Class 9 Science Force and Laws of Motion

Class 9 Science Force and Laws of Motion

I. Multiple Choice Questions
Choose the correct option:
1. The S.I. unit of force is
(a) kgm/s (b) kgm/s²
(c) Newton ( (d) Newton-meter
2. The product of mass and velocity gives a physical quantity
(a) force (b) inertia
(c) momentum (d) Newton
3. The rate of change of momentum of an object is proportional to
(a) mass of the body (b) velocity of the body
(c) net force applied on the body (d) none of these .
4. If two balls of different masses are dropped on sand, the depths of penetration is same if:
(a) heavier ball is dropped faster than lighter ball
(b) lighter ball is dropped faster than heavier ball
(c) the product ‘mi/ is same for both bodies
(d) none of these
5. The coin remains at rest in the figure shown. This is due to

(a) inertia of rest
(b) two forces act on the coin which balance each other
(c) no unbalanced force acts on it
(d) all of these
6. A force of 50 N moves a body.
(a) Frictional force exerted on the body is less than 50 N
(b) Frictional force exerted on the body is more than 50 N
(c) None of these
(d) Both (a) and (b)
7. Fielder giving a swing while catching a ball is an example of
(a) inertia (b) momentum
(c) Newton’s II law of motion (d) Newton’s I law of motion
8. Action and reaction forces
(a) acts on same body (b) act on different bodies
(c) act in same direction (d) both (a) and (c)
9. When we stop pedaling the bicycle it stops because
(a) the earths gravitational force acts on it
(b) it is not accelerated
(c) no unbalanced force acts on it
(d) frictional force acts on it
10. A football and a stone has same mass
(a) both have same inertia (b) both have same momentum
(c) both have different inertia (d) both have different momentum
Answer. 1—(c), 2—(c), 3—(c), 4-(c), 5-(d), 6-(a), 7-(c), 8-(b), 9-(d), 10-(a).

II. Very Short Answer Type Questions
Question 1. Define force.
Answer: It is a push or pull on an object that produces acceleration in the body on which it acts. 4

Question 2. What is S.I. unit of force?
Answer: S.I. unit of force is Newton.

Question 3. Define one Newton.
Answer: A force of one Newton produces an acceleration of 1 m/s² on an object of mass 1 kg. .
1 N = 1 kg m/s²

Question 4. What is balanced force?
Answer: When forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.

Question 5. What is frictional force?
Answer: The force that always opposes the motion of object is called force of friction.

Question 6. What is inertia?
Answer: The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia.

Question 7. State Newton’s first law of motion.
Answer: An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

Question 8. State Newton’s second law of motion.
Answer: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

Question 9. What is momentum?
Answer: The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. The S. I. unit is kg m/s. (p = mv)

Question 10. State Newton’s III law of motion.
Answer: To every action, there is an equal and opposite reaction and they act on two different bodies.

Question 11. Which will have more inertia a body of mass 10 kg or a body of mass 20 kg?
Answer: A body of mass 20 kg will have more inertia.

Question 12. Name the factor on which the inertia of the body depends.
Answer: Inertia of a body depends upon the mass of the body.

Question 13. Name two factors which determine the momentum of a body.
Answer: Two factors on which momentum of a body depend is mass and velocity. Momentum is directly proportional to the mass and velocity of the body.

Question 14. What decides the rate of change of momentum of an object?
Answer: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

Question 15. The diagram shows a moving truck. Forces A, B,
C and D are acting on the truck.
Name the type of forces acting on a truck.

Answer: The forces A, B, C and D acting on the truck are:

III. Short Answer Type Questions
Question 1. State the difference in balanced and unbalanced force.
Answer:

Question 2. What change will force bring in a body?
Answer: Force can bring following changes in the body:

1. It can change the speed of a body.
2. It can change the direction of motion of a body,
3. It can change the shape of the body.

Question 3. When a motorcar makes a sharp turn at a high speed, we tend to get thrown to one side. Explain why?
Answer: It is due to law of inertia. When we are sitting in car moving in straight line, we tend to continue in our straight-line motion. But when an unbalanced force is applied by the engine to change the direction of motion of the motorcar. We slip to one side of the seat due to the inertia of our body.

Question 4. Explain why it is dangerous to jump out of a moving bus.
Answer: While moving in a bus our body is in motion. On jumping out of a moving bus our feet touches the ground and come to rest. While the upper part of our body stays in motion and moves forward due to inertia of motion and hence we can fall in forward direction.
Hence, to avoid this we need to run forward in the direction of bus.

Question 5. Why do fielders pull their hand gradually with the moving ball while holding a catch?
Answer: While catching a. fast moving cricket ball, a fielder on the ground gradually pulls his hands backwards with the moving ball. This is done so that the fielder increases the time during which the high velocity of the moving ball decreases to zero. Thus, the acceleration of the ball is decreased and therefore the impact of catching the fast moving ball is reduced.

Question 6. In a high jump athletic event, why are athletes made to fall either on a cushioned bed or on a sand bed?
Answer: In a high jump athletic event, athletes are made to fall either on a cushioned bed or on a sand bed so as to increase the time of the athlete’s fall to stop after making the jump. This decreases the rate of change of momentum and hence the force.

Question 7. How does a karate player breaks a slab of ice with a single blow?
Answer: A karate player applied the blow with large velocity in a very short interval of time on the ice slab which therefore exerts large amount of force on it and suddenly breaks the ice slab.

Question 8. What is law of conservation of momentum?
Answer: Momentum of two bodies before collision is equal to the momentum after collision.
In an isolated system, the total momentum remain conserved.

Question 9. Why are roads on mountains inclined inwards at turns?
Answer: A vehicle moving on mountains is in the inertia of motion. At a sudden turn there is a tendency of vehicle to fall off the road due to sudden change in the line of motion hence the roads are inclined inwards so that the vehicle does not fall down the mountain.

Question 10. For an athletic races why do athletes have a special posture with their right foot resting on a solid supporter?
Answer: Athletes have to run the heats and they rest their foot on a solid supports before start so that during the start of the race the athlete pushes the support with lot of force and this support gives him equal and opposite push to start the race and get a good start to compete for the race.

Question 11.Why do you think it is necessary to fasten your seat belts while travelling in your vehicle?
Or
How are safety belts helpful in preventing any accidents?
Answer: While we are travelling in a moving car, our body remains in the state of rest with respect to the seat. But when driver applies sudden breaks or stops the car our body tends to continue in the same state of motion because of its inertia. Therefore, this sudden break may cause injury to us by impact or collision. Hence, safety belt exerts a force on our body to make the forward motion slower.

Question 12. Explain how momentum gets conserved in collision of two bodies.
Answer: Consider two bodies i.e., balls A and B, the mass and initial velocities are m_Au_A and m_Bu_Brespectively before collision. The two bodies collide and force is exerted by each body. There is change in their velocities due to collision.

∴ The total momentum of the two balls remains unchanged or conserved provided no other external force acts.

Question 13. When you kick a football it flies away but when you kick a stone you get huh why?
Answer: This is because stone is heavier than football and heavier objects offer larger inertia.
When we kick a football its mass is less and inertia is also less so force applied by our kick acts on it and hence it shows larger displacement but in case of stone, it has larger mass and offers larger inertia. When we kick (action) the stone it exerts an equal and opposite force (reaction) and hence it hurts the foot.

Question 14. If a person jumps from a height on a concrete surface he gets hurt. Explain.
Answer: When a person jumps from a height he is in state of inertia of motion. When he suddenly touches the ground he comes to rest in a very short time and hence the force exerted by the hard concrete surface on his body is very high, and the person gets hurt.

Question 15. What is the relation between Newton’s three laws of motion?
Answer: Newton’s first law explains about the unbalanced force required to bring change in the position of the body.
Second law states/explains about the amount of force required to produce a given acceleration.
And Newton’s third law explains how these forces acting on a body are interrelated.

Question 16. Give any three examples in daily life which are based on Newton’s third law of motion.
Answer: Three examples based on Newton’s third law are :

1. Swimming: We push the water backward to move forward.
   action – water is pushed behind
   reaction – water pushes the swimmer ahead
2. Firing gun: A bullet fired from a gun and the gun recoils.
   action – gun exerts force on the bullet
   reaction – bullet exerts an equal and opposite force on the gun
3. Launching of rocket
   action – hot gases from the rocket are released reaction – the gases exert upward push to the rocket

Question 17. A bullet of m.ass 20 g is horizontally fired with a velocity 150 m/s from a pistol of mass 2 kg. What is the
recoil velocity of the pistol?
Answer:

Question 18. Negative sign indicates that the direction in which the pistol would recoil is opposite to that of bullet.

Two bodies as shown in the figure collide with each other and join thereafter. With what velocity will they move after combining together?
Answer:

IV. Long Answer Type Questions
Question 1. Explain Newton’s second law of motion and with the-help of an example show how it is used in sports.
Answer: Newton’s second law of motion: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
Let us assume:
Object of mass m, is moving along a straight line with an initial velocity ‘u’, It is uniformly accelerated to velocity v in time ‘t by the application of force,

In cricket field, the fielder gradually pulls his hands backward while catching a ball. The fielder catches the ball and gives swing to his hand to increase the time during which the high velocity of the moving ball decreases to zero.
The acceleration of the ball is decreased and therefore the impact of catching the fast moving ball4s reduced.
If not done so, then the fast moving ball will exert large force and may hurt the fielder.

Question 2. State all 3 Newton’s law of motion. Explain inertia and momentum.
Answer:
Newton’s I law of motion: An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.
Newton’s II law of motion: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the-force.
Newton’s III law of motion: To every action, there is an equal and opposite reaction and they act on two different bodies.
Inertia: The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia.
Momentum: The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. Its S.I. unit is kgm/s. p = m x v

Question 3. Define force. Give its unit and define it. What are different types forces?
Answer: Force: It is a push or pull on an object that produces acceleration in the body on which it acts.
A force can do 3 things on a body
(a) It can change the speed of a body.
(b) It can change the direction of motion of a body.
(c) It can change the shape of the body.
The S.I. unit of force is Newton.
Newton: A force of one Newton produces an acceleration of 1 m/s² on an object of mass 1 kg.
1N = 1kg m/s²
Types of forces:

1. Balanced force: When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
2. Unbalanced force: When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced force.
3. Frictional force: The force that always opposes the motion of object is called force of friction.

Question 4. What is inertia? Explain different types of inertia. Give 3 examples in daily life which shows inertia.
Answer:
Inertia: The natural tendency of an object to resist change in their state of rest or of motion is called inertia.
The mass of an object is a measure of its inertia. Its S.I. unit is kg.
Types of inertia:
Inertia of rest: The object at rest will continue to remain at rest unless acted upon by an external unbalanced force.
Inertia of motion: The object in the state of uniform motion will continue to remain in motion with same speed and direction unless it is acted upon by an external unbalanced force. .
Three examples of inertia in daily life are:

1. When we are travelling in a vehicle and sudden brakes are .applied we tend to fall forward.
2. When we shake the branch of a tree vigorously, leaves fall down.
3. If we want to remove the dust from carpet we beat the carpet so that dust fall down.

V. Activity-based Questions
Question 1.

• Make a pile of similar carom coins on a table, as shown in the figure.
• Attempt a sharp horizontal hit at the bottom of the pile using another carom coin or striker. If the hit is strong enough the bottom coin moves out quickly. Once the lowest coin is removed, the inertia of the other coins makes them ‘fall’ vertically on the table.
  Inertia: It is the tendency of a body to maintain its state of rest or of motion.
  

Question 2.

• Set a five-rupees coin on a stiff card covering an empty glass tumbler standing on a table as shown in the figure.
• Give the card a sharp horizontal flick with a finger. If we do it fast then the card shoots away, allowing the coin to fall vertically into the glass tumbler due to its inertia.
• The inertia of the coin tries to maintain its state of rest even when the card flows off.
  

Answer: The force applied on the card due to flicking changes the inertia of the card but the coin resist a change and stay at the rest i.e. inertia of rest and due to gravity falls down in the tumbler.

Question 3.

• Place a water-filled tumbler on a tray.
• Hold the tray and turn around as fast as you can.
• We observe that the water spills. Why?

Answer: The water-filled in tumbler and tray are at rest. On moving/turning around the tray at faster speed the water spills because the tray and the tumbler comes into motion while the water in the tumbler remain at inertia of rest.

Question 4.

• Request two children to stand on two separate carts as shown on the next page.
• Give them a bag full of sand or some other heavy object. Ask them to play a game of catch with the bag.
• Does each of them receive an instantaneous reaction as a result of throwing the sand bag (action)?
• You can paint a white line on cartwheels to observe the motion of the two carts when the children throw the bag towards each other.

Answer. Yes, in this case each of them receives an instantaneous reaction as a result of throwing the sand bag.
This activity explain Newton’s III law of motion i.e., the force is exerted forward in throwing the bag full of sand and the person who is throwing it gets pushed backward.

Question 5.

• Take a big rubber balloon and inflate it fully. Tie its neck using a thread. Also using adhesive tape, fix a straw on the surface of this balloon.
• Pass a thread through the straw and hold one end of the thread in your hand or fix it on the wall.
• Ask your friend to hold the other end of the thread or fix it on a wall at some distance. The arrangement is shown in the figure below.
• Now remove the thread tied on the neck of balloon. Let the air escape from the mouth of the balloon.
• Observe the direction in which the straw moves.
  
  Observation:
  When the air escapes out from the balloon the straw moves in the opposite direction of the air moved out of the balloon.
  This activity explains the law of conservation of momentum and Newton’s III law of motion.
  Initial momentum = Final momentum

Question 6.

• Take a test tube of good quality glass material and put a small amount of water in it. Place a stop cork at the mouth of it.
• Now suspend the test tube horizontally by two strings or wires as shown in the figure on next page.
• Heat the test tube with a burner until water vaporises and the cork blows out.
• Observe that the test tube recoils in the direction opposite to the direction of the cork.
  
  Observation:
  The cork is pushed out in forward direction by the hot steam. The test tube is pushed in the backward direction.
  It explain Newton’s III law of motion and conservation of momentum.

VI. Value-based Questions
Question 1. Class V students were playing cricket with the cork hall in the school campus. Charu a senior student told them about the accidents that can occur due to cork ball in the campus and also advised them to bring soft cosco ball to play the game.
(a) Why it was safe to play with soft ball and not with hard cork ball?
(b) A player pulls his hands backwards after holding the ball shot at high speed. Why?
(c) What value of Charu is seen in this act?
Answer:
(a) The soft ball will have less inertia as compared to the heavy ball and it would not hurt the players.
(b) By pulling the hand backwards it reduces the force exerted by the ball on hands.
(c) Charu showed the value of being responsible and helpful by nature.

Question 2. Saksham saw his karate expert friend breaking a slate. He tried to break the slate but Saksham’s friend stopped him from doing so and told him that it would hurt, one needs lot of practice in doing so.
(a) How can a karate expert break the slate without any injury to his hand?
(b) What is Newton’s third law of motion?
(c) What value of Saksham’s friend, is seen in the above case?
Answer:
(a) A karate player applies the blow with large velocity in a very short interval of time on the slate, therefore large force is exerted on the slate and it breaks.
(b) To every action there is an equal and opposite reaction, both act on different bodies.
Saksham’s friend showed the value of being responsible and caring friend.