Week 07 · Tuesday: Potential Energy: Height and Stretch (Sports Physics 6-8)

Tuesday: Stored Energy Becomes Motion

Rocket holds a tennis ball at shoulder height over the patio. "Stored energy," he says. "Watch it turn into speed."

He lets go. The ball drops, hits the patio and bounces back up, slowing as it rises, then falls again.

Raven follows it with her eyes. "What do you notice? Fast at the bottom, slow at the top."

"At the top it almost stops," Rocket says. "Then it comes back down fast again."

Nova hovers level with the highest point of the bounce. "Where did the kinetic energy go at the top?" she asks.

Rocket thinks. "Back into stored energy. Then back into motion on the way down."

"Back and forth," Raven says, writing. "Like a swing."

Nova hums. "Scientists have a rule for this trading."

From potential to kinetic

The physics text says gravitational potential energy may be converted to other forms of energy, such as kinetic energy.

Release a lifted ball, and gravity does work on it as it falls. The stored energy becomes kinetic energy, and the ball speeds up.

A ball at the top of a ramp has the most potential energy and no kinetic energy. At the bottom it has the most kinetic energy.

In between, the energy is shared. As the ball drops lower, it trades stored energy for speed.

The text gives a roller coaster that starts from rest at the top of a 20 metre hill. At the bottom it moves at about 19.8 metres per second.

Ball at the top: all potential energy, no motion

Ball partway down: some potential, some kinetic

Ball at the bottom: most kinetic energy

Ball rolls on and friction slows it: energy becomes warmth

FOLLOW THE ENERGY

Read the question.
Tap your answer.

Where on a ramp does a rolling ball have the most gravitational potential energy?

Where does the ball have the most kinetic energy?

A ball from a higher ramp reaches the bottom moving faster. Why?

Mass does not change the speed

The roller coaster example has a surprise. When friction is small, the speed at the bottom depends only on the height and the starting speed, not on the mass.

A heavier ball stores more energy when lifted, but it also needs more energy to reach the same speed. The two effects cancel.

So a heavy ball and a light ball released from the same ramp height reach the bottom at about the same speed.

The heavier ball still carries more kinetic energy at that speed, which is why it pushed the cup farther last week.

The text adds one more surprise. With no friction, the speed at the bottom is the same on a straight path or a curvy one. Only the height change matters.

Statement	True or false?
A lifted ball's potential energy can become kinetic energy when it is released.	?
A ball from a higher ramp reaches the bottom faster.	?
A heavier ball from the same ramp height reaches the bottom much faster than a light one.	?
At the top of a bounce, the ball has its most kinetic energy.	?

WHY THIS EXERCISESeparating speed from energy keeps the heavy-versus-light puzzle clear.

A BOUNCING BALL

Tap a card.
Then tap its spot.

1First

2Second

3Third

4Fourth

The roller coaster in the physics text starts at the top of a ____ metre hill.

At the bottom of a ramp, a ball has its most ____ energy.

Try it

Drop a tennis ball from waist height onto a hard floor, outdoors or in a cleared hallway. Watch the top of the bounce.
Notice how the ball nearly stops at the top before falling again. That pause is stored energy.
Sketch the bounce path in your Field Log and label fast and slow spots.

Draw a ramp with a ball at the top, middle and bottom. Beside each ball, draw two bars: one for stored energy and one for motion energy.

Describe a swing or a bounce you have watched. Where was it fastest, and where did it almost stop?Explain to a younger child why a ball from a higher shelf hits the floor harder.Predict tomorrow: you will roll a ball from one, two and three books high. Write your guess for each roll distance.

Strong thinking. Tomorrow is Field Lab: three ramp heights, a tape measure and your own energy data.