Content Writer-SME | Updated On - Oct 4, 2024
A pull or push on an object that changes the original state of the object is called force. In simple words, the definition of force in class 9 is given as “Anything that can change the state of rest or motion of a body is called force”. It is a vector quantity which means a physical quantity which has both magnitude and direction.
The laws of force and motion are related to each other because the famous scientist Sir Isaac Newton gave three laws while studying the concepts of force. Newton's laws of motion in physics give us three rules that can be applied in our real life routine. Here in this article, we will understand Newton's concept of force and laws of motion with the help of real life examples.
What is Force?
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The concept of force and the laws of motion may seem self-evident today, yet it was nothing short of revolutionary centuries ago. The three laws of motion explain how objects behave when they are stationary, moving, or subjected to forces. In terms of change in velocity, the definition of force in class 9 is given as:
| “Force is defined as the push or pull on an object with mass that causes it to change its velocity” |
In general, push and pull caused by objects interacting with one another is considered as force. Push is described as an action of force that causes an object to move from its initial position. Pushing the table, Pushing thumb pins, Walking, etc. are examples of Pushing.
The pull is described as an act that makes anything move by dragging. Plucking the string of a guitar, Opening the drawer, Pulling ropes while playing tug of war, etc. are examples of Pull.- Force is a vector quantity which is represented by F in physics.
- The SI and C.G.S unit of force are Newton and Dyne.
- The dimensional formula of force is [M L T-2].
- 1 Newton is equivalent to 1 kg m/s2.
What is 1 Newton of Force?
| 1 Newton is defined as the force required to accelerate a body of mass 1 kg with acceleration 1 m/s2. |
Types of Force
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Based on laws of motion, different type of forces are mentioned in the NCERT Class 9 Science Book which are as follows:
Contact Force
Contact forces are forces that act on a body directly or indirectly through a medium. The following are some examples of contact forces:
- Muscle Power
- Mechanical Force
- Force of Friction
- Tension Force
- Viscous Drag
Non-Contact Force
Non-contact forces are those that work across spaces without making direct touch with the body. The following are some examples of non-contact forces:
- Gravitation Force
- Electrostatic Force
- Magnetic Force
Balanced and Unbalanced Forces
When balanced forces are applied to a body, there is no net effective force acting upon it. Balanced forces do not cause any changes in motion.Unbalanced forces exerted on a body change its velocity and/or direction of motion. It moves in the direction of the force with the highest magnitude.
Net Force
When many different forces act on a body, they can be resolved into a single component called the net force acting on the body. The net force determines the direction of motion.Weight
Weight is the gravitational force with which the earth pulls an object. It is given by
Weight (W) = mg
Where
- m is the mass of the body
- g is the acceleration due to gravity
Normal Reaction
Normal reaction is a force between the surfaces in contact.
- This force is normal to the surfaces in contact.
- The normal reaction occurs to stop two bodies in contact to merge into each other.
Law of Inertia
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According to the Law of Inertia, “Inertia is an inherent property of a body which has the inability to change its state of rest or uniform motion or direction of motion by itself.”
- Mass is the measure of inertia.
- More is mass more is inertia.
There are three types of Inertia
- The inertia of Rest: It states that if a body is at rest, it will remain at rest.
- The inertia of Motion: It states that if a body is moving with constant velocity, it will keep moving with that velocity.
- The inertia of Direction: It states that if a body moves in a particular direction, will keep moving in that direction and can't change its direction by itself.
Newton’s First Law of Motion
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| “Newton’s first law of motion states that a body at rest or uniform motion will continue to be at rest or uniform motion until and unless a net external force acts on it”. |
The important aspect to remember is that if there is no net force exerted on an item due to imbalanced forces, the object will retain its velocity.
- If the velocity is zero, the item is said to be at rest.
- If an external force is applied, the velocity will change as a result of the force.
- The first law of motion is also known as the Law of Inertia.
Linear Momentum
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Linear momentum is defined as the quantity of motion contained in a body.
- It is measured as the product of the mass of the body and its velocity.
- The SI unit of Momentum is kg m/s.
- It is a vector quantity and its direction is the same as that of the velocity.
Linear momentum is given by the formula
P = MV
Where
- P is the linear momentum of the body
- M is the mass of the body
- V is the velocity of the body
Newton’s Second Law of Motion
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Newton's second law of motion deals with the relationship between force and acceleration.
According to the second law of motion,
| “The rate of change of linear momentum is directly proportional to the applied force and it takes place in the direction in which the force acts”. |
Assume a body of mass m is traveling in a straight line with an initial velocity of u. It is uniformly accelerated to velocity, v, in time, t, by applying a constant force, F, over the entire duration, t.
Initial momentum, p1 = mu
Final momentum, p2 = mv
Now, a change in momentum can be expressed by Newton's second law of motion as:
F ∝ dP/dt
⇒ F ∝ (mv−mu)/t
⇒ F ∝ {m(v−u)}/t
⇒ F ∝ ma
⇒ F = k ma
Substituting k = 1, we get the formula of Newton’s second law of motion
F = ma
Where
- m is the mass of the body
- a is the acceleration of the body
Newton’s Third Law of Motion
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Newton's third law of motion states that
| “To every action, there is an equal and opposite reaction”. |
Whenever two bodies interact, the two forces that they exert on each other are always equal in magnitude and opposite in direction. This means that all-natural forces behave in pairs.
- The magnitude of these two forces is always equal, but their direction is always opposing.
- These forces are always applied to separate objects and never to the same one.
Effects of Force and Laws of Motion
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When a force is applied to an object, it affects its state of motion or rest. The velocity of the body and its direction can be changed by applying force. Furthermore, some more effects of force and laws of motion when applied on any object are as follows:
- Change in Motion:
When force is applied to an object, its speed and direction change. This results in a change in velocity and as the velocity changes, the overall motion of the object also changes. When the ball is bowled by the bowler in a particular direction (towards the batsman), it changes its path and direction when it comes in contact with the bat. This is a real life example of the application of force and change in motion.
- Change in Linear Momentum:
Change in velocity is associated with changes in various physical quantities. One such is linear motion. According to the formula of linear momentum (P=MV), there is a direct relationship between momentum and velocity. Thus, with changes in the speed and direction of the object, the linear momentum also changes. Applying brakes to the car or cycle is a real life example of change in linear momentum.
- Change in Appearance of Object:
Force has the ability to change the shape of an object but it depends on how strong the applied force is. When too much force is applied on an object, its shape changes and it also breaks. This can be best understood with another real life example. Take an elastic rubber band and pull it firmly from both ends, it will eventually change its shape.
Things to Remember
- A push or pull on an object causes acceleration in the body on which it is acting.
- Force is a vector quantity and its SI unit is Newton (N).
- The first law of motion is also known as the law of inertia.
- The second law of motion explain the term linear momentum in physics.
- The third law of motion states that, "For every action there is an equal and opposite force".
Sample Questions
Ques. Define Force. (1 Mark)
Ans. Force is a physical quantity that causes motion or opposes motion or can change the direction of the motion of a body.
Ques. Is force a scalar or a vector quantity? (1 Mark)
Ans. Force is a vector quantity because it has magnitude and direction.
Ques. What is the law of conservation of momentum? (2 Marks)
Ans. The law of conservation of momentum states that in an isolated system, the total momentum of two or more bodies acting on each other remains constant until an external force is applied.
Ques. How many laws of motion are there? (1 Mark)
Ans. There are three laws of motion. These
- Newton’s first law of motion
- Newton’s second law of motion
- Newton’s third law of motion
Ques. What are Newton's laws of motion? (2 Marks)
Ans. The relationship between an object's motion and the forces operating on it is explained by Newton's laws of motion. We know from the first law that until a force acts on an item, it will not affect its velocity. According to the second law, the force on an object is equal to its mass times its acceleration. Finally, the third law argues that every action has an equal and opposite reaction.
Ques. When a moving bus abruptly stops, why does a person sitting in the bus tumble forward? (2 Marks)
Ans. This occurs due to the inertia of his upper body's motion. When a moving bus comes to a stop, the bottom part of the person's body comes to rest with the bus, while the upper section of the person's body continues moving due to inertia.
Ques. Prove that F = ma by defining momentum. (2 Marks)
Ans. From Newton’s Second law of motion, we have F = dp/dt
But, p = mv ⇒ F = d(mv)/dt
Since mass m is constant, therefore we can write
⇒ F = m dv/dt
But, dv/dt = a, acceleration
⇒ F = ma
Ques. Name the three laws of motion. (2 Marks)
Ans. The three laws of motion are
- Newton’s first law of motion: This law states that a body can’t change its state of rest or motion by itself unless some external force acts on it.
- Newton’s second law of motion: This law states that force acting on a body is proportional to the rate of change of momentum.
- Newton’s third law of motion: This law states that, to every action, there is an equal and opposite reaction.
Ques. Why does dust settle on a carpet that has been beaten with a stick? (2 Marks)
Ans. This is due to the law of inertia.
The dust particles on the carpet's surface are at rest, however when we strike it with a stick, the carpet cloth's position changes, but dust particles remain in their location owing to the inertia of rest, and dust comes out as a result.
Ques. Which is the most important Newton's Law? (2 Marks)
Ans. Newton's second law is among the most important laws in physics. For a body with a constant mass m, F = ma can be written, where F (force) and a (acceleration) are both vector variables. A body speeds according to the equation when it is subjected to a net force.
Ques. What is the magnitude of the force acting on a 10 kg body that has an acceleration of 5 m/s2? (2 Marks)
Ans. Given
- Mass of the body, m = 10 kg
- Acceleration of the body, a = 5 m/s2
From Newton’s second law of motion, we have
F = ma
⇒ F = 10 x 5 = 50 N
Ques. What is the acceleration produced on a body of mass 20 kg when a force of 100 N is applied to it? (2 Marks)
Ans. Given
- Force acting on the body, F = 100 N
- Mass of the body, m = 20 kg
From Newton’s second law of motion, we have
F = ma
⇒ Acceleration, a = F/m
⇒ a = 100/20 = 5 m/s2
Ques. What is the magnitude of the force acting on a body of mass 60 kg that has an acceleration of 2 m/s2? (2 Marks)
Ans. Given
- Mass of the body, m = 60 kg
- Acceleration of the body, a = 2 m/s2
From Newton’s second law of motion, we have
F = ma
⇒ F = 60 x 2 = 120 N
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