DIver on a diving board
An 82 kg diver is preparing to dive off the end of a 12 kg diving board. The board is a 4.0 m long plank with two points of support. One point is at the rear of the board and one 1.2 m in from the rear. What are the magnitudes and directions of the forces on the board at the two support points?
Answer
This problem is a static equilibrium problem because there is no net force and no net torque -- nothing is moving.
It's really important that you draw a free body diagram for this problem. Draw the plank and all the forces acting on it. Let's think about how many we have:
- Force of gravity on the plank, pointing down from the center of mass of the plank (the center)
- Weight of the diver, pointing down from the end of the plank
- Force from the inner support, pointing up
- Force from the rear support, pointing ???
Which direction does the force from the rear support point? Our instinct might be to say "up", but imagine what would happen if you took away the rear support. Pretend it's not there and it's clear to see that there is going to be a net torque on the plank; i.e. there would be nothing to keep it from tipping forward! So the force from the rear support must point down to balance out the torques. Once you believe this, set the net force and net torque equal to zero and solve for the forces.
Customer support service by UserEcho
This problem is a static equilibrium problem because there is no net force and no net torque -- nothing is moving.
It's really important that you draw a free body diagram for this problem. Draw the plank and all the forces acting on it. Let's think about how many we have:
- Force of gravity on the plank, pointing down from the center of mass of the plank (the center)
- Weight of the diver, pointing down from the end of the plank
- Force from the inner support, pointing up
- Force from the rear support, pointing ???
Which direction does the force from the rear support point? Our instinct might be to say "up", but imagine what would happen if you took away the rear support. Pretend it's not there and it's clear to see that there is going to be a net torque on the plank; i.e. there would be nothing to keep it from tipping forward! So the force from the rear support must point down to balance out the torques. Once you believe this, set the net force and net torque equal to zero and solve for the forces.