# Force

## Physics

## Chapter - 3: Force

## ðŸ‘‰Â All Chapter ContentsÂ ðŸ‘ˆÂ

- 3.1 Inertia of Concept of Force - Newton's First Law
- 3.2 Nature of Fundamental Forces
- 3.3 Balanced and Unbalanced Forces
- 3.4 Momentum
- 3.5 Collision
- 3.6 Effect of Force on Motion: Newton's Second Law
- 3.7 Gravitational Force
- 3.8 Newton's Third Law
- 3.9 Frictional Force

**Write down the dimension of momentum?**

The dimension of momentum is MLT-1.

**What is the unit of force?**

The unit of force is newton(N).

**What is called weight?**

The amount of force that a body feels because of the attraction of the earth, that is, the amount of gravity felt by a body is called weight.

**What is the value of gravitational constant?**

The value of gravitational constant is, G = 6.673*10-11 Nm2kg-2.

**What is called the attraction force between earth and the apple?**

The attraction force between the earth and the apple is called gravitational force.

**What is the name of attraction between the earth and the moon?**

The attraction between the earth and the moon is called gravitational force.

**State Newton's first law of motion.**

Netwon's firt law of motion is -

"Every object will contine in its state of rest or of uniform motion in a straight line unless an external force is applied to it."

**Write down Newton's second law of motion.**

Newton's second law of motion is -

"The rate of change of motion of a body is proportional to the applied force acting on it and takes place in the direction in which the force acts."

**State Newton's third law of motion.**

Newton's third law of motion is -

"To every action there is an equal and opposite reaction."

**Why is momentum a vector quantity?**

Momentum is a vector quantity, because velocity is a vector though mass is a scalar quantity. So product of mass and velocity will be a vector quantity.

**What is the unit of momentum?**

The unit of momentum is kgms-1.

**What does Newton's second law give?**

Newton's second law gives the equation for the measurement of force.

**Define unit of force?**

The unit of force is Newton (N). 1 N is defined as the force that will produce an acceleration of 1 ms-2 when it acts on a mass of 1 kg.

**What do you understand by a force of 50 N?**

A force of 50 N means that force which acting on a body of mass 50 kg produces 1 ms-2 acceleration.

**What law of motion can be derived from Newton's second law of motion?**

From Newton's second law of motion, first law of motion can be derived.

**What do you understand by the momentumÂ of a body is 500 kgms-1?**

The momentum of of a body is 500 kgms-1. It means that if a body of mass 500 kg moves with a velocity of 1 ms-2, then the momentum will be 500 kgms-1.

**How many types of friction are there? Name them.**

There are four types of friciton-

1) Static friction

2) Sliding friction

3) Rolling friction

4) Fluid friction

**What are called lubricants?**

Oil, mobil and grease like materials are called lubricants or greasy materials.

**What is collision?**

When a moving object hits another body at rest or in motion, then it is said that a collision has taken place.

**What is the dimension of force?**

The dimentions of force is MLT-2.

**What is the inertia of a body?**

The tendency of a body to maintain its own state forever in which the body is at present or the property of a body to preserve that state is defined as inertia. For example, If we put a book on a table the book will be there forever if nobody displaces it.

**Measure of inertia:** Mass is the measurement of its inertia.

**How can the definition of force be found from Newton's first law of motion? Explain it.**

The first law of Newton: "Every object will continue in its state of rest or of uniform motion in a straight line unless an external force is applied to it."

From Newton's first law of motion, we observe that a body cannot change its state on its own. If the body is at rest, it tends to remain at rest forever and if it is in motion it tends to keep on motion with uniform speed for all time. To change the state of a body something external must be applied. That is, the external cause which changes or tends to change the state of an object is called force. Thus from Newton's first law, we get qualitative definition of force.

**Explain the law of conservation of momentum.**

Law of conservation of momentum: "If no external force is applied on two or more bodies except their action and reaction force then the sum of momentum of the bodies remains same."

**Explanation: **When a cannon-ball is short from a cannon, the cannon gives a backward push. Before the cannon-ball was shot, the momentum of cannon and cannon-ball was zero because both were at rest. After the burst, the cannon-ball moves forward with a particular momentum and the cannon moves backward with a momentum of same magnitude but opposite in direction. As a result, the momentum remains same and reaches zero after the burst.

**Explain the backward motion of gun.**

When a gun is fired it gives a backward push, this is called backward motion of gun. When a person fires with a gun he feels a backward push at that time. Before the gun shot, the momentum of the bullet and the gun is zero, because both are in a rest state. But after the gun shot, action and reaction force of the bullet and the gun exists for the same time. According to Newton's second law of motion, the bullet and the gun acquire equal but opposite momentum. As a result the bullet moves forward with a particular momentum and the gun moves backward with a momentum of same magnitude but opposite in direction. Due to this, the gun gives a backward thrust. Which is called backward motion of gun.

**Explain the reason of bodies falling from high places towards earth.**

If a body is dropped from a certain height, if falls on earth due to the influence of gravity. When a body is falling, only gravity, that is, the attraction of earth acts on the body. So the reason behind falling bodies is gravity.

**Explain the attraction force between an apple and earth according to Newton.**

According to Newton, the attraction force between an apple and earth depends on the mass of apple and earth and the distance between them. According to Newton's law of gravitation, the attraction force between apple and earth is proportional to the product of their mass; and proportional to the square of their distance; and it acts on the straight line connecting their centres.

**If we kick a football, it stops after travelling some distance - Explain why.**

When we kick a football, it acquires motion because we exert force on it. It advances in a straight line with a definite velocity. The ball will move as long as it has velocity. After that it will stop. Because of this velocity, the football moves forward for some times and then stops when the velocity is no more.

**Explain - the causes of attraction between the earth and the moon.**

Every particle of this universe is attracted toward each other. The force of attraction between any two bodies or particles in this universe is called 'Gravitation'. If earth is one of the two bodies, then the force of attraction is called gravity, that is, the attraction of the earth on any other body is called gravity. So the causes of attraction between the earth and the moon is gravity.

Question Bank

## 3.1Â Inertia of Concept of Force - Newton's First Law

- State Newton's first law.
- Explain why we never observe that a body in motion keeps its perpetual motion forever?
- What does inertia mean?
- What is inertia of rest?
- What is inertia of motion?
- Mass is the measurement of inertia? Explain.
- What is force?
- What is contact force?
- What is a non-contact force?
- Explain that the concept of inertia is found from Newton's first law.
- Which way does passenger lean when the moving bus suddenly breaks and why?

## 3.2 Nature of Fundamental Forces

- What is fundamental force?
- How many fundamental forces in nature and what are they?
- What do you mean by gravity?
- What do you mean by electromagnetic force?
- What do you mean by weak nuclear force?
- What do you mean by strong nuclear force?
- Discuss the comparative intensity of the fundamental forces.

## 3.3 Balanced and Unbalanced Forces

- What do you mean by balanced force?
- What do you mean by unbalanced force?

## 3.4 Momentum

- What is momentum?
- Determine the unit and dimension of momentum.
- What do you understand by momentum 200 kgm/s?
- Write the law of conservation of momentum.

## 3.5 Collision

- Write the law of conservation of momentum?
- Prove the law of conservation of momentum.
- Write the law of conservation of energy?
- Show that before the collision and after the collision total momentum is equal.
- Explain the necessity of force and motion in the case of safe journey.

## 3.6 Effect of Force on Motion: Newton's Second Law

- Write the second law of Newton.
- Prove that, F = ma.
- Write the unit and dimension of force.

## 3.7 Gravitational Force

- What is gravitational force?
- What is the value of the gravitational constant 'G'?

## 3.8 Newton's Third Law

- Write Newton's third law.
- Explain Newton's third law.

## 3.9 Frictional Force

- What is friction?
- What is frictional force?
- Explain the origin of friction.
- How many types of friction and what are they?
- What is static friction?
- What is sliding friction?
- What is rolling friction?
- What is fluid friction?
- Explain the effect of friction on the motion?
- Why there is a groove on the surface of the car tire?
- Explain how friction can be reduced?
- Explain how friction can be increased?
- Explain why friction is a necessary evil?

Secondary Physics

- Chapter - 1 :Â Physical Quantities and Their Measurements
- Chapter - 2 :Â Motion
- Chapter - 3 :Â Force
- Chapter - 4 :Â Work, Power, and Energy
- Chapter - 5 :Â State of Matter and Pressure
- Chapter - 6 :Â Effect of Heat on Matter
- Chapter - 7 :Â Waves and Sound
- Chapter - 8 :Â Reflection of Light
- Chapter - 9 :Â Refraction of Light
- Chapter - 10 :Â Static Electricity
- Chapter - 11 :Â Current Electricity
- Chapter - 12 :Â Magnetic Effects of Current
- Chapter - 13 :Â Modern Physics and Electronics
- Chapter - 14 :Â Physics to Save LifeÂ

## Mathematical Problems

- What is the force to be applied to a mass of 50 kg to produce an acceleration of 4 ms
^{-2}? [Ans: 200 N]

- A boy pushes a box of mass 20 kg with a force of 50 N. What will be the acceleration of the box? [Ans: 2.5 ms
^{-2}]

- A force of 2000 N acts on a body of mass 20 kg for a time of 0.1 s. What is the change of momentum of the body? [Ans: 200 kgms
^{-1}]

- A bullet of mass 10 g was shot from a gun with a velocity of 500 ms-1. If the mass of the gun is 2 kg, find the backward velocity of the gun. [Ans: 2.5 ms
^{-1}]