What is the net force involved in jumping?

[AZhang]

Ok, so I'm trying to understand the forces involved in jumping, more specifically, right before a person takes off (as in the part where they press into the ground). I know that the weight force is pushing the person down and that the normal force is bigger than the weight force to create a net force that points upwards. But how can the normal force suddenly become larger? I know that the body is pressing down on the floor, but then wouldn't there be both a weight force and the force of the body pressing down, which means that the normal force and the other two forces cancel out, meaning that there is equilibrium? I'm so confused...

 

[Waveparticle]

When your talking about a static object, such as you standing on the sidewalk then the gravitational force pulling you down is equal to the normal force pointing up. When you jump in the air the normal force overcomes gravity and you are lifted in the air. When you come back down to Earth after jumping, the normal force rapidly decelerates according to the flexibility of the sidewalk and then the normal force and gravitational force are equal again.

 

[AZhang]

But how does the normal force increase?

 

[Waveparticle]

Normal force is not an actual force but more of a pseudo force such as centrifugal force. The normal force is more of a supporting force that prevents you from being pulled directly to the core of the Earth. The normal force increases because of the weakness of gravity and your ability to overcome the entire pull of the Earths mass.

 

[AZhang]

But I thought the normal force is the force the surface exerts on the object. So if the normal force is increasing when you're pushing down, then wouldn't there be some kind of force making it increase?

 

[AZhang]

Well, I guess my main question would be:

If a normal force is the force the surface exerts in reaction to the forces exerted on it, then how will there ever be anything that is not at equilibrium? Because won't the normal force always equal the forces exerted on the surface?

 

[Waveparticle]

The force making the normal force increase as you push down is created by the surface you are standing on. Let's say you are standing with 10N of force on a surface. As you increase the force to say 20N to jump, the surface also increases its force to 20N.

 

[AZhang]

So you are adding a force to the floor?

 

[willem2]

Normal force is not an actual force but more of a pseudo force such as centrifugal force. The normal force is more of a supporting force that prevents you from being pulled directly to the core of the Earth. The normal force increases because of the weakness of gravity and your ability to overcome the entire pull of the Earths mass.

A pseudo force is a force that appears because in an accelerating or rotating reference frame, an object will accelerate, seemingly without a force present. To make F=ma valid again, a pseudoforce is invented. In an unaccelerated reference frame, there will be no force and the object will move in straight line, or won't move at all.

The normal force is a real force. It is the force that will hurt you if you fall and reach the ground.

The normal force increases, because you push against the ground with your legs, the ground will be compressed slightly, like a spring that gets compressed, and the ground will push back harder. You'll start to accelerate until you leave the ground.

 

[mrspeedybob]

So you are adding a force to the floor?

Exactly.

 

[AZhang]

But if you are adding one, wouldn't there be equilibrium?

 

[Waveparticle]

But if you are adding one, wouldn't there be equilibrium?

You would be in equilibrium when you are static but as soon as you begin to accelerate upward you are no longer in equilibrium

 

[Integral]

When jumping the body converts chemical energy to motion. As you straighten your body to jump, an additional force due to the upward acceleration of your body is created.

The net force on the floor is the sum of your gravitational normal force and this force of acceleration.

In this I am treating the gravitational normal force as the body's mass times g.

 

[Darryl]

Would the 'force of G acting on our bodies, be the amount of energy to accellerate us at 9.8m/s/s ?

So if it was not for the ground, we would be falling towards to earth, and I am guessing that there would be some way to calculate the escape velocity ?

Would that be the equivalent energy to accellerate a body at 9.8m/s/s ? against gravity, so that the speed of escape would be that accelerating x the distance to 'free' space?