NCERT Class 9 Science Chapter 7 Activities — Work, Energy, and Simple Machines
NCERT Class 9 Science — Work, Energy, and Simple Machines, Activities 7.1 to 7.5 on pages 125 to 133.
Also here. Solutions to the end-of-chapter questions for this chapter and the in-text questions are on their own pages.
Five activities in two groups. Activities 7.1 and 7.2 are about energy — that a raised object has some, and that as it falls the total stays constant. Activities 7.3, 7.4 and 7.5 are about simple machines — the inclined plane and the lever, and what they do and do not give you.
Key insight. A simple machine never reduces the work you have to do. It lets you apply a smaller force over a longer distance, or a smaller weight at a greater distance from the pivot. Activities 7.3 and 7.5 are both demonstrations of that trade, and a student who leaves the chapter thinking a ramp or a lever “saves energy” has missed the one thing it is teaching.
Activity 7.1
Take a heavy ball and a large container of loose sand. Raise the ball about 1 m above the sand bed and drop it. Is a depression made? Now drop it from a greater height and compare the depressions.
What you should observe. The higher the drop, the deeper the hollow.
A depression forms, and it is deeper when the ball is dropped from a greater height. The raised ball has gravitational potential energy, $E_p = mgh$, and the deeper depression shows that more of it was available to do work on the sand.
Why sand rather than a hard floor. The sand records the result. A ball dropped on concrete bounces and tells you nothing you can measure afterwards; a sand bed keeps a depression whose depth is a visible, comparable measure of the energy delivered.
Why height is the variable. Mass and gravity are fixed, so $mgh$ depends only on $h$ — doubling the height doubles the potential energy, and the depression deepens accordingly. Dropping a heavier ball from the same height works as well and is worth doing too.
What goes wrong. Sand that has been packed down by earlier drops. Each run needs the surface raked level and loose, or later drops meet firmer sand and make shallower depressions for reasons that have nothing to do with height.
Activity 7.2
Set up a simple pendulum. Paste a sheet of white paper on the wall behind it and draw a horizontal line at the level of the bob at rest. Pull the bob aside to a measured height and release it. Mark how high it rises on the far side, and watch over several swings.
What you should observe. The bob very nearly regains its starting height on the other side, swing after swing — but not quite, and the shortfall grows.
The bob rises to almost the same height on the opposite side. Its energy shuttles between potential at the extremes and kinetic at the lowest point, with the total — the mechanical energy — staying constant.
$$E_p + E_k = mgh = \tfrac{1}{2}mv^2 = \text{constant}$$
Why “almost”. Air resistance and friction at the support remove a little energy each swing, so the amplitude slowly decays. The chapter’s statement is that mechanical energy is conserved “if no other external forces act on it” — and the slow decay is those other forces showing themselves. Pointing that out is better than pretending the heights match exactly.
What the chapter derives from it. Equating $mgh$ to $\tfrac12 mv^2$ gives $v = \sqrt{2gh}$ for anything falling through a height $h$ — and, as Example 7.8 notes about a child on a slide, that speed “depends only upon the height”, so neither the shape of the slide nor the mass makes any difference. That is a genuinely counter-intuitive result and this activity is the evidence for it.
What goes wrong. A bob that swings in a circle rather than a plane, which happens if it is released with any sideways push. The rise on the far side then cannot be compared with the paper. Release it gently from rest, with the string taut.
Activity 7.3
Take a smooth plank about 1.5 m long, a toy car or cart, and a spring balance. Attach the balance to the cart. Set up an elevated surface — a low stool or a pile of books — about 0.5 m high. First lift the cart vertically with the balance and read the force. Then lay the plank as a ramp and pull the cart up it, reading the force again.
What you should observe. The ramp reading is smaller — but the cart travels much further.
Pulling the cart up the ramp needs a smaller force than lifting it straight up. But the ramp is longer than the height, so the force acts over a greater distance and the work done is about the same.
Why “about the same” and not exactly. Friction on the plank takes a share, so the work up the ramp is slightly more than $mgh$. The chapter specifies a smooth plank to keep that share small. A rough one can wipe out the advantage entirely, which is worth demonstrating deliberately if there is time.
What the machine actually gives you. Not less work — less force. That is the trade, and the ramp’s steepness sets the exchange rate: a gentler slope needs less force over a longer run. It is why loading ramps into lorries are long and shallow rather than short and steep.
What goes wrong. Pulling the cart up at an angle to the plank rather than along it. Part of the reading then goes into pressing the cart onto the plank or lifting it off, and the force measured is not the one doing the useful work. Keep the spring balance parallel to the surface of the ramp.
Activity 7.4
Take a 30 cm scale, a pencil, two or three erasers and a stapler. Rest the scale across the pencil so that the pencil sits closer to one end. Put the stapler on the short end. Put one eraser on the long end — does the stapler lift? If not, add another.
What you should observe. One or two erasers lift a stapler several times their weight.
A much heavier object is lifted by a much lighter one, because the light one sits further from the pivot. The scale acting this way is a lever — “a rigid bar that can rotate about a fixed point”.
Why the pencil goes near one end. That is what makes the two arms unequal, and the inequality is the whole mechanism. Put the pencil in the middle and the erasers lift nothing — which is worth doing first, as the control.
What is being traded. The eraser end has to move much further down than the stapler end moves up. Less force, more distance — the same bargain the ramp offered in Activity 7.3, in a different shape.
What goes wrong. A scale that bends. A flexible plastic ruler absorbs the effort instead of transmitting it, and the stapler does not budge. “A rigid bar” is part of the definition for a practical reason.
Activity 7.5
Take a scale 50 cm or longer, string, two paper cups as pans, tape and identical coins. Tie the string around the centre of the scale so it hangs level, and hang a cup near each end. Put one coin in each pan and check the beam stays horizontal. Add a second coin to the right pan — the beam tilts. Slide the heavier pan towards the centre until it balances, and measure its distance. Repeat with 4 coins, then 8, recording each balancing distance in Table 7.1.
What you should observe. Every time you double the coins, the balancing distance halves.
| Left pan (effort) | Its distance L₁ | Right pan (load) | Its distance L₂ |
|---|---|---|---|
| 1 coin | L | 1 coin | L |
| 1 coin | L | 2 coins | L/2 |
| 1 coin | L | 4 coins | L/4 |
| 1 coin | L | 8 coins | L/8 |
$$n_1 L_1 = n_2 L_2$$
Why doubling and halving is the design. It makes the inverse relationship unmistakable without any calculation. A student who has seen the distance halve three times in a row can state the rule before being told it, which is exactly what the table is for.
Why the product and not the sum. What balances a lever is the turning effect — the weight multiplied by its distance from the pivot. Eight coins close in and one coin far out give the same product and therefore balance, which is why a seesaw works with a child and an adult at different distances.
What goes wrong. Two things worth watching. The pans must weigh the same, or the beam is already unbalanced before any coins go in — which is why the activity starts with one coin in each and checks it hangs level. And the string must be tied at the true centre; an off-centre pivot puts a constant error into every distance measured.
Common mistakes
- Comparing depressions in Activity 7.1 without re-levelling the sand. Packed sand gives a shallower hollow for reasons unconnected with the drop height.
- Expecting the pendulum in Activity 7.2 to rise to exactly the same height forever. It does not — friction and air resistance take a little each swing, and saying so is more honest than glossing over it.
- Thinking the speed at the bottom of a slide depends on its shape or the child’s mass. From $v = \sqrt{2gh}$ it depends only on the height, which is one of the chapter’s most testable results.
- Believing a ramp or a lever reduces the work. It reduces the force, at the cost of a longer distance. The work is the same, or slightly more once friction is counted.
- Pulling the balance at an angle to the ramp in Activity 7.3. The reading then includes a component doing no useful work, and the comparison with the vertical lift is invalid.
- Putting the pivot in the middle in Activity 7.4. Equal arms give no advantage at all, which is the control rather than the experiment.
- Adding the weight and the distance in Activity 7.5. It is the product $nL$ that balances, not the sum — which is why 8 coins at an eighth of the distance work.
Practise next
- The end-of-chapter questions, Revise, Reflect, Refine, where the conservation of mechanical energy from Activity 7.2 is applied numerically and the lever relation from Activity 7.5 is used on seesaws and crowbars.
- The in-text Pause and Ponder questions of this chapter, thirteen across seven boxes, which follow the chapter through work, kinetic energy, potential energy, conservation and simple machines in the same order.
- The activities of Chapter 6, whose cart and spring balance are reused here, and whose forces are what do the work this chapter measures.
Doubts are answered by Shiwam, usually within a day. Ask about this question specifically — a general question about the chapter is better asked in class.