Bridge in the Context of A Spring.

[las2vc]

We have weekly problem sets that lead us through a "story" of sorts. In this week's, a group of tourists are on a newly-built, albeit poorly-designed bridge. The question reads:

"The tourists bend their knees and, at the count of three, jump upward simultaneously. While their feet are in contact with the bridge, as they straighten out their legs during their jump, are the tourists in equilibrium? Why or why not?"

I know that equilibrium means to have a constant velocity. I was thinking that they WERE in equilibrium because even though they're accelerating upward (?), they're not moving and therefore their velocity is constant at zero until their feet leave the bridge. Does this make sense? Is that appropriate physics 'language'? It seems that I'm missing some key terminology or something that would make it sound more articulate.

Thanks!
Laura.

 

[Nate810]

Yes, your reasoning is correct. The tourists are in equilibrium while their feet are in contact with the bridge, since they are not moving. You could say that the tourists are at rest and maintain a state of static equilibrium as they jump.

 

[kyle8921]

I would like to clarify that equilibrium does not necessarily mean constant velocity. Equilibrium refers to a state where all forces acting on an object are balanced, resulting in a net force of zero. In this scenario, the tourists are in equilibrium while their feet are in contact with the bridge because the upward force exerted by their legs is balanced by the downward force of gravity. This results in a net force of zero and therefore, the tourists are not accelerating or changing their velocity.

However, as they straighten out their legs and jump upwards, they are no longer in equilibrium as the upward force from their legs becomes greater than the downward force of gravity. This causes them to accelerate upwards until their feet leave the bridge, at which point they are no longer in contact with the bridge and are no longer affected by its forces.

In terms of terminology, it would be more accurate to say that the tourists are in equilibrium while their feet are in contact with the bridge and are in a state of acceleration once they jump upwards. I hope this clarifies the concept of equilibrium in this context.