You are Here: Home >< Physics

# Free fall, why doesn't it fall? watch

1. okay so the thing in orbit has a centripetal force acting on it, directed towards the earth, why doesn't it fall?
so many theories around

1. the curvature of the earth drops at same rate
2. force at right angles to motion and as work done= force x distance, when force is in direction of displacement no overall work is done.
2. work is done, but changing velocity not bringing oject closer to earth

what would you write and what would get me the mark?
2. (Original post by Tanzie)
okay so the thing in orbit has a centripetal force acting on it, directed towards the earth, why doesn't it fall?
so many theories around

1. the curvature of the earth drops at same rate
2. force at right angles to motion and as work done= force x distance, when force is in direction of displacement no overall work is done.
2. work is done, but changing velocity not bringing oject closer to earth

what would you write and what would get me the mark?
The first number 2.

2. Force at right angles to motion and as work done = force x displacement in direction of force, when force is in direction of displacement no overall work is done.
3. ta muchly
4. (Original post by Tanzie)
ta muchly
Actually both of 1 and 2.1 are correct. 1 is I believe is the first method of explain the idea of orbits. Newton stated that a shell fired from a great hight above the earth surface would travel further as you increased the firing velocity, until it reaches a point where the shell would travel a path where the earth dropped off as quickly as the shell does.

In fact tho, 2.1 is correct but doesn't actually prove anything. It brings up yet another problem, WHY is the force perpendicular to motion. Gravity is a central potential force and not like the magnetic field acting on a charged particle.

The best way to describe orbits I believe is to paraphrase from Douglas Adams, "[Orbits are] learning how to throw yourself at the ground and miss."

Another model is that in orbits the gravitational force is canceled by the centrapetal acceleration. The idea of this is that angular momentum is conserved. So like water in a plug hole as a planet falls closer to the 'plug hole' it spins faster. As it spins faster, unlike the plug hole, not having any resistance it misses the plug hole and gets 'spit out' the other side. Then it loses speed and once again falls down into the plug hole etc etc.
5. so which would i write? in an exam?
6. I think the question is wrong, it does fall!

but as the aceleration of free fall is equal to the centripetal acceleration of the circular path (a=v^2/r), it travels in a circular path (maybe elliptical aswell but hey im not a physicist)

maybe if you made your question a bit clearer you would get answers more like what you want.
7. (Original post by Tanzie)
so which would i write? in an exam?
The one I said, as that's what they're looking for in the AQA mark schemes, anyway. The first sounds horribly vague and the last I feel doesn't really answer the question why; it just re-states the point.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: June 11, 2006
Today on TSR

### Cambridge interviews

Find out who is getting invitations

### University open days

• University of Roehampton
Sat, 17 Nov '18
• Edge Hill University
Faculty of Health and Social Care Undergraduate
Sat, 17 Nov '18
• Bournemouth University