Push, Pull and Pressure

You need: a paper or plastic cup · a rubber band · a paper clip bent into a hook · a ruler · sticky tape · bags of rice weighing 100 g, 200 g, 300 g

1. Punch two holes near the top of the cup and tie string through to make a handle.
2. Hang the cup from the rubber band and hook the rubber band onto a pencil held over a table edge.
3. Tape the ruler upright behind it. Mark where the bottom of the cup sits.
4. Add each rice bag and mark the new stretch. You now have a scale.
5. Use it to measure the pull needed to drag a shoe across the floor.

How you'll know: The band stretches further for heavier bags in a steady pattern, so you can read a force from the stretch.

Safety: Goggles if you are stretching bands hard. Do not aim bands at people.

Go further: Add a 400 g bag. Does the pattern stay the same, or does the band start to behave differently?

You need: rubber bands · a spring from an old pen · paper clips · thin and thick garden wire · plasticine · a sponge · a ruler

1. Measure each object before you start.
2. Stretch, bend or squash it with a steady force, then let go.
3. Measure it again and record whether it went back to its shape.
4. Bend thin wire and thick wire by the same amount and compare how hard it was.

How you'll know: Your table sorts every object into elastic (springs back) or plastic (stays changed), and names the internal force: tension or compression.

Safety: Wire ends can be sharp: bend them over with pliers. Goggles on for springs.

Go further: Find something that is elastic for a small force but plastic for a big one. Where is the change-over point?

You need: a small wooden block or a shoe · a force meter or spring balance · sandpaper, carpet, lino, plastic sheet, a wet towel · a results table

1. Lay the surfaces side by side on a flat table or deck.
2. Pull the block slowly across each surface with the force meter.
3. Read the force just as the block starts to move.
4. Repeat three times on each surface and work out the mean.

How you'll know: The roughest surfaces need the biggest pull, and your repeats are close to each other, which makes the pattern believable.

Go further: Add a second block on top. How does doubling the weight change the friction?

You need: identical kits: card or construction-kit chassis, 2 axles (skewers), wheels (bottle tops, CDs or kit wheels in two sizes), straws as bearings, thick and thin elastic bands · masking tape start line · a tape measure · pause sheets

1. Build a car. Loop one end of the band around the rear axle and anchor the other end to the front of the chassis.
2. Wind the rear axle backwards exactly the agreed number of turns.
3. Put the rear axle on the start line and let go without pushing.
4. Measure the distance from the start line to the rear axle. Record it.
5. Fill in the pause sheet: what you saw, one change, and why.
6. Make only that change. Run again. Repeat for three runs.

How you'll know: Your pause sheets show one change at a time, and you can point to the run where the distance jumped and say why.

Safety: Tape over skewer points. Keep run lanes clear so cars do not trip anyone.

Go further: Use gears or pulleys between the band and the axle and explain what changed.

You need: a plank · books to prop it at three heights · a toy car or your best elastic-band car · carpet, lino, grass, sand areas · tape measure

1. Set the ramp at the lowest height. Let the car roll from the same mark each time.
2. Measure how far it travels past the bottom of the ramp. Repeat three times.
3. Change only the height. Repeat.
4. Then keep the height the same and change the floor surface.

How you'll know: Your bar graph shows a clear pattern for height and for surface, and you name the independent and dependent variables correctly.

Safety: Keep ramps stable. Outdoors, watch for other classes and slippery grass.

Go further: Explain in terms of gravity and friction why a skateboard slows faster on grass than on concrete.

You need: a slab of plasticine or playdough smoothed flat · a 500 g bag of rice or a full drink bottle · blocks or lids of three different base sizes · a ruler · a toothpick for measuring depth

1. Measure the base of each block and work out its area (length times width).
2. Stand the first block on the plasticine and balance the rice bag on top for 30 seconds.
3. Lift it off and measure the depth of the dent with a toothpick marked in millimetres.
4. Smooth the plasticine and repeat with each block.

How you'll know: The smallest base makes the deepest dent even though the weight did not change: same force, smaller area, more pressure.

Go further: Work out the pressure for each block: force (about 5 newtons for 500 g) divided by area in square centimetres.

You need: an apple · a blunt butter knife · a rolling pin · a wooden skewer · playdough · photos of a toki (adze), a ngira (needle), a hammer and a drawing pin

1. Try to cut the apple with the flat side of the knife, then with its edge. Compare.
2. Press the rolling pin and then the skewer point into playdough with the same gentle push.
3. For each photo, circle the part that meets the material and the part your hand pushes on.
4. Decide: is this tool designed to make pressure bigger (cut, pierce) or smaller (spread, flatten)?

How you'll know: You can explain every tool with the words force, area and pressure, and you notice that most tools have a wide end for your hand and a narrow end for the job.

Safety: Blunt knives only. Cut on a board, away from your fingers. Check for food allergies before handling food.

Go further: Design your own tool for a real job in the garden or kitchen and label where it concentrates and where it spreads the force.

You need: a sandpit or beach · a ruler · different footwear: gumboots, jandals, sneakers, bare feet · a flat board about 30 cm square

1. Smooth a patch of damp sand.
2. Stand still on one foot for five seconds in each type of footwear, then step off.
3. Measure the depth of each footprint.
4. Stand on the board and measure again.

How you'll know: The board makes the shallowest print because your weight is spread over a bigger area. That is how snowshoes and wide tractor tyres work.

Safety: Check the sand for sharp objects first. An adult nearby at the beach, and stay away from the water's edge.

Go further: Trace each shoe on grid paper, count the squares to find its area, and see whether area predicts depth.