Physics Help Please

[FiddleOnTheRoof]

I could spend another hour asking google, but no answer makes much sense. Please help....

What is the difference between

F=ma and F=mg

And when will they be used respectively. ???

If you have time to illustrate both with a problem question, this would be greatly appreciated!!!

I suppose g (gravity) would be a constant figure as things falling towards earth fall at the same rate regardless of mass

www.school-for-champions.com/science/force.htm#.XdLQ7fSnzqs

www.school-for-champions.com/science/gravity_overview.htm#.XdLRVfSnzqs

Do these help?

The a is for acceleration. The g is acceleration due to gravity.

You could use F=ma for stuff going horizontal, but F=mg is a special case for falling items.

OK, here's a couple of problems:

What is the braking force needed to stop a car of 1000 kg, slowing down at a rate of 5 m/s^2?

F= ma so F= 1000 kg x 5 m/s^2
Thus braking F=5000 N

What force will a coin of 5 g hit the floor with if it falls straight down from a skyscraper? Take g to be 9.8 m/s^2.

F=mg so F= 0.005 kg x 9.8 m/s^2

In both these equations, F is force (measured in Newtons, N) and m is mass (measured in kg).
a is acceleration (measured in metres per second per second, or ms^-2)
g is just a specific type of acceleration, which is acceleration due to gravity, usually given as 9.81 ms^-2

So if you drop a ball from a height, it will accelerate towards the ground at 9.81 ms^-2. If that ball weighs 0.1 kg, the force exerted on the mass, by way of the acceleration due to gravity, is F=mg, F = 0.1 x 9.81, F = 0.981 N

a is a general acceleration. Imagine pushing your ball along a smooth horizontal surface. If you exert a force of 2 N on your 0.1 kg ball, it will accelerate according to F=ma, a = F/m, a = 2/0.1, a = 20 ms^-2

hth

Oops
F= 0.049 N

That is brill... thanks to all!