CBSE Class 8 Physics Force Notes Set 02

Force and Its Effects

A force is a push or a pull. It can start or stop a body, change its speed or direction, or change its shape. It is a vector, measured in newton (N); 1 N = 1 kg m/s² = 10⁵ dyne.
Contact forces (muscular, friction, tension, normal reaction) need touching. Non-contact forces (gravity, magnetic, electrostatic) act through space. Balanced forces do not change the state of motion; an unbalanced net force does.

Newton's Laws of Motion

First law: a body stays at rest or in uniform motion unless an external force acts on it. This tendency is inertia, and it is greater for a body of greater mass. Examples: passengers fall backwards when a bus starts and forwards when it stops.
Second law: the rate of change of momentum is proportional to the force, which gives F = ma. Momentum p = mv (kg m/s). Impulse = F × t = change in momentum, so a longer time of contact means a smaller force, as when a fielder draws his hands back while catching.
Third law: for every action there is an equal and opposite reaction. They act on different bodies. This explains the recoil of a gun, walking, rowing and rocket propulsion.

Solving Numerical Problems

Convert all quantities to SI units first (grams to kilograms, km/h to m/s by dividing by 3.6). Find the acceleration from a = (v − u)/t and then use F = ma, or find the change in momentum and divide by the time. In recoil problems use conservation of momentum: total momentum before firing equals total momentum after.