In this topic, I shall write about the physics syllabus on how things move under forces, and how they move when they collide. We also learn exactly what is "momentum", a word that has come to be commonly used in English.

The way things move obeys Newton's three laws motion. Of course, this does not mean that before Newton was born around 1643, they did not obey Newton's laws. Newton understood these laws and wrote them down.

In simple English, Newton's three laws of motion goes something like this :

1. An object will stay at rest or move in a straight line unless a force acts on it.

2. The momentum of an object changes when there is an overall force on it. If we double the force, the momentum changes twice as fast.

3. If we push on an object, the object pushes back with an equal and opposite force.

If we push an heavy object on a perfectly smooth ground, we would still get tired after a while - even though there is no friction on the ground. It cannot be because of the weight since we are not lifting the object up.

So what is it that makes us feel tired?

There must be something inside the object. This something is called "mass". The SI unit for mass is kilogram, same as the unit we normally use for weight !

At this point in the physics course, we finally learn that kilogram is not the unit for weight. Weight is the force the Earth pulls on the object.

The unit of force is not kilogram - the unit of force is Newton (N). Earth pulls down at a 1 kg object with a force of 9.8 N. So if we want to be scientifically correct, the weight of a 1 kg object is 9.8 N.

So after learning physics in school, the next time we go to a shop to buy 1 kliogram of rice, should we ask for 9.8 Newtons of rice instead ??

Newton called "momentum" the "quantity of motion". Momemtum is mass times velocity. That means if an object has more mass or more velocity or both, then it has more momentum. This makes sense - a moving object that has more mass or more velocity are both harder to stop.

A google search gives the following meanings for the word "impulse" :

- a sudden strong urge to act
- something that causes something to happen quickly

In physics, the meaning is more specific. If a force is pushing body for a time, then the impulse is force x time.

This can be used if a big force hits an object for a short time, which is more like the everyday meaning of impulse. But we can also use the word "impulse" when a small force pushes at the object for a long time.

If a force pushes at an object, the object can move. If a few forces push at the object, it can also move. The combined effect of all the forces is called a "resultant force".

We can think of this as just like a single force pushing at an body. This would cause the momentum of the body change. A bigger force would give bigger change in momentum over the same period of time.

We have seen from the above that a bigger force on a body means bigger acceleration.

For the same force, a bigger mass means smaller acceleration - harder to push.

From the above, we have seen that force is directly related to the mass and acceleration of an object. If force is bigger, acceleration if bigger. If mass is bigger for the same force, acceleration is smaller.

Since a force has direction, this would also determine the direction of acceleration. Mass does not have direction, so we don't worry about that. If there is only one force, then the acceleration is always in the same direction as the force.

Have you ever played with marbles?

When one marble hits a second marble, the second marble can move off in some direction. The first marble can bounce off in another direction. Good fun ! Even adults like it, though they use bigger balls, long sticks, expensive tables and call it billiards.

The part that always interests me is when one marble hits another head on - the first marble stops, the second marble flies off in the same speed and direction. Almost as like the first marble has passed on the "motion" to the second marble.

And indeed, that is exactly what happened! It is called conservation of momentum.

It just means that if you add up the momenta (plural for momentum) of the colliding balls, this total must remain the same after collision. This is true even if some of the kinetic energy (energy of motion) of the balls are lost to heat during collision and the rebounding balls move more slowly - the total momentum is still the same!

As mentioned above, when a marble hit an identical marble, the first one can stop completely and the second one fly off at the same speed and direction.

What if one marble is heavier than the other marble? Then both may move after colliding, but their relative speed would stay the same. This means that before colliding, if their distance apart closes in at say 10 cm per second, then after colliding their distance would separate at cm per second also.

The example above using collision of marbles are kind of an ideal case where there is no energy loss. In real life, there is always some energy during collision - at least to heat and sound. This energy has to come from somewhere - so there is always some loss in total kinetic energy.

But curiously, no matter how little or how much kinetic energy is lost in a collision, the total momentum still stays the same. Of course, this is true only if there is no friction on the ground.

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