Force is a push or pull that changes or tends to change the state of rest or motion of a body. It is measured in newtons (N). Sir Isaac Newton described how forces affect motion in his three laws.
Newton’s First Law (Law of Inertia)
A body remains at rest, or continues moving at constant velocity in a straight line, unless acted upon by an external force.
- Inertia is the tendency of a body to resist a change in its state of motion.
- Example: passengers are thrown forward when a moving vehicle stops suddenly, because their bodies tend to continue moving.
Newton’s Second Law
The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force.
Newton’s Third Law
For every action, there is an equal and opposite reaction.
- Example: a rocket pushes exhaust gases downward, and the gases push the rocket upward with an equal and opposite force.
Momentum
Momentum = mass × velocity (p = mv). It is a vector quantity, measured in kg m/s.
Impulse = force × time = change in momentum. Impulse explains why crumple zones and airbags reduce injury — they increase the time over which the force acts, reducing the force.
Worked Example
A force of 20 N acts on a mass of 4 kg. Find the acceleration.
F = ma → a = F/m = 20/4 = 5 m/s²
Summary
Newton’s first law describes inertia, the second gives F = ma, and the third states that forces always occur in equal and opposite pairs. Momentum (p = mv) and impulse (Ft) are key companion concepts.
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