9th Class Science Force and laws of motion Force and Laws of Motion

Force and Laws of Motion

Synopsis

• Force

Force is that cause which produces an acceleration in the body on which it acts.

A force or a set of forces can change the speed of body, change the direction of the motion of the body and change the shape of the body.

• Balanced and unbalanced forces

If a set of forces acting on a body produces no acceleration in it/then the forces are called balanced forces. If the set of forces produces an acceleration, they are said to be unbalanced forces.

• Types of forces Contact force.     2. Non-contact force.

• Newton's first Law of Motion

A particle remains at rest or moves in a straight line with a constant speed unless it is compelled to change that state by an external unbalanced force. The first law of motion gives the definition of inertia.

• Inertia of rest: The tendency of a body to continue in its state of rest is due to inertia of rest, e.g., when a bus starts suddenly, the passengers fall backwards.

Inertia of motion: The tendency of a body to continue in its state of motion is due to inertia of motion, e.g., a bicycle is observed to move forward even when pedalling is stopped.

Inertia of direction: The tendency of a body to continue to move with uniform motion in a linear direction.

• Newton's Second Law of Motion

The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts.

Mathematically

\[F\propto \frac{\Delta P}{\Delta t}\Rightarrow F=k\left[ \frac{{{P}_{2}}-{{P}_{1}}}{\Delta t} \right]\Rightarrow k\left( \frac{mv-mu}{\Delta t} \right)=k\left( m\left( \frac{v-u}{\Delta t} \right) \right)=km\left( \frac{\Delta v}{\Delta t} \right)F=kma\]

The value of the constant of proportionality can be taken as 1 when a unit of forces is chosen in such a way that it produces a unit acceleration by a unit mass then F = ma.

• Impulse: Impulse is defined as the product of force which acts on a body and the time for which the force acts. When a large force acts on a body for a very short duration of time then this large force is called impulsive force.