EN हिं

NCERT Solutions for Class 9th Science Chapter 7 .6.3 Lever — Pause and Ponder

Book page 1357 Updated on2026-09-08

Q11.
Why is it easier to open the lid of a can by using a spoon as shown in Fig. 7.35?
Answer

The spoon works as a class-I lever with a very large mechanical advantage: the rim of the can is the fulcrum, the edge of the lid is the load and your hand at the far end of the handle supplies the effort.

Load arm = distance from the can's rim to the lid edge ≈ 1 cm
Effort arm = distance from the rim to your hand ≈ 10 cm
Mechanical advantage = effort arm / load arm = 10 cm / 1 cm = 10

So a hand force of 20 N produces a lifting force of
F2 = F1 × (d1/d2) = 20 N × 10 = 200 N on the lid
Why it happens: the lid is held by a rim that grips all round, and prising it needs a force far larger than fingers can give. The spoon does not reduce the work, but it multiplies the force ten times by making your hand travel about ten times as far as the lid edge moves. F1d1 = F2d2 is satisfied throughout.
Try This: hold the spoon halfway along the handle instead of at the end. The effort arm halves, the mechanical advantage halves, and the lid becomes noticeably harder to lift — proof that it is the arm lengths that matter.
Q12.
Why do you push an object closer to scissors fulcrum when you want to cut an object which is hard?
Answer

Because moving the object towards the fulcrum shortens the load arm, and a shorter load arm means a larger mechanical advantage — so the blades press on the object with a much bigger force.

Scissors are a class-I lever: the rivet is the fulcrum.
Mechanical advantage = effort arm / load arm
Effort arm (rivet to your fingers) stays the same, say 8 cm

Object near the tip: load arm = 6 cm → MA = 8/6 = 1.3
Object near the rivet: load arm = 1 cm → MA = 8/1 = 8

With a 30 N squeeze of the fingers:
near the tip: cutting force = 30 N × 1.3 = 40 N
near the rivet: cutting force = 30 N × 8 = 240 N
Why it happens: the blades turn about the rivet, so a point close to the rivet moves through a very small arc while your fingers move through a large one. By F1d1 = F2d2, a small movement at the load must come with a large force. Hard materials such as thick cardboard or a wire need that large force; soft paper does not, so it can be cut anywhere along the blade.
Did you know? Wire cutters and pliers are built with permanently short jaws and long handles for exactly this reason — their mechanical advantage is high by design.
Q13.
Throughout history, many designs of perpetual machines (using wheels, weights or magnets) have been proposed but none actually work. Why do all real machines eventually slow down and stop? Explain in terms of work and energy.
Answer

Every real machine has friction and air resistance acting on its moving parts. These forces do negative work continuously, draining the machine's mechanical energy into heat and sound — forms that cannot climb back into the moving parts. With no fuel or electricity to top up the store, the mechanical energy runs down to zero and the machine stops.

Work–energy theorem applied to the machine:
change in mechanical energy = work done by all forces
Friction and drag always oppose the motion, so their work is negative
mechanical energy after one cycle = mechanical energy before – (energy lost as heat and sound)
Each cycle the store is smaller → motion becomes slower → finally zero

A perpetual machine would have to be one of two impossible things:

  • A machine that creates energy out of nothing, so that it can keep doing useful work forever. That breaks the conservation of energy — energy can change form but never appear from nowhere.
  • A machine with absolutely no friction, air resistance or sound, which merely keeps moving. Even that would only keep going; it could do no useful work, because every joule of useful work taken out would have to come from its own store.
Why friction cannot be undone: the chapter's Ready to Go Beyond box on page 126 makes the key point — work done against gravity, or against electric and magnetic forces, is stored as potential energy and can be returned. Work done against friction is not stored. It becomes disordered thermal energy spread through the parts and the air, and no arrangement of wheels, weights or magnets can gather it back.
Tip: good design can make losses small — smoother bearings, oil, streamlined shapes — so a machine runs longer. It can never make them zero.
Was this helpful? Report an error