# Acceleration of gravity and Newton's laws of motion

1. Jul 27, 2015

### pinochet

hello i've just started learning physics my question is why do force increase while falling and stay constant while on the ground.i know force is increasing because in free fall things accelerate which according to newton first law force must also go up. the force of gravity of earth is 9.8N per kilogram so i figure if my mass is 83.28 kg than the force of gravity should stay 822 newton which it does if something support your weight so why do thing accelerate i the way i see it free shall should not change there velocity if the only force is gravity which is porportional to mass .

2. Jul 27, 2015

### jbriggs444

You seem to be under the impression that force is proportional to velocity -- that is, that the faster you are going, the more force is required to keep you going that fast. That turns out not to be correct. Newton's first law says that it takes no force at all to keep a moving object moving.

Newton's second law says that it takes force to make an object accelerate.

3. Jul 27, 2015

### Svein

I have trouble understanding your question but..
1. If you are falling, the "force of gravity" is working on you and making you accelerate.
2. As long as we are talking about things happening in the close vicinity of Earth, the force does not "increase while falling".
3. When you are on the ground, the ground applies a force on you that is equal to the "force of gravity", but in the opposite direction. Net force on you: 0

4. Jul 27, 2015

### pinochet

yes force has to increase for you to go fast i dont mean to keep from slowing down, if force stay the same ,velocity should stay the same. so if im given a force of 822N due to my mass in earth gravity if i were to fall i show fall with a force of 822N
this force that is acting on me is it 9.8N per kilogram

5. Jul 27, 2015

### jbriggs444

That is simply not true. If net force is non-zero and stays the same, you go faster and faster without bound.

Edit: There is a bound if you invoke special relativity. But let's not go there.

6. Jul 27, 2015

### Staff: Mentor

Right, and it is always the same, no matter whether you are falling, jumping upwards, standing on the ground or doing whatever, as long as you don't go to space.

Force and velocity are different things. While falling, your speed is increasing because there is a force accelerating you downwards. While standing on ground, the downwards force from gravity is in balance with the upwards force from the floor, so you do not accelerate.

7. Jul 27, 2015

### Staff: Mentor

That is incorrect. Acceleration is a function of force, as the equation A = F/M states. If there is a net force applied to an object, then there is acceleration, which means that the velocity is changing. A net force of zero means that there is zero acceleration and velocity does not change. This doesn't mean velocity is zero, only that it is is not changing.

If you drop an object near the surface of the Earth, the object accelerates at effectively a constant rate as the net force applied to the object by gravity is very close to constant.

8. Jul 28, 2015

Force doesn't increase while falling.

Force = mass * acceleration. The acceleration is a constant given by g = 9.8 m/s^2. And the mass is a constant too. Thus the force is a constant too.
What is increasing is the velocity. A constant acceleration (different from 0) means that the velocity is increasing linearly in time, while the position is increasing quadratically.

The force of gravity (the weight) while falling is the same as while on the ground. When falling, the only force acting on the object is the force of gravity => constant 9.8 m/s^2 acceleration => velocity increases.
When on ground, the forces that act upon the object are the force of gravity pointing downwards and the normal force pointing upwards. Thus the net force is 0 => acceleration is 0 => velocity stays at 0.

Last edited: Jul 28, 2015
9. Jul 28, 2015

### Staff: Mentor

I think you guys may be missing the issue based on the grammar issues in the OP. It appears to me that what the OP is saying is true:

If you are standing on the ground, the net force on you is zero. If you are falling, the net force on you is 9.8N/kg.

So, pinochet, how can this be? Hint: standing on the ground.

10. Jul 28, 2015

### pinochet

Alrigh
so newton is wrong if an object
well
while on a diving board i according to netwon's third law my weight is coming back to me from the diving platorm netforce on me is zero so no acceleration.but where i get confused is when you jump from the diving platform,and the forces become unbalance so my velocity would become to increase downward.i have trouble seeing how with a constant force of 822 Newtons i can continue to increase velocity downward without force on me increasing as well .well if force dont increase while falling then
okay so a constant unbalance force causes a constant rate of accleration and not a constant velocity as i seem to have thought

11. Jul 28, 2015

### pinochet

okay i think i get it unbalance forces will cause a constant acceleration and not a constant velocity as i previously thought.

12. Jul 28, 2015

### jbriggs444

This is a frequently seen misunderstanding of the third law. The pressure that the soles of your feet exert on the diving board is equal and opposite to the pressure that the diving board exerts on the soles of your feet. That is all that Newton's third law says.

Your weight (the downward force of gravity on you) is not always equal and opposite to the force of the diving board on the soles of your feet. If you are accelerating upward on the diving board, it is almost certainly because the force of the diving board on the soles of your feet is greater than your weight.

13. Jul 29, 2015

### Finny

exactly.

As Drakkith posted, F = Ma.

Think about the forces you feel in a car: when accelerating and in contrast when proceeding at a constant velocity.