Question

Work done by a force on an object equals the magnitude of force along displacement of object times the displacement of point of application of force by the force. Since internal forces cannot do work on any body, which force exactly does work on our body when we walk?

Answer 1

@Vincent-Lyon.Fr

Answer 2

Internal forces cannot "accelerate" the system.

There is no restriction on whether or not they are capable of doing work.

In case of walking -
work is done by our body's internal forces
acceleration is provided by static friction force

Answer 3

What LastDayWork has just said is exactly what I would have answered.

Internal forces DO WORK, and their work has to be taken into account when you apply the work-energy theorem.

Caution: most English-speaking books have wrong statements in this respect when they state that the friction force can provide you with energy, which is wrong. The work of the internal forces by your muscles provides you with energy.

Answer 4

Ok Thanks. I did not know that. So the work done is by the internal forces and the acceleration caused is due to the frictional force which is a consequence of the internal force when we walk?

Answer 5

@MayankD
Consider the situation (my teacher first explained it to us this way)

You participated in a marathon. After successfully completing it (wont talk about your position :P) you looked exhausted.

Your friend approaches you and asks - "Why do you look so exhausted? Whatever work was done was performed by the friction force. :D" What would be your response??

Answer 6

..sorry to inturrupt.. But work done by friction is negative right??

Answer 7

@AnthonyStark
Net work done is zero.

Answer 8

then how do we move forward?

Answer 9

Lol :P . Net work is 0. How?

Answer 10

same doubt here....

Answer 11

Work done by internal friction forces is:
- nil if the moving parts roll on each other
- negative if they are slipping

Work done by external friction forces is:
- nil if the system rolls on a fixed base
- negative it slip on the fixed base
- positive, negative or nil if the base is moving (think of a conveyor belt for a positive work done by friction)

Answer 12

So while walking we do not slip. So work done by friction is 0?

Answer 13

Net work done by static friction is zero.
Net work done by kinetic friction is negative.

While walking we experience both (although we study only about static).

We walk because of work done by our body's internal forces.

Answer 14

But when we walk , i guess friction doesnt do any work because there is no relative motion between our feet and ground...

Answer 15

^^^ i meant in 1 step

Answer 16

oohk i get it ...work done by static friction is 0...thanks @LastDayWork

Answer 17

Ok here's my doubt. What force does the internal force from our body contribute to?:
1)Normal force on ground by our body
2)static friction acting on our body

Answer 18

While walking, when the forward foot touches the ground; it slips (and kinetic friction perform negative work).

After that static friction in our forward foot de-accelerate our body where as that on our backward foot accelerate it.

Answer 19

Work is done by our body in sustaining the angular motion of our legs.

Answer 20

Q1: So while walking we do not slip. So work done by friction is 0?
Q2: But when we walk , i guess friction doesnt do any work because there is no relative motion between our feet and ground...
A1+2: Correct!

Q: oohk i get it ...work done by static friction is 0...thanks
A: yes and no, as I said this is true only on a fixed ground, not on a conveyor belt.

Q: What force does the internal force from our body contribute to?: 1)Normal force on ground by our body 2)static friction acting on our body
A: Both, actually. Imagine you are jumping forward: what you muscles do will create by reaction an increase in the normal force by the ground and a friction force that will accelerate you forward.

Answer 21

^^
1st para - agreed (in ideal case)
2nd para - agreed
3rd para - apparently true (but I am not convinced how can we alter the normal reaction force)

Answer 22

Ok you guys lost me. @LastDayWork can you please re-explain the process of walking and @Vincent-Lyon.Fr can you please explain how the normal reaction changes when we change our posture(i'm guessing its something to do with the center of mass and torque)

Answer 23

@Vincent-Lyon.Fr
In every other case of push and pull, there exists another force (except the concerned action-reaction pair) like friction or normal reaction from a 3rd body, which helps us in performing the task.

Out body's strength depends on its ability to remain rigid.

But in case of walking (running/jumping) - how can we independently alter the normal reaction force being exerted on our body by the ground??

Please explain..

Answer 24

Q: 3rd para - apparently true (but I am not convinced how can we alter the normal reaction force)

A: Imagine you are standing on a balance. The reading of the balance gives you an idea of the normal force it is acting on you.
Now if you jump, you push on the balance, which reacts with an extra push and the reading increases.
The sum (Weight + normal force) is not balanced anymore and you are accelerating upwards because normal force is greater in magnitude than your body weight.

Answer 25

And how do we generate that extra push?

Answer 26

In terms of physics

Answer 27

Some muscles contract, pulling on some ligament or tendon which make your legs unfold like a lever. Your feet press on the support.

Answer 28

@MayankD
http://en.wikipedia.org/wiki/File:Blender3D_NormalWalkCycle.gif

Watch the animation in the URL and read my earlier post.

forward foot - the foot at front of the plane of torso
backward foot - the foot at back of the plane of torso

Answer 29

Ok so the trick is to not assume the body to be a point mass but a rigid body.

Answer 30

But when we walk there is no slipping(relative motion ) with the ground else we'd fall. So how does kinetic friction do work

Answer 31

Compare the landing of forward foot to the landing of an aircraft, you'll surely get the answer.

Answer 32

There is no need in invoking kinetic friction to explain why you are tired when you walk or run.
It is just that our muscles are not reversible machines and we do not get back the work we have done by going up when we go down again. In other words, we are not elastic enough.
This is why there was such a big controversy when Oscar Pistorius claimed he had no advantage running with his blades. Perfectly elastic blades do indeed improve the legs' efficiency.

Answer 33

@Vincent-Lyon.Fr
I referred to kinetic friction simply to complete the discussion. It doesn't explains why our muscles get tired.

I want to learn about the dynamics of walking with respect to the work done by our body (as I cannot understand how the internal forces of a system can alter a singular external force). Can you recommend me some book(s) and/or tell me which branch of Physics deals with this concept?