Key Points

A force is defined as a push or pull applied on an object, which results from the object's interaction with another object.

The SI unit of force is newton, written with a small 'n', and its symbol is $N$.

A force applied on an object can make it move from rest, change its speed or direction of motion, or bring about a change in its shape.

Forces come into play only when two objects are interacting in some way; at least two objects must interact for a force to be present.

Contact forces are types of forces that act only when there is physical contact between the objects, either direct or indirect.

Muscular force is a type of contact force that results from the action of muscles in our body, used for activities like walking, lifting, or chewing.

Friction is a contact force that comes into play when an object moves or tries to move over another surface, always acting in a direction opposite to the motion.

Non-contact forces are forces whose effect can be experienced even if the objects are not in physical contact with each other.

Magnetic force is a non-contact force exerted by a magnet on another magnet or a magnetic material, which can be attractive or repulsive.

Electrostatic force is a non-contact force exerted by a charged body on another charged or uncharged body, with like charges repelling and unlike charges attracting.

Gravitational force is a non-contact force with which the Earth attracts objects towards itself, and it is always an attractive force.

The weight of an object is the gravitational force with which the Earth pulls it towards itself, and its SI unit is newton ($N$).

Mass is the amount of matter in an object and remains constant everywhere, whereas weight is a force that can vary slightly depending on gravitational pull.

A spring balance is a device used to measure the weight (force) of an object by observing the stretch in a spring, calibrated in newtons ($N$).

Upthrust or buoyant force is the upward force applied by a liquid on an object immersed in it, which opposes the gravitational force.

An object sinks if its gravitational force is greater than the buoyant force, but it floats if these two forces are equal.