What are Newton's three laws of motion?
Newton's laws say: an object stays at rest or keeps moving unless a force acts on it; force equals mass times acceleration; and every action has an equal and opposite reaction.
The first law, the law of inertia, says that an object will stay still or keep moving at the same speed in a straight line unless an outside force acts on it. That is why passengers lurch forward when a car brakes suddenly, and why seat belts are important.
The second law states that force equals mass times acceleration, written F = ma. A heavier object needs more force to speed it up by the same amount. The third law says that for every action there is an equal and opposite reaction. When a rocket pushes gas downward, the gas pushes the rocket upward.
Isaac Newton
English scientist Isaac Newton published these laws in 1687 in his book Philosophiæ Naturalis Principia Mathematica. They became the foundation of classical mechanics, used to design bridges, cars and spacecraft. Einstein's theories later refined them for extremely high speeds and strong gravity.
Did You Know?
The unit of force, the newton, is named after Isaac Newton. One newton is roughly the weight of a small apple on Earth.