# Question about acceleration and gravity

1. Jul 21, 2012

### raeshun

I am having trouble understanding g-force.So if a object with a mass of 100 kg is sitting on a platform in space.If the platform was accelerating at 10 m/s^2 upwards would the weight of the object be 1000 kg? If so why?

2. Jul 21, 2012

### CWatters

The force on the platform would be

F=ma
=100kg x 10m/s2
=1000 Newtons (not Kg)

On earth the force on the ground would be

F=mg
=100kg x 9.8m/s2
=980 Newtons.

About the same. So if this 100kg mass on the platform was a man he would feel roughly the same as he does on earth (eg 1g).

To be precise he would be experiencing 10/9.8 = 1.02g

3. Jul 21, 2012

### haruspex

The kg is a measure of mass, not force. Weight is a force. So a "weight of 100kg" is scientifically incorrect. It is used in everyday speech to stand for "the force exerted by gravity at the earth's surface on a 100kg mass", but in scientific terms that's (approximately) 1000 Newtons. Likewise, the force required to accelerate a 100kg mass at 10m/s2 in a weightless environment is 1000 N.

4. Jul 21, 2012

### raeshun

So if this happened on earth ignoring air resistance would the person on the platform feel weightless?

5. Jul 21, 2012

### CWatters

What might be confusing is that weighing scales measure the force an object puts on them so they should really be marked in Newtons not Kg. However since we don't normally take weighing scales to the moon they are calibrated for g=9.8 and marked in kg.

If you took kitchen weighing scales to the moon with a 1kg test mass you would discover they would read INCORRECTLY. For example they would under read by a factor of six yet the mass has not changed.

6. Jul 21, 2012

### CWatters

EDIT: haruspex is correct below. If the platform was on earth and accelerating upwards at 10m/s2 then would feel about 2g.

Last edited: Jul 22, 2012
7. Jul 21, 2012

### haruspex

If by "this" you mean the platform accelerating upwards at 10m/s2, they would feel a force of about 2g: 1g to stay put plus another to accelerate upwards.