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NCERT Solutions for Class 9th Science Chapter 7 Project work — The Journey Beyond

Book page 139 Updated on2026-09-08

Q1.
Remove both the ends from a pen so that the refill can slide freely through the barrel (Fig. 7.40). Fix the pen cap to the side of the barrel and attach a rubber band to the clip of the cap. Connect the free end of the rubber band to the refill using a safety pin. Stretch and release the rubber band. The refill shoots out, showing the conversion of elastic potential energy into kinetic energy. Repeat with different amounts of stretch, and observe how the distance travelled changes. Is there a relationship between the stretch and the distance travelled?
Answer

Yes. The farther you stretch the band, the farther the refill flies — and the distance grows much faster than the stretch does, roughly as the square of the extension.

How to do the experiment properly

  1. Mark the barrel in centimetres so you can pull the refill back by a measured amount x each time.
  2. Fire the refill horizontally from the same height and from the same launch spot every time — tape the pen to the edge of a table.
  3. Measure the horizontal distance R from the table edge to where the refill first lands.
  4. Take three shots at each stretch and use the average, to reduce the error.
Stretch x (cm)Distance R (cm) — sample readingsR ÷ xR ÷ x²
2402010
3903010
41604010
Why R goes as x2
Elastic potential energy stored in the stretched band grows as the square of the extension: U ∝ x2
All of it becomes kinetic energy of the refill: ½mv2 = U ∝ x2
so v2 ∝ x2, i.e. v ∝ x

For a horizontal launch from a fixed height, the time of flight t is the same every shot,
so for a horizontal launch off a table the time of flight t is fixed and R = v × t ∝ v ∝ x
but for a launch at an angle the projectile formula gives R ∝ v2 ∝ x2
What to expect in your own data: if you fire horizontally off a table, R rises in proportion to x. If you fire at an angle across the floor, R rises roughly as x². Either way, doubling the stretch more than doubles the reach — and a graph of R against x² (or against x) that comes out as a straight line is the proof.
Safety: never aim the refill at anyone's face, and wear spectacles if you have them. Fire along the floor or into a cardboard box.
Q2.
Construct one or more simple machines, or a combination of them (lever, pulley, and inclined plane) using easily available materials, such as cardboard, wooden strips or rulers, pencils or bolts (to act as a fulcrum), thread or rope, small pulleys (or two bottle caps stuck together), and paper cups to hold small weights. Be imaginative in your design. Use your model to lift or move a small load, measure the effort and the load, and calculate the mechanical advantage.
Answer

Build any one of these three, then measure the effort with a spring balance (or by counting identical coins) and use MA = load ÷ effort.

MachineHow to build itWhat to measureExpected mechanical advantage
LeverA 30 cm ruler resting across a pencil. Load (a stone in a paper cup) near the pencil, effort at the far end.Load arm d₂, effort arm d₁, and the effort neededMA = d₁/d₂; with d₁ = 24 cm and d₂ = 6 cm, MA = 4
Inclined planeA stiff cardboard ramp of length L from the floor to a stack of books of height h. Pull a toy car up with a spring balance.L, h, and the spring balance readingMA = L/h; with L = 60 cm and h = 15 cm, MA = 4
PulleyTwo bottle caps glued back to back on a nail axle, hung from a hook. Thread over the groove, load on one side, effort on the other.Weight of load, force needed on the free endMA ≈ 1 for a single fixed pulley — it changes only the direction
Sample answer — the ruler lever
Load = a 200 g stone, so load = mg = 0.2 kg × 10 m s–2 = 2 N
Load arm d2 = 6 cm, effort arm d1 = 24 cm
Predicted effort: F1 = F2 × d2/d1 = 2 N × 6/24 = 0.5 N
Measured effort (spring balance) = 0.6 N
Mechanical advantage = load ÷ effort = 2 N ÷ 0.6 N = 3.3
Ideal MA = d1/d2 = 24/6 = 4
Why the measured MA is always a little less than the ideal: some of your effort goes into overcoming friction at the fulcrum, the ramp surface or the pulley axle, and into lifting the ruler or rope itself. That part does no useful work on the load. Smoother surfaces and lighter parts bring the two numbers closer together — and no machine ever beats the ideal value.
Try This: combine two machines — use a lever to lift a load onto a ramp, then roll it up. The mechanical advantages multiply, which is how real cranes and jacks reach very large values.
Q3.
Computer simulations can help in visualising physical quantities that are difficult to observe directly. The PhET simulations (https://phet.colorado.edu) provide interactive models, such as Energy Skate Park, Energy Forms and Changes, Pendulum Lab, and Masses and Springs. Use these to explore how different forms of energy change as parameters, such as mass, height, and friction are varied.
Answer

Each simulation shows an energy bar chart alongside the motion. Change one quantity at a time and watch which bar grows and which shrinks — that is the whole of this chapter, on screen.

SimulationWhat to changeWhat to look forWhich idea it proves
Energy Skate ParkSet friction to zero; drop the skater from different heightsThe KE and PE bars swap size, but the total bar never movesConservation of mechanical energy (Section 7.4.3)
Energy Skate ParkNow turn friction onA thermal-energy bar appears and grows; the skater's highest point falls each passWhy the roller-coaster humps get lower (Question 8, page 129)
Energy Skate ParkChange the skater's mass, keeping the heightAll bars scale up together, but the speed at the bottom is unchangedv = √(2gh) does not depend on mass (Think It Over, page 116)
Pendulum LabRelease the bob from different angles, with and without frictionWithout friction it returns to the same height every swing; with friction, a little less each timeActivity 7.2, page 127
Masses and SpringsStretch the spring by different amountsThe elastic-PE bar grows much faster than the stretchEnergy stored by deformation (Section 7.4.2)
Energy Forms and ChangesConnect the water wheel, the generator and the bulbEnergy symbols pass along the chain; some always leak away as heatForms of energy, Fig. 7.10; the gharat on page 136
What to record: for Energy Skate Park with friction off, note the skater's speed at the lowest point for drop heights of 2 m, 4 m and 6 m. You should find them close to 6.3, 8.9 and 10.8 m s⁻¹ — exactly √(2gh) with g = 9.8 m s⁻². Getting the simulation to agree with your own calculation is the point of the exercise.
Tip: if the site is blocked or slow, the same idea works with a marble in a length of flexible plastic pipe held as a U. Release it from one arm and see how high it climbs in the other.
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