# Day 40: BFPM Stumper

A student working in his capstone wants to know: what is the force with the question mark? In other words, what force balances the friction that propels a runner moving at constant speed? (Or is this a case where the particle model breaks down?)

HS Physics Teacher constantly questioning my teaching.

### 5 responses to “Day 40: BFPM Stumper”

1. Brian Frank says :

Here’s my best idea. I think when your foot first plants down, the ground applies a force backwards on the runner’s feet. This force causes a torque about the center of mass, allowing the COM of mass to get ahead of your foot plant. As this shift in com happens, the force shifts from backwards to forward… accelerating the body forward… so I don’t think that it’s a balanced force model… but unbalanced force model moving rapidly back and forth between unbalanced backwards and unbalanced forward… any one else?

2. Frank Lee says :

what he said ^

3. Frank Lee says :

hey this just turned into a triple frank post up in here

4. Noah Segal says :

Sorry to break the Frank streak. The PASCO force plates can measure transverse as well as normal force on them. At an AAPT meeting we ran over a few and saw exactly what Brian suspected: when planting a foot, there is a backward force and during the second part of the stride there is a forward force producing equal impulse.

5. Frank Lee says :

When I was in track, the coach said we should sprint on our toes, so I did. And I was faster. I wondered why. Figured when the heel strikes the ground, it sorta "stops" you, while if you land on your toes… it sorta just keeps you up while you’re moving forward. Wonder if I can measure that with the PASCO force plates hm…