Forces

Definition and Nature of Force

A force is a push or pull that is applied to an object in order to move it, stop it, or change its direction. Forces are fundamental to the study of physics as they are responsible for changes in the motion of objects.

• Force is a vector quantity, meaning it has both magnitude and direction.

• The SI unit of force is the newton (N).

• Forces can act through direct contact (contact forces) or at a distance (field forces, such as gravity or magnetism).

Examples of Forces:

• Pushing a box across the floor (applied force)

• Gravity pulling an apple toward the ground (gravitational force)

• Friction slowing down a sliding object (frictional force)

• Magnet attracting a paperclip (magnetic force)

Formula:

• The general formula for force is given by Newton's Second Law: where is force, is mass, and is acceleration.

Newton's First Law

Law of Inertia

Newton's First Law states that an object at rest will remain at rest, and an object in motion will remain in motion at constant velocity, unless acted upon by a net external force. This law is also known as the Law of Inertia.

• Objects resist changes to their state of motion.

• If the net force on an object is zero, its velocity will not change.

• This law explains why seatbelts are necessary in cars: if the car stops suddenly, your body tends to keep moving forward due to inertia.

Example: A hockey puck sliding on ice will keep moving in a straight line at constant speed unless friction or another force slows it down.

Mass and Weight

Definitions and Differences

• Mass is a measure of the amount of matter in an object. It is a scalar quantity and does not change with location. The SI unit of mass is the kilogram (kg).

• Weight is the force exerted on an object due to gravity. It is a vector quantity and depends on both the mass of the object and the acceleration due to gravity.

Formula for Weight:

• where is weight (in newtons), is mass (in kilograms), and is the acceleration due to gravity (on Earth, ).

Example: An object with a mass of 1 kg has a weight of N on Earth.

Comparison Table: