Newton’s laws of motion

English: Isaac Newton Dansk: Sir Isaac Newton ...

Sir Isaac Newton (1642-1727) (Photo credit: Wikipedia)

Sir Isaac Newton was an English Physicist and Mathematician who worked on many areas of science and maths in the seventeenth century. The number of things he worked on was quite impressive, ranging from gravity to optics (he discovered that white light is made up of many colours) to his work in developing calculus. One of the things Newton is most remembered for is laying the foundations of Classical (sometimes called Newtonian) Mechanics, the physics of moving objects. The basis of this branch of physics are Newton’s laws of motion, so without further ado, let’s take a look at the first law.

Newton’s First Law of Motion:

An object will remain in a state of either rest or constant velocity unless acted upon by a resultant force.

Let’s unpick this a little. Everything is either at rest – so not moving (in which case it has zero velocity), or in motion, in which case it has some velocity. Velocity is simply speed in a certain direction. If I drive north at 40 miles per hour, my velocity is 40 mph north. I could also say I have velocity of negative 40 mph to the south. The first law tells us that if no resultant force acts on an object it’s velocity will not change. If it is at rest it will stay at rest. If it is moving it will keep moving at the same speed in the same direction in a straight line forever – unless a resultant force acts on it. What do we mean by resultant? Think of a shopping trolley being pushed along. As you push it in a straight line, you are exerting a force on the trolley by pushing it, right? But it isn’t changing speed or direction, so what gives? What’s happening is that your push is not the only force at work. As you push the trolley there is also friction between the wheels and the floor and air resistance to work against you. As it happens, the forces trying to push the trolley the other way balance out the force you are creating – there is no resultant (net) force acting on the trolley, so it’s velocity must remain constant.

Newton’s Second Law of Motion:

The force acting on a body is equal to its mass multiplied by its acceleration.

This is usually expressed in the form of an equation, F = ma. It tells us many things. Firstly, that you need a bigger force to give the same acceleration to an object with more mass, or to give an object with the same mass a bigger acceleration. It also tells us that both a force and the acceleration that results from it are in the same direction as each other. It is important to note here that in physics, acceleration means a change in velocity. This could be speeding up, slowing down (negative acceleration), or changing direction. It could even be changing speed and direction. Finally, Newton’s Second law gives us the SI unit (confused? see the post on SI units here) for force. Mass is measured in kg, while acceleration is measured in ms-2, so the unit for force must be kgms-2. We call this unit the Newton (I think you can guess why).

Newton’s Third Law of Motion:

When a body exerts a force on another body, the second body exerts a force – of the same type, of equal magnitude, and in the opposite direction – on the first body.

Wow – that’s a lot of jargon. Allow me to simplify: Newton’s third law is sometimes stated as “every action has an equal and opposite reaction”. So when you walk into a lamppost, it pushes back against you. When you jump off the ground, you push the Earth away from you just as it pushes your feet – and when you fall back down due to gravity, your own gravity pulls the Earth up. This seems a little crazy until you take a look at it in the context of the second law – even though the force you exert on the Earth is the same as its pull on you, its mass is much, much bigger, so it only accelerates by a tiny amount.


2 comments on “Newton’s laws of motion

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