Nowhere in this activity is there a mention of kinetic energy-- energy of motion
ID: 1332245 • Letter: N
Question
Nowhere in this activity is there a mention of kinetic energy-- energy of motion (KE). A little study of energy conservation tells us that the gain in KE of the ball as it rolls down the ramp is equal to the decrease in PE as the ball loses height-- in short, PE=KE. Why were we able to bypass KE in our analysis here? (The answer to this question underlies the reason that physics types use energy principles to solve problems-- intermediate steps can be skipped!)
Step 1: Mark the ramp at 30, 60, 90, and 120 cm from the bottom end. Assemble the ramp so that when you roll a ball down it, the length of carppet it rolls onto is sufficient to stop it. Experiment to see how far this is.
Step 2: Release the steel ball at each of the intervals along the ramp. Measure the vertical height from the floor or table. Roll the ball three times from each height and record the stopping distances in the Step 2 Data Table.
Step 3: Change the angle of the ramp, but launch the ball from the same vertical height that you did for the previous ramp position.
Explanation / Answer
Here, ball has some potential energy initially. This potential energy is converted to kinetic energy on reaching the end of ramp. Now friction works against the ball till the kinetic energy is reduced to zero and then ball stops.
We didn't use KE energy here as KE gained by ball at end of ramp was equal to potential energy of ball when it was at top of ramp. Energy carried by ball was same at any instant until it reached the end of ramp. Even if we had made use of KE in intermediate steps, that would have resulted in same answer as Initial potential energy of ball at top of ramp equals final kinetic energy at end of ramp. Friction would have done same work against the ball and results would be the same.
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