# a level physics

#1
hi

what's the difference between mg=mv^2/r simplified to v=root(gr)
and
mv^2/r=-gmm/r^2 simplified to v=root(gm/r)
and
1/2mv^2=gmm/r simplified to v=root(2gm/r) ?

and when are the scenarios when you'd use them ?

thank you
Last edited by hannahco; 4 weeks ago
0
4 weeks ago
#2
Weight = centripetal force is where there is no resultant force acting on the object.

the second one is for gravitational fields. So basically eg the moon revolves around the earth. So what is causing it to stay in orbit
there is a centripetal resultant force causing it to move in that circular motion.

I cant explain the third one have you typed it correctly.
0
4 weeks ago
#3
(Original post by hannahco)
hi

what's the difference between mg=mv^2/r simplified to v=root(gr)
and
mv^2/r=-gmm/r^2 simplified to v=root(gm/r)
and
1/2mv^2=gmm/r simplified to v=root(2gm/r) ?

and when are the scenarios when you'd use them ?

thank you
The last one is about escape velocity - it's equating the initial KE of an object with the amount of work required to move it to infinity.

In the case of the last two, it would help to distinguish between the mass of the object anf the mass of the planet it is orbiting around/trying to escape from. Use m and M rather than two m's.

Although the equations have some superficial similarities, they are about quite different things - better to know how to derive them when you need them than to try and memorise them.
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