How Newton’s three laws predict the effect of forces
Newton’s first law says an object stays at rest or keeps moving in a straight line at constant speed unless an unbalanced force acts on it. This tendency to resist change is called inertia, and heavier objects have more of it. Passengers lurch forward when a bus brakes suddenly because their bodies tend to keep moving.
The second law connects force with change in momentum. Momentum is p = mv, and the force is equal to the rate of change of momentum, which gives F = ma for a constant mass. A force of one newton gives a 1 kg mass an acceleration of 1 m/s². The third law states that every action has an equal and opposite reaction acting on a different body. When a gun fires, the bullet moves forward and the gun recoils backward, and the total momentum before and after stays the same. For example, a 5 kg ball accelerating at 2 m/s² needs a net force of 10 N.
Why it matters
Seat belts and airbags protect passengers by applying inertia and increasing the time taken to stop, which reduces the force. Cricketers pull their hands back while catching for the same reason. Rockets rise by pushing exhaust gases downward, using the third law. These laws form the base of mechanics and lead into gravitation, work and energy.
Key terms
- Inertia
- The natural tendency of an object to resist any change in its state of rest or motion.
- Momentum
- The product of an object’s mass and its velocity.
- Newton
- The force that gives a mass of one kilogram an acceleration of one metre per second squared.
- Conservation of momentum
- In the absence of external forces, the total momentum of a system remains constant.