Common misconceptions › Class 9 › Science › Work, Energy, and Simple Machines Class 9 Science · Chapter 7 · NCERT (iesc107)
Work, Energy, and Simple Machines: common misconceptions
See the tempting mistake students make in Work, Energy, and Simple Machines, why it feels right at first glance, and a worked visual example that shows the right idea.
Worked visual “If your child says…” (6) Teach & practice The tempting mistake & worked visual Does holding a heavy load do mechanical work on it? Wrong
I do mechanical work on a bag because my arms get tired while I hold it still.
Why it feels right. Effort feels like work.
Right
Work by a force on an object depends on its displacement.
Worked example
↑ Held · 0 m ↑ Lifted · 1 m The held bag stays at the same height. Lifting the other bag moves it 1 m along the force direction. Hold the bag at the same height. Its displacement is zero. Lift it 1 m and the upward force does work on the bag. If your child says… (6 concept checks in this chapter)
As you work through Work, Energy, and Simple Machines , listen for these everyday misunderstandings and use the prompt alongside each one.
If your child says…
“I'm pulling upward, so I'm doing work on it.”
Ask together How far has the bag moved? What’s true Without displacement, the supporting force does zero work on the bag. If your child says…
“Twice the speed means twice the energy.”
Ask together What happens to speed × speed when both factors double? What’s true At unchanged mass, doubling speed makes kinetic energy four times as large. If your child says…
“The book's potential energy has one fixed value.”
Ask together Is the book equally far above the floor and tabletop? What’s true The chosen zero determines which height enters the calculation. If your child says…
“Even without air resistance, some of the ball's total energy disappears as it falls.”
Ask together How do the total energy bars compare as the ball falls? What’s true Energy changes form during a fall without disappearing. If your child says…
“Taking longer means doing less work.”
Ask together Did lifting more slowly change the cup's final height? What’s true Raising the same cup through the same height requires unchanged work against gravity. If your child says…
“The door supplies extra energy.”
Ask together Which part of the door moved farther during the easier push? What’s true A lever allows less force over more distance, while your push supplies the energy. Use this chapter three ways Previous chapter How Forces Affect Motion Next chapter Journey Inside the Atom
In the app, your child answers a short check-in with confidence taps, and you see which of these misunderstandings showed up—and which ones are gone.
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