Discussion
Which diagram best represents the force diagram for the student while he is on the roof?
*This question is included in 04. Gravitation, question #45
| (A) |  |
| (B) | ... |
| (C) | ... |
| (D) | ... |
| (E) | ... |
| (F) | ... |
The solution is
Posted: 07/19/2013 18:44
Why isn't it D should friction to the left balance out the force of the ground to the right?
Posted: 07/22/2013 15:53
Q: Should friction to the left balance out the force of the ground to the right?
A: Actually, the force of the ground to the right IS equal in amount to a friction force, but it is static friction not the kinetic one you are thinking about. Every time the runner's foot hits the ground, the runner is acting upon the ground a horizontal force equal to the friction force (the Normal force x static friction coefficient), and the ground in reaction is pushing the runner back using the same amount (3rd Newton's law).
On a slippery floor, the force against the ground by the foot may overcome static friction, hence the foot will slip, and when it slips, as it moves forward there will be a kinetic friction force to the left that will slow down the person.
A: Actually, the force of the ground to the right IS equal in amount to a friction force, but it is static friction not the kinetic one you are thinking about. Every time the runner's foot hits the ground, the runner is acting upon the ground a horizontal force equal to the friction force (the Normal force x static friction coefficient), and the ground in reaction is pushing the runner back using the same amount (3rd Newton's law).
On a slippery floor, the force against the ground by the foot may overcome static friction, hence the foot will slip, and when it slips, as it moves forward there will be a kinetic friction force to the left that will slow down the person.
Posted: 07/22/2013 15:55
Note that in the force diagram, we are concerned only about the system of forces acting on the student / runner, and not about the ones acting on the floor. Hence we don't draw the force against the floor, vertical (equal to the weight of the student) or horizontal (equal to the static friction force).
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