Gravitational potential energy question - normal force on us

In summary, the conversation discusses the concept of normal force and why we feel a greater force when falling from a certain height despite our weight remaining the same. It is explained that our acceleration is not the same when standing still and falling, and that the normal force depends on the acceleration relative to free fall. The example of a bullet shot from a gun causing more damage than a bullet thrown by hand is used to further illustrate this concept.
  • #1
Vash25
4
0
Hi,

If we are standing on the ground, the Earth applies a force equal to our weight to us, but why do we feel a greater force when we fall to the ground from a certain height? Our weight is the same along this small height because our mass and acceleration are the same and, even so, the normal force is greater.

Thanks
 
Physics news on Phys.org
  • #2
To keep you at rest while you are at rest relative to the ground, the ground must exert a force mg on you directed up. To stop you from moving and cause you to be eventually at rest relative to the ground, the ground must exert a force greater than mg directed up. Once you come to rest, the force that the ground exerts is back to being mg up.
 
  • Like
Likes Delta2
  • #3
Vash25 said:
Summary:: why do we feel a greater force when we fall to the ground from a certain height?

Our weight is the same along this small height because our mass and acceleration are the same and, even so, the normal force is greater.
No, your acceleration is not the same. When you're standing still your acceleration is zero. When you fall to the ground, you have an acceleration between the time that you first make contact with the ground and the time you come to a stop.

You may be under the misconception that when you are standing still the upward force equals ##mg##, but that does not mean that your acceleration is ##g##.

Your acceleration is ##g## when you're in free fall, but when you are standing still on the ground you are not in free fall.
 
  • #4
One way of thinking about it is that the average supporting force holding you up over time comes out to mg. If it were less, you'd wind up accelerating downward on average. It it were more, you'd wind up accelerating upward on average.

If you fall further, you take longer to fall. When you hit bottom, the ground has to make up that missing supporting force. The longer the interval of support force you miss while falling, the more extra support the ground has to make up when you smash down. [And you make a bigger hole in the ground and get more broken bones in your body]
 
  • #5
If you remove gravity, it might be easier to understand.

Why does a bullet shot horizontally from a gun cause more damage than a bullet thrown by hand?
The answer has nothing to do with gravity.
 
  • Like
Likes Lnewqban and nasu
  • #6
Vash25 said:
If we are standing on the ground, the Earth applies a force equal to our weight to us, but why do we feel a greater force when we fall to the ground from a certain height? Our weight is the same along this small height because our mass and acceleration are the same and, even so, the normal force is greater.
The acceleration relevant for the support force is the acceleration relative to free fall (proper acceleration that an accelerometer measures):

- If you are standing on the ground your proper acceleration is 1g upwards, hence the support on you from the ground is mg upwards.

- When reducing vertical downward speed your proper acceleration is more than 1g upwards, hence the support on you from the ground is more than mg upwards.
 
  • #7
Mister T said:
No, your acceleration is not the same. When you're standing still your acceleration is zero. When you fall to the ground, you have an acceleration between the time that you first make contact with the ground and the time you come to a stop.

You may be under the misconception that when you are standing still the upward force equals ##mg##, but that does not mean that your acceleration is ##g##.

Your acceleration is ##g## when you're in free fall, but when you are standing still on the ground you are not in free fall.
Thank you Mr. T.

I have some doubts about your answer: I understand that the vertical net force when we are standing on the ground is zero, therefore our acceleration is also zero. Ok, in free fall our acceleration is "g". Imagine that we fall from two different heights, when we fall to the ground we feel a greater force under our feet when the height is greater, but the force that we exert on the ground must be the same "mg" for both heights, therefore the normal force on us should be "mg" regardless of height. I know something in my reasoning is wrong but I don't know what. Thanks
 
  • #8
kuruman said:
To keep you at rest while you are at rest relative to the ground, the ground must exert a force mg on you directed up. To stop you from moving and cause you to be eventually at rest relative to the ground, the ground must exert a force greater than mg directed up. Once you come to rest, the force that the ground exerts is back to being mg up.
Thanks Mr. Kuruman,

I still have a doubt. Imagine that we fall from two different heights, when we fall to the ground we feel a greater force under our feet when the height is greater, but the force that we exert on the ground must be the same "mg" for both heights, therefore the normal force on us should be "mg" regardless of height. I know something in my reasoning is wrong but I don't know what. Thanks
 
  • #9
Vash25 said:
but the force that we exert on the ground must be the same "mg" for both heights, therefore the normal force on us should be "mg" regardless of height.
Newton's third law: The downward force of you on the ground is equal to the upward force of the ground on you.

Newton's second law: The net force on you is equal to your acceleration times your mass.

The net force on you is the vector sum of the upward force of the ground on your feet and the downward force of gravity on your whole body.

When you are in the process of landing on the ground, you are going from a state of falling down to a state of standing still. You are accelerating upward.

Put this all together. The upward force on you from the ground has to be enough more than the downward force of gravity to account for your upward acceleration. So naturally this upward force has to exceed mg. It follows that the downward force of you on the ground has to exceed mg.

If you fall farther then when you land, you either have to accelerate upward more strongly or accelerate upward for a longer interval or both.
 
  • #10
Vash25 said:
the force that we exert on the ground must be the same "mg" for both heights
This is not correct. Can you explain your reasoning why you believe this?
 
  • #11
Vash25 said:
Imagine that we fall from two different heights, when we fall to the ground we feel a greater force under our feet when the height is greater, but the force that we exert on the ground must be the same "mg" for both heights, therefore the normal force on us should be "mg" regardless of height.
The force that your feet exert on the ground is equal in magnitude to the force that the ground exerts on your feet (Newton's Third Law).

The magnitude of these forces is greater when you fall from a greater height.
 
  • #12
Vash25 said:
Imagine that we fall from two different heights, when we fall to the ground we feel a greater force under our feet when the height is greater, but the force that we exert on the ground must be the same "mg" for both heights, therefore the normal force on us should be "mg" regardless of height.
The exact impact force depends on the acceleration of the center of mass during the impact, which depends on many factors, like deformation of the ground and of the object. Estimating this can get very complicated, especially for complex objects like humans, which can bend their joints to dampen the fall, and thus control the maximal impact force to a certain degree.

So impact force is not a fixed value for given fall height, but must be greater than mg for some time period.

Vash25 said:
I know something in my reasoning is wrong but I don't know what.
Yes, but you don't really present the reasoning, just its result, so we cannot point out the error in it more precisely.

Try drawing a free body diagram and applying Newton's Laws to it.
 
Last edited:
  • Like
Likes Dale
  • #13
You have additional momentum when you fall to the ground from a certain height (because your potential energy has been converted to kinetic energy). The force from the ground has to counter both your weight and the additional momentum. To be more precise it is the change (from having velocity ##v=\sqrt{2gh}## where h the height, you come to a full stop) in your momentum that requires the additional force.
 
  • Like
Likes Lnewqban

1. What is gravitational potential energy?

Gravitational potential energy is the energy that an object possesses due to its position in a gravitational field. It is the potential for an object to do work when it is moved from one position to another in the presence of gravity.

2. How is gravitational potential energy calculated?

Gravitational potential energy is calculated using the equation PE = mgh, where m is the mass of the object, g is the acceleration due to gravity, and h is the height of the object above the ground.

3. How does the normal force affect gravitational potential energy?

The normal force does not directly affect the gravitational potential energy of an object. However, it is necessary to keep an object in equilibrium when it is on a surface, and this equilibrium is maintained by the normal force.

4. How does the normal force on us change with gravitational potential energy?

The normal force on us does not change with gravitational potential energy. The normal force is a reaction force to our weight, which is determined by our mass and the acceleration due to gravity. As gravitational potential energy increases or decreases, our weight and the normal force remain constant.

5. Can gravitational potential energy be negative?

Yes, gravitational potential energy can be negative. This occurs when an object is below the reference point, such as when an object is on the ground. In this case, the gravitational potential energy is considered to be zero, and any position below the reference point will result in a negative value for gravitational potential energy.

Similar threads

Replies
12
Views
2K
Replies
9
Views
2K
Replies
10
Views
934
Replies
27
Views
11K
Replies
12
Views
898
Replies
9
Views
1K
Replies
11
Views
2K
  • Mechanics
Replies
3
Views
1K
Back
Top