Isaac Newton was an English mathematician, physicist and astronomer who is widely considered to be the most influential scientist of all time. He is best known for having an apple fall on his head (which didn’t really happen) but his work in physics and maths went much further than discovering gravity. Through the story of Isaac Newton, we’ll explore the physics of forces and work and understand how Newton’s insights changed the world.
Sir Isaac Newton was born on Christmas day 1642 in Linconshire, England. From the age of about twelve until he was seventeen, Newton was educated at The King’s School, Grantham, which taught Latin and Greek and probably imparted a significant foundation of mathematics. He soon became the top-ranked student in his class, distinguishing himself mainly by building sundials and models of windmills. Throughout his career, Newton worked on a wide variety of topics in physics including optics, gravitation and mechanics. He was one of the first people to use a prism to split light into its 7 colours of the rainbow. He also once stuck a needle in his eye to see how the retina worked! At the age of just 26, Newton invented calculus, the mathematical techniques of integration and differentiation that are used in almost all science and engineering applications – and even in things like the stock market and economics. He used this form of maths to explain how the planets move in the solar system. Most physics students will know Newton because of the three laws of motion named after him. Using newton’s laws, a physicist can explain how an object will move and accelerate when a force is applied to it. Newton’s discoveries are too many to mention here, but every single mathematician and physicist in the world has him to thank for helping them understand how our world works.
What is a Force?
If you think of applying force to something, you probably think of pushing or pulling. In physics, a force is anything that causes the motion of an object to change. There are many different types of forces in physics, including friction, gravity, electric force, magnetic force and weight. The unit of force is the newton (N), which is named after Sir Isaac Newton. Isaac Newton’s experiments on the physics of forces and motion led him to write his famous laws:
- A body at rest will remain at rest, and a body in motion will remain in motion unless acted upon by an external force.
- The force felt by an object is directly proportional to the acceleration and mass of an object (Force = Mass x Acceleration)
- For every action, there is an equal and opposite reaction.
Weight vs. mass
In everyday conversation we use the word weight to refer to how heavy something is. In physics though, there is a big difference between weight and mass. Mass is the amount of matter in an object, it does not change depending on where the object is, it is a property of an object. Weight is the force of gravity acting on an object (Weight = Mass x Gravity) and it can change depending on where you are.
Think of astronauts floating around in space, they have the same mass as they would have on earth, but they appear to be weightless. If you watch footage of men walking on the moon, they seem like they can jump extremely high very easily. This is because they weigh nearly six times less on the moon, than they do on earth. But remember, their mass has not changed. This can also be seen on earth. If you stood on a weighing scales at the base of Mt. Everest, and then weighed yourself at the top of the mountain, the scales would read differently. This is because the strength of gravity gets weaker as you get higher.
Friction as a force
Another important force is friction. Friction is the force that opposes motion. When you want to stop a moving car, you step on the brake pedal. This stops the wheels turning by holding a pad against the spinning wheel. Without friction, an object in motion would travel forever. Another way to produce friction is to rub things together, you can try this yourself by rubbing your hands together. Odds are you’ll feel your hands heating up – this is an effect of friction. Most machines with moving parts want to minimise the effects of friction because it is a waste of energy. To reduce friction, you need to use a technique called lubrication. Lubrication makes moving parts slide past each other, rather than rub against each other.
Extension of a spring
Have you ever stretched out a spring and let it go? What happened? Does the same thing happen when you squash a spring? There’s some sort of force acting here that seems to react to an applied force but in the opposite direction. If you pull a spring downwards it will get longer (i.e. change its length), this generates an upward force inside the spring that opposes it and wants to bring it back to its original length. As the downward force gets bigger, the extension gets bigger at the same rate. We call this a direct proportion relationship. In a spring, this relationship was discovered by the scientist Robert Hooke – so we call it Hooke’s law.
When a scientist talks about work, they don’t mean their job. In physics, work is a way of measuring how much energy is used. Imagine you are pushing an empty shopping trolley around a super market, it’s fairly easy if there’s nothing in it. If your trolley is full and gets really heavy, it will require more energy to push it around, so more work will have to be done to move it. Another factor is how far you have to push it. It’s going to take a lot more energy to push the trolley 100 metres, rather than 5 metres, right? So, if we were to make a formula for work, it would involve the force applied, and the distance travelled (Work = Force x Distance).