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Sports Physics 6-8 / Week 06 / Wednesday
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Week 06 · Kinetic Energy: Mass and Speed

Wednesday

Field Lab: The cup slide
// The energy of motion
⏱ about 20 min

Wednesday: Field Lab: The Cup Slide

Rocket has a ruler propped on a book to make a tiny ramp. A paper cup lies at the bottom, open end facing it.

"Ten centimetres up, twenty up, thirty up," he says. "Higher start, faster ball, farther cup. I know it already."

Raven taps the tape measure. "Knowing is a guess until you measure. Three rolls each."

Nova hovers over the cup. "What else could you change besides the start height?" she asks.

"The ball," Rocket says. "A heavier one from the same spot."

He lets the ball go from the 10 mark. It rolls into the cup, and the cup scoots a few centimetres.

"What do you notice?" Raven asks. "The ball stopped and the cup moved. The energy changed hands."

Nova's light brightens. "Now measure how far it travelled."

Your mission

Today you measure kinetic energy by what it does. A ball rolls down a short ramp into a cup, and the cup slides.

The farther the cup slides, the more energy the ball transferred to it. You will test three start heights and then a heavier ball.

This follows a take-home investigation in an OpenStax college physics text. That version uses a ruler, a book, a marble and a foam cup.

  • A ruler with a groove, or a strip of cardboard folded into a shallow channel, and a book to prop it on
  • A paper or foam cup lying on its side, open end facing the ramp
  • A small light ball, such as a ping pong ball or a small foam ball
  • A heavier small ball, such as a tennis ball
  • A tape measure and masking tape to mark the cup's starting spot
  • A smooth, level floor and your Field Log
Safety first
Set the ramp on the floor, not on a table edge. Nothing rolls toward stairs or a road.
Use soft or very light balls only. Keep small balls away from younger children.
Keep hands and feet out of the lane while a ball is rolling.
  1. Prop one end of the ruler on the book. Place the cup on its side at the bottom with its open end facing the ruler.
  2. Mark the cup's starting spot with tape.
  3. Hold the light ball at the 10 centimetre mark and let it go. Do not push it.
  4. Measure how far the cup slid from the tape. Write it in the table. Put the cup back on the tape.
  5. Repeat twice more from the 10 centimetre mark.
  6. Now roll three times from the 20 centimetre mark, then three times from the 30 centimetre mark.
  7. Switch to the heavier ball. Roll it three times from the 10 centimetre mark and record the slides.
  8. Clean up and compare your rows.
PREDICT BEFORE YOU ROLL
  • Read the question.
  • Tap your answer.
From which mark will the ball push the cup farthest?
The heavier ball rolls from the same 10 centimetre mark as the light ball. What do you expect?
What stopped the cup after it slid?
TestSlide 1 (cm)Slide 2 (cm)Slide 3 (cm)Longest slide
Light ball from 10 cm
Light ball from 20 cm
Light ball from 30 cm
Heavier ball from 10 cm

Reading the result

The physics text asks the question you just answered with a tape measure. What happened to the kinetic energy of the ball at the bottom of the ruler?

The ball slowed and the cup sped up. Energy was transferred from the ball to the cup.

Then the cup slid and friction with the floor slowed it. The text says this experiment turns stored energy into motion and then into thermal energy.

A higher start should give a longer slide. The text also asks whether the slide distance for a heavier ball is proportional to its mass.

Your numbers will wobble. The cup can tip or spin. Three rolls and the longest slide give you a fair comparison.

What the lab showsTrue or false?
A ball from the 30 centimetre mark reaches the cup faster than one from 10 centimetres.?
The cup gained kinetic energy from the ball.?
The cup kept sliding forever on the smooth floor.?
The heavier ball from the same mark carries less kinetic energy than the light ball.?
WHY THIS EXERCISEThe cup is a kinetic energy meter: farther slide, more energy transferred.
Sketch your ramp, ball and cup. Mark the 10, 20 and 30 centimetre starts and draw the cup's slide for each.

Excellent measuring. Tomorrow you will read real kinetic energy numbers for a basketball, a walker and a sprinter.

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