IX- Physics - Laws of Motion - All Theory Questions

Use Newton's second law of motion to explain the following instances: (a) You pull your hands back while catching a fast moving cricket ball.

(a) We pull our hands back while catching a fast moving cricket ball, because by doing so, we increase the time of catch, i.e. increase the time to bring about a given change in momentum, and hence, the rate of change of momentum decreases. Thus, a small force is exerted on our hands by the ball.

Explain the following: (a) When a train suddenly moves forward, the passenger standing in the compartment tends to fall backwards.

(a) When a train suddenly starts, the passengers tend to fall backwards. This is because the lower part of the body, which is in contact with the train, begins to move while the upper part of the body tends to maintain its position of rest. As a result, the upper part tends to fall backwards.

Explain the following: (b) When a train suddenly starts, the sliding doors of some compartments may open.

(b) The frame of the sliding door being in contact with the floor of the train also comes in motion on start of the train, but the sliding door remains in its position due to inertia. Thus, the frame moves ahead with the train, while the door slides opposite to the direction of motion of the train. Thus, the door may shut or open accordingly. .

Use Newton's second law of motion to explain the following instances: (b) You prefer to land on sand instead of hard floor while taking a high jump.

(b) When an athlete lands from a height on a hard floor, his feet comes to rest instantaneously, so a very large force is exerted by the floor on his feet, but if he lands on sand, his feet push the sand for some distance; therefore, the time duration in which his feet comes to rest increases. As a result, the force exerted on his feet decreases and he is saved from getting hurt.

Explain the following: (c) People often shake branches of a tree in an attempt to cause the fruits to fall.

(c) When the branches of the tree are shaken, they come in motion, while the fruits due to inertia remain in a state of rest. Thus, the larger and weakly attached fruits get detached from the branches and fall down due to the pull of gravity.

Explain the following: (d) After alighting from a moving bus, one has to run for some distance in the direction of bus in order to avoid falling.

(d) When people alight from a moving bus, they continue to run alongside the bus to avoid falling. If they were to stop at once, the feet would come to rest suddenly but the upper part of the body would still be in motion and they would tend to fall forward.

Name and state the action and reaction in the following cases: (e) A person moving on the floor,

(e) Action: Force exerted by the feet on the ground in backward direction. Reaction: Force exerted by the ground on feet in forward direction.

Explain the following: (e) Dust particles are removed from a carpet by beating it.

(e) The part of the carpet where the stick strikes comes in motion at once, while the dust particles settled on it remain in the state of rest due to inertia of rest. Thus, a part of the carpet moves ahead with the stick leaving behind the dust particles that fall down due to gravity.

Name and state the action and reaction in the following cases: (f) A moving train colliding with a stationary train.

(f) Action: Force exerted by a moving train on a stationary train. Reaction: Force exerted by a stationary train on a moving train.

Explain the following: (f) It is advantageous to run before taking a long jump.

(f) When running, the athlete brings his body in the state of motion. When the body is in motion, it becomes easier to take a long jump.

The force of attraction between two masses each of 1 kg kept at a separation of 1 m is which one of the values shown below: (a) 9.8 N (b) 6.7 N (c) 980 N (d) 6.7 x 10-11 N

6.7 x 10-11 N

The value of G is one of the values listed below: (a) 9.8 N m2 kg-2 (b) 6.7 x 10-11 N m2 kg-2 (c) 6.7 x 10-11 m s-2 (d) 6.7 N kg-1

6.7 x 10-11 N m2 kg-2

Give one example in each of the following cases where a force: i. Stops a moving body. ii. Moves a stationary body. iii. Changes the size of a body. iv. Changes the shape of a body.

A fielder on the ground stops a moving ball by applying a force with his hands. The pull exerted by horse makes a cart moves. In a cycle pump, when the piston is lowered, the air is compressed to occupy a less volume. On pressing a piece of rubber, its shape changes.

State the effects of a force applied on i. A non-rigid, and ii. A rigid body. How does the effect of the force differ in the two cases?

A force when applied on a non-rigid body changes the inter-spacing between its constituent particles and therefore causes a change in its dimensions and can also produce motion in it. On the other hand, a force when applied on a rigid body, does not change the inter-spacing between its constituent particles and therefore it does not change the dimensions of the body but causes motion in it

A person pushing a wall hard is liable to fall backwards. Give reason.

A person pushing a wall hard (action) by his palm, experiences a force (reaction) exerted by the wall on his palm in the opposite direction; thus, he is liable to fall backwards.

The property of inertia is more in the following vehicle: (a) A car (b) A truck (c) A horse cart (d) A toy car.

A truck

How are g and G related?

Acceleration due to gravity (g) is directly proportional to universal gravitational constant (G).

State Newton's first law of motion.

According to Newton's first law of motion, if a body is in a state of rest, it will remain in the state of rest, and if the body is in the state of motion, it will remain moving in the same direction with the same speed unless an external force is applied on it.

State the Newton's second law of motion. What information do you get from it?

According to Newton's second law of motion, the rate of change of momentum is directly proportional to the force applied on it and the change of momentum takes place in the direction in which the force is applied. It gives the quantitative value of force, i.e. it relates the force to the measurable quantities such as acceleration and mass.

State Newton's second law of motion. Under what condition does it take the form F = ma?

According to Newton's second law of motion, the rate of change of momentum is directly proportional to the force applied on it, and the change of momentum takes place in the direction in which the force is applied. The relation F=ma holds for the following conditions: (i) When the velocity of the body is much smaller than the velocity of light. (ii) When the mass remains constant.

State Newton's third law of motion.

According to Newton's third law of motion, to every action there is always an equal and opposite reaction. The action and reaction act simultaneously on two different bodies.

Action and reaction are equal in magnitude'. Is this statement true?

Action and reaction are equal in magnitude'. Is this statement true?

The gravitational force between two bodies has the following property: (a) Always repulsive (b) Always attractive (c) Attractive only at large distances (d) Repulsive only at large distances.

Always attractive

A force is needed to do the following actions: (a) Change the state of motion or state of rest of the body. (b) Maintain the body in motion. (c) Maintain the body at rest. (d) Maintain constant velocity of the body.

Change the state of motion or state of rest of the body.

Classify the following amongst contact and non - contact forces: a. Frictional force b. Normal reaction force c. Force of tension in a string d. Gravitational force e. Electrostatic force f. Magnetic force

Contact force: (a) frictional force (b) normal reaction force (c) force of tension in a string Non-contact force: (d) gravitational force (e) electric force (f) magnetic force

Action and reaction act on: (a) Same body in opposite directions. (b) Different bodies in opposite directions. (c) Different bodies, but in the same direction. (d) Same body in the same direction.

Different bodies in opposite directions

Give one example of each of the following cases of inertia: (b) Inertia of motion.

Example of inertia of motion: A ball thrown vertically upwards by a person in a moving train comes back to his hand.

Give one example of each of the following cases of inertia: (a) Inertia of rest

Examples of inertia of rest: A coin placed on top of a card remains in place when the card is slightly and quickly jerked horizontally.

Give two examples to show that greater the mass, greater is the inertia of the body.

Examples to show that greater the mass, greater is the inertia of the body are as shown below: (i) If you want to start a car by pushing it, you find that it takes a very large force to overcome its inertia. On the other hand, only a small force is needed to start a child's express wagon. The difference between the car and express wagon is the difference in mass. The car has a large mass, whereas the wagon has a small one. (ii) A cricket ball is more massive than a tennis ball. The cricket ball acquires a much smaller velocity than a tennis ball when the two balls are pushed with equal force for the same time.

Newton's third law does which of the following actions: (a) Defines the force qualitatively. (b) Defines the force quantitatively. (c) Explains the way the force acts on a body. (d) Gives the direction of force.

Explains the way the force acts on a body.

Explain the law of inertia (or Newton's first law of motion)

Explanation: Newton's first law can be explained in the following two parts: (i) Definition of inertia: The 1st part of Newton's first law of motion gives the definition of inertia, according to which an object cannot change its state by itself. Example: A book lying on a table will remain in its position unless it is displaced. (ii) Definition of force: The 2nd part of Newton's first law defines force, according to which force is that external cause which can move a stationary object or which can stop a moving object. Example: A book lying on a table is displaced from its place when it is pushed.

Name the physical quantity that causes motion in a body.

Force causes motion in a body.

The non - contact force is : a. Force of reaction b. Force due to gravity c. Tension in string d. Force of friction

Force due to gravity is a non-contact force.

Is force needed to keep a moving body in motion?

Force is not needed to keep a moving body in motion.

Name the two factors on which the force needed to stop a moving body in a given time depends.

Force needed to stop a moving body in a given time depends on its mass and velocity.

Which of the following is a contact force: a. Electrostatic force b. Gravitational force c. Frictional force d. Magnetic force

Frictional force is a contact force.

What is Galileo's law of inertia?

Galileo's law of inertia states that a body continues to be in its state of rest or of uniform motion unless an external force is applied on it.

Give the qualitative definition of force based on Newton's first law of motion.

Give the qualitative definition of force based on Newton's first law of motion.

State whether the gravitational force between two masses is attractive or repulsive?

Gravitational force is always attractive.

Why does a person tend to fall when he jumps out from a moving train and tries to stop immediately?

If a person jumps out of a moving train and tries to stop immediately, he falls due to inertia of motion. This is because his body tends to move forward with the velocity of the train while his feet are stationary.

A ball is moving on a perfectly smooth horizontal surface. If no force is applied on it, then will its speed decrease, increase or remain unchanged?

In absence of any external force, its speed shall remain unchanged.

Name the factor on which the inertia of a body depends and state how does it depend on the factor stated above.

Inertia of a body depends on its mass. Inertia is directly proportional to mass, i.e. greater the mass of a body, greater is its inertia.

More the mass, the more difficult it is to move the body from rest'. Explain this statement by giving an example.

It is difficult, i.e. a larger force is required to set a loaded trolley (which has more mass) in motion than an unloaded trolley (which has less mass).

Define linear momentum and state its S.I. unit.

Linear momentum of a body is the product of its mass and velocity. Its SI unit is kgms-1.

Distinguish between mass and weight.

Mass is a scalar quantity, but weight is a vector quantity. Mass is the measure of the quantity of matter contained in a body, but weight is the measure of force with which the Earth attracts the body. Mass of a body is always constant but weight varies from place to place.

Which of the quantity is constant for a body: mass or weight?

Mass of a body is always constant.

The acceleration produced in a body by a force of given magnitude depends on the following factor: (a) Size of the body (b) Shape of the body (c) Mass of the body (d) None of these.

Mass of the body

Define the terms mass and weight.

Mass: The mass of a body is the quantity of matter it contains. Weight: The weight of a body is the force with which the Earth attracts it.

Write the mathematical form of Newton's second law of motion. State the conditions if any

Mathematical expression of Newton's second law of motion is as shown below: Force = Mass × Acceleration Above relation holds for the following conditions: (i) When the velocity of the body is much smaller than the velocity of light. (ii) When the mass remains constant.

The linear momentum of a body of mass m moving with velocity v is given by the following relation: (a) v/m (b) m/v (c) mv (d) 1/mv

Mv

The unit of linear momentum is : (a) N s (b) kg m s-2 (c) N s-1 (d) kg2 m s-1

N s

How does Newton's second law of motion differ from the first law of motion?

Newton's first law of motion gives the qualitative definition of force. It explains the force as the cause of acceleration only qualitatively but Newton's second law of motion gives the quantitative value of force. It states force as the product of mass and acceleration. Thus, it relates force to measurable quantities such as acceleration and mass.

State Newton's law of gravitation.

Newton's law of gravitation: Every particle in the universe attracts every other particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them, and the direction of the force is along the line joining the masses.

State the purpose of Newton's third law of motion.

Newton's third law explains how a force acts on an object.

Two equal and opposite forces act on a stationary body. Will the body move? Give a reason to your answer.

No, the body will not move because the net force acting on it is zero. Hence, it will remain stationary due to inertia of rest.

Name and state the action and reaction in the following cases: (a) Firing a bullet from a gun,

Reaction: Recoil experienced by the gun.

State the law of inertia (or Newton's first law of motion)

Statement of Newton's first law: If a body is in a state of rest, it will remain in the state of rest, and if the body is in the state of motion, it will remain moving in the same direction with the same speed unless an external force is applied on it.

What is the C.G.S. unit of force? How is it defined?

The C.G.S. unit of force is dyne. One dyne is the force which acts on a body of mass 1 gramme and produces an acceleration of 1 cms-2, i.e. 1 dyne = 1 g × 1 cms-2.

Name the S.I. and C.G.S. units of force. How are they related?

The S.I. unit of force is newton and the C.G.S. unit of force is dyne. 1 N = 105 dyne.

Name the S.I. unit of force and define it.

The S.I. unit of force is newton. One newton is the force which acts on a body of mass 1kg and produces an acceleration of 1 m/s2, i.e. 1 N = 1 kg × 1 m/s2.

State the S.I. units of (a) mass and (b) weight.

The S.I. unit of mass is kg and that of weight is newton.

Write down the average value of g on Earth's surface.

The average value of 'g' on the Earth's surface is 9.8 m/s2.

A ball moving on a table eventually stops. Explain the reason.

The force of friction between the table and the ball opposes the motion of the ball

What do you understand by the term force due to gravity?

The force with which the Earth attracts a body towards its centre is called the force due to gravity.

Comment on the statement 'the sum of action and reaction on a body is zero'.

The given statement is wrong. Reason: According to Newton's third law of motion, the action and reaction act simultaneously on different bodies. Hence they do not cancel each other.

Define gravitational constant G.

The gravitational constant is defined as the force of attraction between two bodies of unit mass separated by a unit distance.

How does the gravitational force of attraction between two masses depend on the distance between them?

The gravitational force of attraction between two masses is inversely proportional to the square of distance between them.

What is the importance of law of gravitation?

The gravitational force of attraction is significant to explain the motion of heavenly bodies, e.g. motion of planets around the Sun, motion of the Moon around the Earth etc.

The separation between two masses is reduced to half. How is the magnitude of gravitational force between them affected?

The magnitude of gravitational force between two masses will become four times as gravitational force varies inversely as the square of distance of separation.

State one factor on which the magnitude of a non-contact force depends. How does it depend on the factor stated by you?

The magnitude of non-contact force on two bodies depends on the distance of separation between them. The force decreases as the distance of separation increases. The force is inversely proportional to the square of the distance of separation.

Two equal and opposite forces act on a moving object. How is its motion affected? Give reason.

The motion remains unaffected because the net force acting on it is zero.

An aeroplane is moving uniformly at a constant height under the action of two forces (i) Upward force (lift) and (ii) Downward force (weight). What is the net force on the aeroplane?

The net force on the airplane is zero or the upward force is equal to the downward force.

What is meant by the term inertia?

The property of an object by virtue of which it neither changes its state nor tends to change the state is called inertia.

Define the term acceleration due to gravity? Write its S.I. unit.

The rate at which the velocity of a freely falling body increases is called acceleration due to gravity. Its S.I. unit is m/s2.

When a ball is thrown vertically upwards from a moving train it comes back to the thrower's hand. Why does this happen?

The reason is that when the ball is thrown, the ball is in motion along with the person and train. Due to the inertia of motion, during the time the ball remains in air, the person and ball move ahead by the same distance. This makes the ball fall back into the thrower's hand.

Why does a coin, placed on a card, drop into the tumbler when the card is rapidly flicked with a finger?

The reason is that when the card is flicked, a momentary force acts on the card, so it moves away. However, the coin kept on it does not share the motion at once and it remains stationary at its place due to the inertia of rest. The coin then falls down into the tumbler due to the pull of gravity.

Write the numerical value of gravitational constant G with its S.I. unit.

The value of G in the S.I. system is 6.67 x 10-11Nm2kg-2.

To move a boat ahead in water, the boatman has to push the water backwards by his oar. Explain this statement.

To move a boat, the boatman pushes (action) the water backwards with his oar. In this response, the water exerts an equal and opposite force (reaction) in the forward direction on the boat due to which the boat moves ahead.

Name the two kinds of inertia.

Two kinds of inertia are as listed below: (i) Inertia of rest. (ii) Inertia of motion.

A light ball falling on ground, after striking the ground rises upwards. Explain the reason.

When a falling ball strikes the ground, it exerts a force on the ground. The ground exerts a force back at the ball in the opposite direction. This is the reason the ball rises upwards.

Why does a glass vessel break when it falls on a hard floor, but it does not break when it falls on a carpet?

When a glass vessel falls from a height on a hard floor, it comes to rest almost instantaneously, i.e. in a very short time, so the floor exerts a large force on the vessel and it breaks. However, if it falls on a carpet, then the time duration, in which the vessel comes to rest, increases, so the carpet exerts less force on the vessel and it does not break.

When you step ashore from a stationary boat, it tends to leave the shore. Explain.

When a man exerts a force (action) on the boat by stepping into it, its force of reaction makes him step out of the boat, and the boat tends to leave the shore due to the force exerted by the man (i.e. action).

Explain the motion of a rocket with the help of Newton's third law.

When a rocket moves in space, it pushes gases outside, i.e. the rocket applies force on the gases in the backward direction. As a reaction, the gases put equal amount of force on the rocket in the opposite direction and the rocket moves in the forward direction.

The action and reaction both act simultaneously. Is this statement true?

Yes, action and reaction act simultaneously.

Give one example in each case where : a. The force is of contact, and b. Force is at a distance

a. Force exerted on two bodies during collision. b. Magnetic force between magnetic poles.

Explain giving two examples each of : a. Contact forces b. Non - contact forces

a. The forces which act on bodies when they are in physical contact are called contact forces. Example: frictional force and force exerted on two bodies during collision. b. The forces experienced by bodies even without being physically touched are called non-contact forces. Example: Gravitational force and Electrostatic force.

Name and state the action and reaction in the following cases: ((b) Hammering a nail,

b) Action: The force exerted by the hammer on the nail. Reaction: The force applied by the nail on the hammer.

Name and state the action and reaction in the following cases: (c) A book lying on a table,

c) Action: Weight of the book acting downwards. Reaction: Force acted by the table upwards.

Name and state the action and reaction in the following cases: (d) A moving rocket,

d) Action: Force exerted by the rocket on the gases backwards. Reaction: Force exerted by outgoing gases on the rocket in forward direction.