What is Force?
In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (accelerate). Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity.
The concept of force is fundamental to classical mechanics and was first described by the English physicist Sir Isaac Newton in the 17th century. According to Newton’s laws of motion, force is directly proportional to the acceleration it produces in a body of given mass.
SI Unit of Force
The SI unit of force is the Newton (N), named after Sir Isaac Newton. One Newton is defined as the force required to accelerate a mass of 1 kilogram at the rate of 1 metre per second squared (1 N = 1 kg m/s2).
| Quantity | Symbol | SI Unit | Unit Symbol |
|---|---|---|---|
| Force | F | Newton | N |
| Mass | m | Kilogram | kg |
| Acceleration | a | Metre per second squared | m/s2 |
Types of Forces
Forces can be broadly classified into two main categories:
1. Contact Forces
These forces act between objects that are physically in contact with each other.
- Frictional Force: The force that opposes the relative motion between two surfaces in contact.
- Normal Force: The perpendicular force exerted by a surface on the object resting on it.
- Tension Force: The pulling force transmitted through a string, rope, or cable.
- Applied Force: A force applied to an object by a person or another object.
- Spring Force: The restoring force exerted by a compressed or stretched spring.
2. Non-Contact Forces (Action at a Distance)
These forces act between objects without physical contact.
- Gravitational Force: The attractive force between any two masses in the universe.
- Electromagnetic Force: The force due to electric charges and magnetic fields.
- Nuclear Force: The strong force that holds protons and neutrons together in the atomic nucleus.
Newton’s Second Law – The Mathematical Definition of Force
According to Newton’s Second Law of Motion, the net force acting on an object is equal to the product of its mass and acceleration:
F = ma
Where: F = Force (N), m = mass (kg), a = acceleration (m/s2)
This equation is the cornerstone of classical mechanics. If we know any two of the three quantities (force, mass, acceleration), we can calculate the third.
Effects of Force
A force applied to an object can produce the following effects:
- It can set a stationary object into motion (e.g., kicking a football).
- It can stop a moving object (e.g., brakes stopping a bicycle).
- It can change the speed of a moving object (accelerate or decelerate it).
- It can change the direction of a moving object (e.g., a ball hit by a bat).
- It can change the shape or size of an object (e.g., stretching a rubber band).
Balanced and Unbalanced Forces
Balanced Forces
When two or more forces act on an object and their resultant (net force) is zero, the forces are said to be balanced. A balanced force does NOT change the state of rest or uniform motion of an object.
Example: A book resting on a table – gravity pulls it down while the normal force from the table pushes it up equally. Net force = zero.
Unbalanced Forces
When the net force acting on an object is not zero, the forces are called unbalanced forces. Unbalanced forces cause a change in the motion (acceleration) of the object.
Example: Pushing a trolley – your applied force exceeds friction, so the trolley accelerates.
Solved Example
Problem: A car of mass 1200 kg accelerates from rest to a velocity of 20 m/s in 10 seconds. Calculate the net force acting on the car.
Solution:
Given: m = 1200 kg, u = 0 m/s, v = 20 m/s, t = 10 s
Step 1 – Find acceleration:
a = (v – u) / t = (20 – 0) / 10 = 2 m/s2
Step 2 – Find Force:
F = ma = 1200 x 2 = 2400 N
Therefore, the net force acting on the car is 2400 Newtons.
Key Points to Remember
- Force is a vector quantity – it has both magnitude and direction.
- SI unit of force is Newton (N).
- Formula: Force = Mass x Acceleration (F = ma)
- Force can cause motion, stop motion, change direction, or deform an object.
- Balanced forces result in NO change in motion; unbalanced forces cause acceleration.
- The four fundamental forces of nature are: Gravitational, Electromagnetic, Strong Nuclear, and Weak Nuclear.
This article is part of the Class 11 Physics notes series on EdustudySite. For more chapters, visit our Academic Notes section.