# Homework Help: How can gravity act as a force on stationary objects

1. Mar 8, 2013

### jayjay55531

...if objects at rest have an acceleration of 0, and the net force on an objet equals m*a, then doesn't that mean there must be no net external force on the object? What am I missing here...??

2. Mar 8, 2013

### SammyS

Staff Emeritus
Gravity can be acting on an object that's at rest. It happens all the time here, near the surface of the earth.

If gravity acts on an object, but the object is at rest, then there must be some other force or forces acting on the object to counteract the effect of the gravitational force.

3. Mar 8, 2013

### jayjay55531

What would these forces be, in the example of a person standing on the ground?

4. Mar 8, 2013

### vela

Staff Emeritus
What's the difference between a ball falling through the air and one sitting on the ground?

5. Mar 8, 2013

### lep11

It doesn't really matter whether the object is moving or not. If the object is stationary or moving at constant speed, then the sum of all forces must be zero. When object is on e.g. the surface of the Earth or on a table there is always normal force preventing the object from sinking through the surface as per Newton's III law.

Last edited: Mar 8, 2013
6. Mar 8, 2013

### SteamKing

Staff Emeritus
If a person is standing on the ground, what is keeping him from falling through the ground?

7. Mar 8, 2013

### Wiemster

Objects at rest have a velocity of 0. If a net force acts on them their acceleration, the time-derivative of their velocity, will be non-zero making them move.

8. Mar 8, 2013

### SammyS

Staff Emeritus
For a person standing on the ground, the ground itself is pushing upwards on the person, supporting the person.

If a person is standing on a trap door, then as long as the trap door remains closed, it supports the person, by pushing up on the person. If the trap door is opened, and falls away, then the only force acting on the person is gravity, and the person will accelerate downward.