Question

A bicyclist initially travels at a steady 8 m/s for 100 seconds on a straight level road, and then takes 40 seconds to slow to 5 m/s. Three English majors discussing this situation make the following contentions about the bicycle's kinetic energy. With which, if any, do you agree?


A.Axel: "The bicycle is just going to slow down naturally. It doesn't take any work for something to slow down."
B.Bram: "I disagree. The speed of the bike decreased, so there is a change in kinetic energy. That means work was done on the bike."
C.Cassie: "I think Axel is right that no work was done, but I don't agree with his reason. There is no work being done here because there are no external forces being exerted."
D.None of them are correct. Back to Physics for them all!

Answer 1

Answer:

rB. Bram is correct

Explanation:

Let's list the situation presented, and then analyze the comments of English learners

The bike is going at an initial speed, so it has a kinetic energy of

       K₀ = ½ m v²

       K₀ = ½ m 8²

       K₀ = (½ m) 64

After a time order goes to another speed

      $K_{f}$ = ½ m v₂² = ½ m 5²

      $K_{f}$ = (½ m) 25

Let's divide the two energies

     K₀ / $K_{f}$ = 64/25

     K₀ / $K_{f}$ = 2.56

We see that an amount of kinetic energy has been lost, by the law of energy conservation it must have been transformed into work, as this decrease the work must be of a force contrary to the movement, the force of friction.

let's analyze the comments:

rA.Axel. False. There must be external force work rubbing

rB. Bram True, you agree with the deduction from the top

rCc: Cassie. False. There is an external force rubbing