Forces can be:

  1. pushes,
  2. pulls, or
  3. twists.

Forces are measured in Newtons.

My mass is 65 Kg. This is true wherever I go in the universe. I might like to go into outer space for my next holiday. It might be very expensive and I am not sure that my travel agent could fix it for me anyway. However, I am absolutely sure that  if I went to the moon for my holiday, my mass would remain at 65 Kg.

My weight is 650 Newtons. This is only true whilst I stay on Earth. If I did to up into outer space, I would become weightless because there is no gravity out there (at least not enough for me to notice). On the moon, I would be very light and jumping would be very easy. On one of the very big planets like Jupiter or Saturn I would be so heavy that I would not be able to stand up! My legs would not be strong enough to take my weight. That is because gravity is so much bigger on a big planet.

Meanwhile, back on Earth, gravity is 10 Newtons per Kg. My weight is my mass multiplied by the force of gravity:

Weight    =    Mass     times   Force of Gravity

650 Newtons    =    65 Kg    times    10 Newtons/Kg

Gravity always pulls me down to Earth !!!

Fortunately, there is always an equal and opposite force called a Reaction which pushes me up. Since the force of gravity pulling me down and the reaction pushing me up are equal and opposite, I stay in the same place. If there was no reaction, I would fall down through the ground until I got to the centre of the Earth.

Here is a more general equation:

Force    =    Mass     times   Acceleration

When we talk about weight we should refer to the force of gravity actin on us The force of gravity is really the acceleration due to gravity.

I hope that you will be able to remember most of the facts on this page, but you must memorise the definition of forces. Write it on a small index card. Put the red words on one side of the card and the blue ones on the other. Add the card  to your revision pack.

Forces can be pushes, pulls or twists. They are calculated by multiplying the mass times its acceleration.

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