• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Work question

  • Thread starter tony873004
  • Start date


Science Advisor
Gold Member
6. At a local playground, a child on a swing has a speed of 3.4 m/s when the swing is at its lowest point.

a.) To what maximum vertical height (above the bottom of the swing) does the child rise, assuming he sits still and "coasts"?

b.) Reanswer part a, with the initial speed of the child being half as much.

c.) During this process, how much work is done by the tension in the rope?

I got part A and B using [tex]\Delta K = - \Delta U[/tex]

But for part C, [tex]W = \Delta K = \frac{1}{2}mv^2[/tex] and [tex]W = Fd[/tex] or [tex]W=mad[/tex]
But the mass of the child is not given? How can I compute the work? Unless this is a trick question and the rope is doing no work. But the rope is what is responsible for the child rising, otherwise the child would continue on a straight path parallel to the ground. So the rope is doing work, right? :confused:


Science Advisor
Homework Helper
Since you have used conservation of mechanical energy in parts a) and b), you already implicitly used the fact that the rope doesn't do work.
Consider the direction in which the force from the rope acts.


Science Advisor
Gold Member
Is that a real Einstein quote in your signature? That's funny :smile:

I guess you're right. The rope pulls towards the center of the circle but the child never gets closer to the center. D = 0, so regardless of mass, W must be 0.


Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving