# Force Body Diagram

1. Nov 3, 2013

### jackiemendez

This may be a stupid question, but could any of you help me with the force body diagram of a person running? Before the foot hits the ground, once the foot hits the ground, and after the foot hits the ground?

I'm having trouble trying to figure out what forces are being applied to the person running...

The person is 115 pounds.

2. Nov 3, 2013

### flyintiger

I believe there is a force down only (force of gravity * mass) when the runner is in the air, and then after striking the ground there is an added force of kinetic friction applied in the opposite direction of the foot movement.

3. Nov 3, 2013

### tiny-tim

hi jackiemendez!

running is a bit complicated

there is a period when neither foot is on the ground

during that period, the body is rotating forwards (ie the angle is changing), and is falling

each push with the foot has to reverse that fall and that rotation, as well as push the whole body forwards

4. Nov 3, 2013

### haruspex

You mean static, right? And of course there's the normal force from the foot on the ground.
To add to tiny-tim's post, when a foot hits the ground it is in front of the CoM, so the normal and frictional forces have a moment tending to tip you backwards. As tiny-tim says, that reduces the falling forwards. But as the CoM moves forwards, the torque from the normal force on the foot reverses, and this exceeds the torque from the frictional force. So now the body is tipped forwards again, completing the cycle.
There are distinct anatomical features to cope with this tipping backwards and forwards. These include the gluteus maximus (so it's not primarily for sitting around) and the external occipital protuberance (that lump on the back of your skull just above the cervical column) and the tendon that attaches to it. The bony features are used in analysing how our distant ancestors moved.

5. Nov 3, 2013

### jackiemendez

Thank you to all who responded! I think I sort of get it... Hopefully

I was curious to how to draw the diagram since I have an assignment to do on FBD, but our class never really touched on bio-mechanics and I was hoping to do a FBD on a runner, I just wasn't sure how to actually start it.