# Direction of frictional force when running

1. Jun 30, 2015

### sach963

When a person is running is frictional force forward or backward?
I do not have good knowledge of

2. Jun 30, 2015

### BiGyElLoWhAt

Well, they're moving forward, pushing their legs backwards. Newton's 3rd law says the force A exerts on B is equal and opposite of the force B exerts on A. What does this say to you?

3. Jun 30, 2015

### A.T.

Both, but for steady speed the net momentum transfer from the ground is forward to counter air resistance (unless on treadmill).

4. Jul 1, 2015

### sach963

So, this means that friction between ground and shoe is forward?

5. Jul 1, 2015

### A.T.

The horizontal force (friction) on the shoe is backward in early stance phase, and forward in late stance phase, See the smaller component in the plot in the video above: negative = backwards, positve = forward. The bigger, always positive, component is the vertical force on the shoe (normal force).

6. Jul 1, 2015

### Chandra Prayaga

Much of the confusion in all similar situations arises because of using imprecise language. For example, regarding the question by sach963, "So this means friction force between ground and shoe is forward?", there are TWO forces of friction between ground and shoe. One is the force by shoe on ground, and the other by ground on shoe. It is the force between ground and shoe that needs to be ultimately forward so that the person is propelled forward by it.This is clearly stated by A.T. above, "force on the shoe". When your shoe pushes back on the ground, the ground pushes forward on your shoe.

7. Jul 1, 2015

### Chandra Prayaga

Sorry, the wrong words got cut and pasted in my response, here is the correct one:
Much of the confusion in all similar situations arises because of using imprecise language. For example, regarding the question by sach963, "So this means friction force between ground and shoe is forward?", there are TWO forces of friction between ground and shoe. One is the force by shoe on ground, and the other by ground on shoe. It is the force BY ground ON shoe that needs to be ultimately forward so that the person is propelled forward by it.This is clearly stated by A.T. above, "force on the shoe". When your shoe pushes back on the ground, the ground pushes forward on your shoe.

8. Jul 1, 2015

### Chandra Prayaga

Incidentally, A.T., that was a very nice video. Can I show it in my class?

9. Jul 1, 2015

### A.T.

With air resistance or during acceleration the average force will be forward. On the treadmill the average horizontal force is approximately zero.

It's not mine. It comes from here:
http://www.smu.edu/Simmons/Research/LPL

10. Jul 1, 2015

### Chandra Prayaga

Thanks

11. Jul 2, 2015

### sach963

Thank you all for your replies.