are used to launch artificial satellites.
a) State the law of motion related to the rocket launching.
b) Write down the life situations related to this law.
c) Write down a class room activity to illustrate this law.
Answer:
a) For every action there is an equal and opposite reaction
b) When a man jumps from a boat to the shore, the boat moves backward. When a bullet is fired from a gun the gun recoils.
c) Pass a long string through a straw and tie the string between two windows of the class room. Paste an inflated balloon on the straw. Then deflate the ballon and release it suddenly release of air causes the balloon to move in the opposite direction. Newton’s third law of motion is thus illustrated.