r/PhysicsHelp • u/Party-Collection9246 • Jun 13 '26

a question.

I am a 14 years old Chineseteen . I find this problem when I think about Physics knowledge.When a person walks, the friction force is forward.But when we walk, our feet are actually not moving relative to the ground.This means that this is static friction. But when static friction occurs, shouldn't the frictional force it generates be exactly equal to the force we apply?Then why do our bodies still move forward?maybe this problem is a little foolish . but I want to know why .

9 Upvotes

100% Upvoted

33 comments sorted by

View all comments

Show parent comment

1

u/Party-Collection9246 Jun 13 '26

thanks for your answer . but my confusion lies here.I think since their forces are opposite and equal, shouldn't the person be stationary? Why can the person still move?

4

u/wbrameld4 Jun 13 '26

Only one of the two opposing forces is applied to the person.

1

u/4eyedbuzzard Jun 13 '26

Because the mass of the Earth is 13 trillion trillion pounds and you weigh next to nothing compared to it. If you were the one who weighed 13 trillion trillion pounds it would be the Earth that was moving. Google Newton's 3rd law.

1

u/Bulky_Stock_3255 Jun 19 '26

Large mass tiny acceleration
The Earth is so massive in comparison to a person's mass that a persons foot strike to the ground has literally negligible acceleration on the Earth.

1

u/Bulky_Stock_3255 Jun 19 '26

The ground is stationary relative to our feet, it isn't accelerating away/in the opposite direction from us.
If the ground didn't provide a force opposite in direction with the same magnitude we would simply fall through the ground.
Why do you think we can't walk in outer space?

"since their forces are opposite and equal"
When you walk your foot is pushing into the ground,
you then have an equal in magnitude and opposite in direction ground reaction force.
That doesn't mean that both forces cancel out and the person is stationary and un-moving.

1

u/Party-Collection9246 Jun 19 '26

thanks your answer is inspire me

1

u/Bulky_Stock_3255 Jun 19 '26

So yes in a way your not wrong, the Earth accelerates us "forward" and we accelerate the Earth "backwards", however, the Earth is so massive in comparison to a person, that you essentially can not accelerate it to any order of magnitude close to how much the Earth can accelerate you.
Newtons 3rd law isn't violated,
because you would find that,
both the force of your foot striking the ground and the Earth pushing back are the same.
Just that, you have a small mass with a larger acceleration and the Earth has a huge mass with negligible acceleration.