Work, Energy and Power: common misconceptions

If your child says… “Holding something heavy must do work.”

Ask together

What distance has the book moved while you hold it still?

What’s true

Supporting a stationary load takes effort but does no mechanical work on that load.

If your child says… “Only my push changes its kinetic energy.”

Ask together

Does the energy change depend on your push alone, or every force together?

What’s true

Work by every force counts.

If your child says… “All areas add positively.”

Ask together

Does opposing force add energy?

What’s true

Below-axis forces oppose increasing position.

If your child says… “Compression stores negative energy.”

Ask together

Where is the potential-energy curve compared with zero on the compression side?

What’s true

We choose zero potential energy when the spring is relaxed.

If your child says… “Reaching the top suffices.”

Ask together

At zero top speed, what could stop gravity pulling the mass inside the circle?

What’s true

The string can pull inward but cannot push outward to cancel gravity.

If your child says… “Multiply output by efficiency for input.”

Ask together

Can useful output exceed input?

What’s true

Input equals useful output divided by efficiency, expressed as a fraction.

If your child says… “Sticking carts keep all their kinetic energy.”

Ask together

If the joined mass doubles but its speed halves, can its kinetic energy stay unchanged?

What’s true

An isolated moving cart sticking to an identical stationary cart leaves half the initial kinetic energy.

If your child says… “Leaking trolleys speed up.”

Ask together

Does departing sand lose its forward momentum?

What’s true

Without horizontal thrust or external horizontal force, a trolley keeps its speed as sand leaves.