# Kinematics problem

1. Nov 20, 2014

### gracy

if any object touches the ground i.e standing on the ground,will there be acceleration due to gravity?if yes,then how much more can object go downward due to this acceleration as it is already on the floor or ground?

2. Nov 20, 2014

### Brul

The 'acceleration due gravity' is simply the force between two masses; in this case the object and Earth. When standing on the floor, floor provides the force in opposite direction according to Newton's 3rd law. But this force works on contact, so when falling in the air you accelerate towards the Earth.

3. Nov 20, 2014

### gracy

how can 'acceleration' be equal to force?

4. Nov 20, 2014

### Staff: Mentor

I don't quite understand your question. If you are standing on the ground at rest, then you are not accelerating. This is because the net force on you would be zero: Your weight would be balanced by the supporting force of the ground.

Perhaps you can rephrase your question if I am missing your point.

5. Nov 20, 2014

### Brul

If you're falling from the sky or something it's because there is a force pulling between you and Earth -> Fgravitational = mobject*g'acceleration due gravity' and there is no opposite force stopping you (except friction of air). In dynamics (or at least in inertial systems) the only force that works remotely is gravity, others on contact. Thus when standing on the solid floor, they will exert an opposite force on you, so you won't sink in. Of course, Earth still pulls on you with the same force F=mg.

6. Nov 20, 2014

### Staff: Mentor

Careful here. The force provided by the floor is not a Newton's 3rd law pair with gravity. (It's Newton's 2nd law that is relevant here.)

7. Nov 20, 2014

### Pharrahnox

The the action reaction pair is gravity acting on m1 by m2, and the gravity acting on m2 by m1. A rule (not sure if it's actually a rule) that I have found very useful for Newton's 3rd Law is that the action reaction forces must be of the same type, i.e. gravitational and gravitational.

The actual answer to your question has already been stated several times, so I won't bother to reword it again, unless you want further clarification.

8. Nov 20, 2014

### Taimoor Qamar

"if any object touches the ground i.e standing on the ground,will there be acceleration due to gravity?"
yes, there is always an acceleration due to gravity; if there wasn't you would have no "weight"
If there was no acceleration due to gravity, when you go to stand on a scale, there would be no force pushing down on the scale, and thus, it would read "0"

Newton's second law states Force=mass*acceleration; Your weight is a force;

"If yes,then how much more can object go downward due to this acceleration as it is already on the floor or ground?"

If it is already on the floor or ground, it cannot go any further. That does not mean it isn't still "accelerating"

The force of your weight is canceled out by an opposite force from the floor; If you think about this logically, its obvious, as people dont just go falling through the ground; There is an equal and opposite force from the matter that the ground is made up of, pushing up against the force of your weight, and thus, balancing out all the forces to a net force of 0;

9. Nov 21, 2014

### Pharrahnox

Actually, I would steer away from saying that you are accelerating. It could cause confusion. I believe the best definition for the OP is that the NET acceleration is zero, acceleration is a change in motion, because the forces cancel out, hence dv/dt (change in velocity over change in time) is zero.

Sorry to be nitpicky, especially if I'm actually wrong, just trying to avoid confusion for the OP.

10. Nov 21, 2014

### Staff: Mentor

I would say that there is always a force of gravity, not necessarily an acceleration due to gravity.

Best written as $\Sigma F = ma$ to emphasize that it's the net force that determines the acceleration.

Sure it does. The acceleration is zero.

Good!

11. Nov 24, 2014