WEIGHT, MASS AND GRAVITY
We always talk about "measuring our weight". We say things like "This dumbbell weighs 2 kilograms," or "I weigh 100 kilos" (ooh, time to do some workouts!). Unfortunately, the tendency to see 'kilogram' as a measurement of WEIGHT is not accurate.
Kg is the SI unit for mass, which measures how much substance is inside an object.
So what's up with weight? Well, remember that gravity pulls everything on Earth. This means that we're constantly accelerating downwards. Yes, accelerating. Gravity does not cause things to fall at a constant speed. Unless a falling object is stopped, it will continue to fall faster, and faster andfaster andfasterandfaster... you get the idea. This acceleration is 9.81 m s-2. Yea, it's a difficult number to remember so they sometimes round it to 9.81 m s-2 or even 10 m s-2 in school to make counting easier. (Check with your teacher to see which rounding is accepted)
Now, imagine a 60 kg person standing on a bathroom scale. Ok, let's consider this formula: F = ma ...Force equals mass times acceleration (Daya = jisim X pecutan)
How convenient! We have a value for mass (60 kg) and a value for acceleration (gravity: 10 m s-2). So,
F | = 60 X 10 |
= 600 N |
The person exerts a force of 600 N onto the bathroom scales. The mechanisms in the scales uses this force to turn the dial to point at the number to show the person's "weight". The more force is provided, the more the dial turns. Of course, do not exert too much force or your scales will go kaput. (Replace 60 kg with 6000 kg and see what you get)
Notice the inverted commas around "weight"? Strictly speaking, scales show you your mass -- how much bone, flesh, water etc. is inside you. If the scale was to show "real weight", the reading for the 60 kg guy/girl would be 600 N. I guess he/she wouldn't be too happy to see that number.
In conclusion, weight is a measurement of the force that an object exerts on the floor (or whatever it is resting on), due to the acceleration of gravity towards the Earth's core. Therefore, objects with more mass are heavier. Also, if an object is brought to a planet/moon with weaker gravity, it will experience reduced acceleration and thus become lighter, and vice-versa.