Newton’s Laws of Motion

There are a lot of things happening in the world. Objects move, fall, and rest all around us. You may think you understand why things move – or don’t move – but did you know that the forces acting on these objects can be measured, and their motion predicted?

Renowned English scientist Sir Isaac Newton wanted to understand the world better. He thought he could describe the movements of the universe through mathematical observation. In 1687 he published these observations in a book, which outlined his understanding of the laws of motion. These three laws of motion are still in use today and can help us understand the relationship between a body and the forces acting on it.

Newton’s First Law of Motion is often stated as ‘An object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an outside force.’ This law is also called the law of inertia – since inertia is the tendency of an object to keep doing what it’s doing. You have experienced Newton’s first law if you have ever used a swing on a playground. In order to start swinging, you have to use a lot of force – but once you get going, it can be hard to stop! Here on Earth, there are some forces working on objects all the time. When you throw a ball upward, it doesn’t keep going up forever because the force of gravity pulls it downward. Once it hits the ground, friction – or resistance caused by surfaces rubbing together – will eventually bring it to a stop.

Newton’s Second Law says that force is the product of mass times acceleration. Mass is the amount of matter – or ‘stuff’ – in an object. To make something accelerate, or speed up, you have to apply a force. The amount of force it takes to move an object changes depending on how massive it is. Think about it: you would need more force to move a bowling ball than to move a beach ball of the same size. The greater the mass of an object, the more force it will take to accelerate the object, and applying more force will make the object move faster.

Newton’s Third Law of motion says that for every action, there is a equal and opposite reaction. If you exert a force on an object, it exerts an equal force back on you. For example, a ball hitting the ground exerts force downward. The floor exerts an equal force upward. This is the principle that allows rockets to launch, or jellyfish to swim! By exerting a force downward, an equal and opposite force pushes upward, and the object accelerates. We can also see the Third Law in action when birds fly. Their wings press down against the air. The air pushes up against their wings, allowing them to move upwards. Another good example of this law is skateboarding. The skater pushes backwards against the ground with their foot. The ground pushes forward with equal force. Since the wheels of the skateboard reduce the friction between the person and the ground, they are able to move forward for some time before forces slow them down again. Although these three laws were written down more than 300 years ago, they are still important now for understanding the way things move, and represent the foundation of classical mechanics.