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Physics Theory : Force

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Force is an external agent (push or pull) which changes or tends to change the state of rest or uniform motion of a body, or changes its direction or shape.
There are different types of forces including frictional forces, gravitational forces, mechanical forces, magnetic forces and electrical forces.
For instance, frictional force acts at the surface of contact of two bodies.
Force of friction always acts in the direction opposite to the direction of the applied force.
Frictional force always opposes the motion of bodies and ultimately stops them.
Force is defined as the rate of change of momentum.
∴ Force = Change in momentum / Time taken for that change


Momentum of a body is the product of its mass and its velocity.
∴ Momentum = Mass × Velocity
Substituting the above equation into the definition of force gives
Force = Mass × (Change in velocity / Time taken for that change).
Since rate of change of velocity is acceleration, force is also defined as the product of the mass and the acceleration.


Newton's Second Law of Motion is the definition of force.
∴ Force = Mass × Acceleration or F = m a


Weight of a body is the force (or pull) by which it is attracted toward the earth by gravity (or the gravitational acceleration).
Weight is directly obtained from Force = Mass × Acceleration (F = m a) when a = g.
∴ Weight = Mass × Gravitational acceleration or W = m g
Gravitational acceleration on the earth is 9.81 m/s2 (approximately 10 m/s2).
An object will weigh more at the poles than at the equator.
This is so because an object will weigh more, the closer it is to the center of the earth.
The earth is not a perfect sphere (and is flatter at the poles and bulges at the equator).


Newton (whose symbol is N) is the SI unit of force.
It is named afer the famous scientist, Sir Isaac Newton.
One Newton is the force required to accelerate a mass of one kilogram at 1 meter/second2.
∴ 1 N = 1 kg × 1 m/s2 = 1000 g × 1 m/s2 = 1000 g m/s2 = 100 g × 10 m/s2.
So, one Newton approximately equals the force to lift vertically upwards a 100 g mass against the gravitational acceleration (assumed about 10 m/s2).
Another commonly used unit of force is kgf (kilogram force), which is the force required to lift vertically upward a mass of 1 kg.
1 kgf = 1000 gf = 9.81 N = 10 N (approximately).


Scalar is a quantity that has only magnitude but no direction.
Examples of scalars are mass, density, speed and time.


Vector is a quantity that has both magnitude and direction.
Examples of vectors are velocity, acceleration, momentum, weight and force.


Physics Quiz on Force.
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