Newton's Three Laws of Motion Explained

Newton's three laws of motion explain how forces change the way objects move. A force is a push or a pull, measured in newtons (N). When several forces act on an object, they combine into one resultant force. If the resultant force is zero, the forces are balanced; if not, they are unbalanced and the object's motion changes. The three laws, published by Isaac Newton in his Principia, are still the basis of the mechanics taught in school physics today.

If the resultant force on an object is zero, an object at rest stays at rest, and a moving object keeps moving at the same speed in a straight line. This resistance to changes in motion is called inertia, and more massive objects have more of it.

The first law corrects a common belief: that something moving needs a force to keep it going. A ball rolling across a field slows down because friction and air resistance act on it. In deep space, far from other objects, a probe keeps moving without its engines running. Seat belts work because of the first law: when a car brakes, your body carries on moving forwards until the belt pulls it back.

When the resultant force is not zero, the object accelerates. The second law links the three quantities:

• F = ma, with force in newtons, mass in kilograms and acceleration in metres per second squared
• 1 N is the force that gives a 1 kg mass an acceleration of 1 m/s²

For a fixed mass, doubling the force doubles the acceleration. For a fixed force, doubling the mass halves it. A 1,200 kg car with a resultant force of 2,400 N accelerates at 2 m/s². Weight is also a force: W = mg, where g is about 9.8 N/kg near the Earth's surface.

Whenever one object exerts a force on another, the second object exerts a force on the first that is equal in size, opposite in direction and of the same type. A swimmer pushes backwards on the pool wall and the wall pushes the swimmer forwards. When you walk, your foot pushes back on the ground and the ground pushes you forwards.

The two forces act on different objects, so they never cancel each other out. That is different from balanced forces, which all act on the same object.

Print the poster on A4 or US Letter for handouts, or on A3 or Tabloid for a classroom wall. Ask students to add a fourth example for each law from their own lives, or to label the force pairs in a photo of a sport. The matching LearnBySlides lesson has full slides, speaker notes, a quiz and a safe trolley demonstration.