When I stand up is the normal force causing the acceleration upwards?

Question

I am confused at how forces work. If I am stationary on hard ground there is force of gravity and the normal force.
If I am squatting down and then I stand up I am accelerating as I start to stand up. I don't think Fgravity can change so how does the Fnormal change because of the acceleration? or is it the Fnormal that does not equal Fgravity anymore that is causing the acceleration?

As you can see I am quite confused. All I know is that since there is movement there is now an acceleration butI dont really know how the forces work to cause the acceleration.
thx!

so why does standing up cuase a greater force downwards?

Answer 1

Ok, I'm kind of confused on how you asked this. I will explain the basic concepts of gravity and acceleration. Gravity for us on the scale that you're using is the Earth's pull on objects on the earth or around the earth (such as the moon). If you are stationary on hard ground, then there is a force of gravity acting down on your body. This causes you to remain on the ground and gives you weight. The real measurement using force would be the force of gravity times your mass. This would give you your weight in Newtons since force is measured in Newtons. You are pushing down on the Earth with your weight in newtons but there is also a force pushing back up on you, the Earth! If there was not this equal force pushing back on you, you would go through the Earth or at least sink in until an equal force could meet your force.
With your example, if you are squatting down and then stand up you are, indeed, accelerating as you start to stand up. Gravity does not change. However the force that you exert changes. Your body exerts a force upward that is greater than the force of the gravity acting down upon your body. This is what allows your body to stand up. The acceleration is caused by your body putting exerting more force than the gravity is exerting on you to the Earth.
Acceleration itself is caused by an increasing or decreasing force. A car is an excellent example of this. If the gas pedal is pushed on a car, the car is no longer going at a steady velocity, it is now accelerating or speeding up. The force of the pedal is causing the car to change its velocity, this is acceleration. If the brake pedal is pushed on a car, friction is the force that causes the car to accelerate negatively, or slow down. This is also acceleration.
So, to tie it all together. If you are stationary and you are squating down, you are not moving and you have no velocity. When you begin to stand up, your body is exerting a force greater than that of gravity that allows your body to change its velocity and accelerate upwards.

Answer 2

Ordinarily the normal force is equal and opposite to your weight (W=mg).

When you stand up, you are exerting a greater-than-usual force on the ground (F' > mg), and thus the normal force is greater.

When you have fully stood up, your body has upward motion and decelerates; the corresponding force reduces the apparent weight briefly (F'' < mg) until your body come to rest, when again F = W = mg.

You can prove all this by squatting on a weighting scales, standing up and watching your apparent weight change.

Answer 3

Its about transfer of weight and the normal force.

When you sit (on the ground), your center of mass is close to your hips and the normal force is acting on you at the contact point.

When you stand up, the internal forces of your body move you center of mass and your feet become your point of contact. So the point of application of the normal force changes.

Sitting to standing is actually a very complex process of shifting your center of mass, applying the right forces at the right angles to the ground, so that you get the right reaction (read normal forces).

No wonder Robots can't do that!!

Answer 4

Well, as you are standing up, you are adding another force, force caused by accelerating your own body up, that force, added to the normal force would be your mass times the acceleration of your body, plus your normal force(force that your body pushes on the ground caused by acceleration of gravity at rest).
When you are at rest, the force of gravity is your normal force, because that doesn't change, it's constant.
Final equation when you are standing up, say at a constant acceleration(1m/s(squared)) is
(Acceleration(standing up)+acceleration of gravity) multiplied by your mass.
or
(1m/s(squared)+9.81m/s(squared))times (your mass)=Total force