Force, energy, power.

I don't have much force. He does not have much power. She has a lot of energy. Force, energy and power. When people use these words in everyday life, they often use them interchangeably.

Here are the some of the meanings given by https://dictionary.cambridge.org/

force - physical, especially violent, strength or power
energy - the power and ability to be physically and mentally active
power - ability to control people and events

So it looks like force is power. Power is energy. They are all just one and the same !

But as you can see in my earlier and later blogs, many common everyday words are given uncommon, very specific, and sometimes quite different meanings in physics. Students often need a lot of effort to unlearn the meanings they are used to, and relearn the physics meanings of these words.

And if you guess that force, energy and power have quite different meanings in physics, you would be right!

In this blog, I shall talk about what force means in physics, and what we learn about force in school physics. In two other blogs - one on energy and another on power, I shall explain what those mean, and how they are related to force.

So we start with something that might be familiar - a spring, or maybe a rubber band. Suppose that I get a small spring. I find some place to hang one end of the spring.

Then I find a small stone and tie it to the free end of that spring. The spring becomes a bit longer, and the stone hangs at rest on the spring.

Simple?

With this example, we get the first idea of what force means in physics. The weight of the stone is a force that pulls the stone down. The stretched spring produces a force that pulls the stone up.

When the spring stretches long enough, its force can become as big as to stone's weight. The two forces then balances, and the stone stops moving.

The above example involves 2 forces - the weight on the stone is gravitational force from Earth, the upward pull is mechanical force from the spring.

At some point in the course, we would also learn about other types of forces. For example:

- The weight of the stone comes from gravity from Earth pulling on it.
- Forces from electrical charges can attract of repel at at a distance, without touching eachother.
- Magnets attracting and repelling each other is familiar example of magnetic force.

In the above example, the spring and stone are in contact with each other. We call this type of force a "contact force", a force that happens when two bodies are touching each other.

In contrast, gravity, electrical and magnetic forces are non-contact forces. The pull on a stone by Earth happens even though Earth is not in contact with the stone.

Apart from the contact force when a object directly pushes another object, another way this can happen is when one object rub against another.

The a box is pushed over the floor, there is usually friction between box and floor. When a stone drops in water, it does not fall as fast as it dones in air. This is because the water gives some resistance to the stone moving through it. This is like friction - called viscosity in water and other fluids.

In air, this resistance is normally called air resistance. Air resistance is most obvious when we drop a piece of paper - it falls quite slowly compared with the same paper when crumpled into a ball.

Many people would have heard of the word "centre of gravity". It seems to be a physics idea that is drifted into everyday use.

So if a book case is really tall, it could fall more easily. If an object has a wide bottom, it would be harder for it to tip over.

The reason for this has to do with the centre of gravity. This is the idea that an object, whatever the shape, has a point where all its weight seem to act.

If you think this sounds strange and abstract, so do I.

One simple way to get an idea of what this means if to try a game that you might have played - to balance a ruler on one finger. If you have never tried this before, try it now with a 30 cm ruler. it can be quite fun.

Most people can do it after a minute or two. But have you ever wondered why this is possible?

The reason is that the ruler behaves as if it has no weight at all - except for a single point at the centre, where all of its weight is concentrated.

Sounds crazy? Why else can you balance the ruler at its centre with 1 finger?

But of course every part of the ruler has weight. We only say "behaves as if". We know that it balances because of the weight on two sides of the finger.

But what is harder to understand is where this "centre" of balance - what we call "centre of gravity" - is for more complicated shapes.

That is where the idea of "moment" of a force comes in. Yes - moments as in levers, if you have heard about that.

What has levers got to do with centre of gravity?

The answer is - everything.

Have you ever played on a seesaw?

It is possible for two children with different weight to play, but the heavier child has to sit closer to the centre. Have you ever wondered why?

It is because then the heavier child would have a smaller "turning effect" on the seesaw. When the "turning effects" of both children are similar, then seesaw is more fun.

In physics, "turning effect" is called "moment".

Apart from being more fun, notice that this also means the seesaw with the two children - the whole lot - can now be balanced at the centre or pivot of the seesaw.

In other words, the pivot is now the centre of gravity.

Apart from playing on a seesaw, what is the use of the "moment" idea?

Think of a book lying on one side of the table. Suppose that I want to use just one finger to push it to the other side. I placed my finger on one corner of the book and start pushing.

After a short distance, the book would likely rotate and my finger would slip off.

Suppose people need to move large machineries of building structures, or build small moving parts in mechanical devices. The same thing can happen and there would be problems.

In order to know what to do about this issue of moving an object, we need to know what are the effects of a force on it. When we push the book, we see that it both move forward and rotate. So there is a translation effect (moving in straight line) plus a rotation effect.

Now, try this : try pushing with your finger on different points on the edge of the book. You should be able to find a point where the book does not rotate anymore when your finger pushed it.

This happens when your finger is point straight at the book's centre of gravity.

Another aspect about force is pressure.

The above examples have just 1 or 2 forces on an object. Lets look at the other extreme - an object immersed in a pail of water.

Water has pressure. You might have experienced this if you have gone swimming in a pool before.

In physics, we learn about how the forces from the water help to keep us afloat. It is all about the pressure of water.

As we go deeper under water, the water pressure gets higher. If you have put your head under the water surface in the swimming pool, you would feel this. It is because there is more water above you.

So the reason why we float is quite simple. It is because the force from water below us is higher that the force from water above us. That's all !

This resultant force has a name - it is called "upthrust".

When I studied in Raffles Institution, there was a science teacher I really liked. His name was Mr Paul. He always looked serious, but somehow all my classmates found him quite funny.

And one of the funniest class was when explained how Archimedes discovered the "Archimedes' principle" when taking a bath. Archimedes was having a hard time from his king, who ordered him to find out whether the crown was made of pure gold.

Ever heard of the phrase "Eureka moment"? Archimedes apparently thought of the answer and jumped out of his bath tub naked, shouting "Eureka" ! Which means "I have found it!"

We had a good time laughing about it after the class.

But that lesson did not explain how Archimedes' principle can tell us whether the crown was made of gold. Nor did we get tested on that in exams.

Come to think of it, it is not Archimedes' principle that decides the crown's material. We really just need to use the water displaced by the crown to find its volume. Then we can calculate the crown's density and check if it is the same as gold's.

So what is Archimedes' principle ?? In simple words :

"If an object is immersed in water, it displaces or pushes aside some water - the same volume as itself. The object also gets an extra upward force from the water - called upthrust. This upthrust is as big as the weight of the water pushed aside by the object."

So what is the use of Archimedes’ principle?

Example 1 - Ship design. Part of a ship in under water, part of it above water. The principle can be used to calculate the volume of the ship that is under water - because the weight of this value of water would be equal to the weight of the ship.

Example 2 - It is used to check the purity of milk with an instrument called lactometer. This is just a tube with some air and liquid inside, with scale markings outside. When floating on milk, the marking near the milk surface can tell us the milk density.

Example 3 - It is used in diving gears and submarines to adjust the depth under water.

So much for forces! It forms the basis for the next topic on "Work, Energy and Power"

You can learn these concepts and more at Dr Hock's maths and physics tuition.