Free body diagram question

Question

How would a free body diagram look like during the push-off of ones vertical jump (that is, the jump is vertical). Would it simply be a box with Fg pointing downwards or a box with both Fg and Fn (the latter pointing directly upwards with a greater magnitude)?

Answer 1

Hi Bob,

Your answer depends on the situation given. There are two cases:

Case 1: Feet are still in contact with the ground. In this case, the FBD will have Fg pointing down and Fn pointing up with Fn > Fg. Keep in mind that this is _only_ true when the feet are in contact with the ground. As soon as the feet leave the ground, Fn = 0 (as Fn is a *contact* force and only exists while two objects are touching).

Case 2: Feet are no longer in contact with the ground. In this case, the FBD will only have Fg pointing downwards (for the reason described above).

I hope this makes sense. If you have any questions, feel free to ask me. :-)