Preparing to Teach

Physics • Topic 2 • Forces and effects

Improving practical work

1 Overview of practical work in this topic

There are many practical activities at Key Stage 3 level that can be carried out by students themselves, and these can involve everyday situations such as motion, sport, use of toys or machines. In the main activity here (video and worksheet) students identify forces in different situations and investigate the effect of balanced and unbalanced forces on motion.

Other key investigations that students could carry out are described briefly below.

l Investigating friction and drag

· To investigate friction, students can measure the force needed to start an object moving, for example a 500 g mass (standard equipment), block or small brick. Change the surface type, contact area, or the weight of the block. Pull the block along a flat surface using a forcemeter, or place the block on a ramp and measure the ramp’s angle when the block starts moving. The results are more reliable if the object is reasonably heavy (500 g).

· Ask students to predict whether a sheet of paper will fall slower, faster or at the same rate as a book. They often predict that the paper will fall slower than the book. Place a sheet of paper flat on top of a book and drop the book. The book and the paper fall at the same rate – the book reduces the effect of drag on the paper. (A paper ball falls faster than a flat paper sheet – see Teaching strategies in 1 Topic 2 Preparing to teach.)

· Students could compare how quickly an umbrella falls if it is dropped closed and dropped open.

l Investigating unbalanced forces and motion

· Attach two pieces of strong thread (each 1.5-2m long) to either side of a small object, for example stiff card showing an image of a runner. Place a clamp stand with a pulley at either end of the bench and run one thread through each pulley, keeping the runner between them. Hang 10g slotted hangers from the ends of these threads so that they hold each thread in tension, but can fall freely. Release the masses simultaneously. If the weights are equal the runner stays still, or moves at a steady speed. If the weights are unequal, the runner will accelerate. Pupils can investigate the effect of unbalanced and balanced forces on motion.

l Investigating non-contact forces

· Pupils can investigate the effect of non-contact forces on objects (e.g. magnetism, electrostatic forces and gravity). Pupils can investigate what is required for a non-contact force to act, for example: gravitational acts on a mass; magnetic forces acts on a magnetic object; electrostatic forces acts on a charged object.

· Pupils can compare the relative strength of different non-contact forces for example, comparing magnetic forces and gravity by investigating if a magnetic object can be lifted or levitated using a magnet; and similarly if a charged object can be lifted by another charged object. They can also investigate the distance over which these forces act, and if the relative size of both objects involved has an impact.

l Investigating elastic objects

· Students measure the extension of elastic objects when a force is applied. A simple method is to suspend the object (for example strong rubber band, spring) from a clamp stand and hang masses from it on a slotted hanger. Use a ruler in a fixed position to measure the length of the elastic object, and calculate the extension as the difference between the new length and the original length. Check that the maximum force pupils can apply is well below the force that causes the object to snap or deform. Extend this using other stretchy objects, like strawberry shoelaces, stretchy toys, hair to see if the rule that extension is proportional to force applies in all cases.

2 Practical work in everyday teaching

Here are some ideas to help you incorporate practical skills into your everyday lessons on forces.

l Forces outside the classroom

Ask students to: Ask students to identify balanced and unbalanced forces at home; find examples of different types of force at home; find examples of unbalanced forces causing changes in motion in toys (e.g. marble run, remote-controlled cars, playground equipment, or pull-back toy cars), in sport (e.g. ball games, on trampolines) and in transport (e.g. car or cycle journeys).

l Brief starter demonstrations or experiments

· Newton’s cradle

· Students link forcemeters together and pull – how do the readings on each forcemeter compare?

l What happens next?

Just after setting up a demonstration, pause and ask ‘What happens next?’ so that students can suggest answers before you carry on. For example:

· Ask students to predict which object will fall to the ground faster (e.g. book and paper; sheet of paper and ball of paper).

· Ask students to predict which elastic objects stretch furthest when the same force is applied to them, and which of these objects will catapult a small object (like a raisin) furthest when released.

· Place a beaker of water on scales and ask whether the weight will change if you dip your finger in the water.

l Challenges

Set students a challenge using basic equipment and a time limit, for example:

· Finding pairs of forces that are not balanced, but are equal in size for example different objects with the same weight, or a ping-pong ball suspended in a stream of air from a hairdryer; or a levitating paperclip.

· Make one object move or lift another object without touching it.

l Longer investigations

Groups of students can work well if each person has an allocated task, for example to set up and clear away equipment; to write down readings; to make the measurements. Experiments might include:

· Investigate factors affecting friction, for example the area in contact, the surface types or the weight of the moving object.

· Investigate how to make a toy vehicle move the furthest distance, using limited equipment or an elastic object.

Teach Better KS3 Physics Dynamic Learning © Hodder and Stoughton 2013 2

Physics • Topic 2 • Forces and effects

3 Activity: Drawing force diagrams

In Topic 2 Improving Practical Work you will find a video of the science behind and the techniques used in this practical.

This work is best done in small groups, each with a set of equipment, to encourage discussion. Students should be able to explain their choices of force arrows and draw a force diagram for each example.

Keep the forces involved in the examples simple, as students find it hard to use several concepts at once: they need to name forces; identify their direction and what they act on; and in the case of unbalanced forces, link resultant force with motion.

This lesson works at the start of KS3 (cover balanced forces in one lesson and unbalanced forces in the next lesson) and later on in KS3 (use more complex examples; students measure the forces and draw scale diagrams, and predict the effect of resultant forces on motion). You can also use the experiments individually at the start of lessons on specific topics (for example showing forces on a ball in water to introduce floating and sinking).

To make this more interesting, you could use unusual or themed objects in experiments, such as toys.

Select from the following experiments. Provide each group with the equipment, the laminated instructions (see the Technician notes below) and the uActivity: Drawing force diagrams worksheet from Topic 2 Improving Practical Work. Encourage students to choose from the card arrows to show the size of each force and stick the arrows on the beaker/other object to show the direction in which each force acts, before they draw their force diagram.

Experiments / Alternative ideas / Forces involved
Balanced forces
Float a ping-pong ball in water. Repeat with a squash ball that floats but weighs more. / Float a plastic cube in water. Repeat with a wooden cube.
Or use pieces of fruit/vegetables. / weight and upthrust
Place an object on a bench, e.g. a book.
Repeat with a heavier weight. / weight and reaction force
A pendulum or small object suspended from string about 50 cm long.
Repeat with a heavier weight. / weight and tension
A balloon filled with helium tethered to a small weight. / tension and upthrust
A hairdryer clamped so the air flow is upwards with a ping-pong ball balanced in the air. / weight and air resistance/drag
A toy car attached to string, with 20–50 g mass tied to the other end. Let the weight fall, pulling the object at a steady speed across the bench (or pull directly). / Balanced forces: friction and pulling force/weight
Experiments / Alternative ideas / Forces involved
The pages of two magazines are interleaved together. Try to pull
them apart. / two pulling forces in opposite directions
Unbalanced forces
Untether a helium-filled balloon from its weight so it floats up. / Push a floating ball underwater. Watch as it is released. / weight and upthrust
Observe a lava lamp. / weight and upthrust
Place a heavy object on a sponge/soft surface such as mud and watch it sink. / weight and reaction force (unbalanced initially)
Drop a crumpled sheet of paper or small object. / weight and air resistance / drag
Charge two balloons by rubbing them on hair/wool. Suspend them on cotton threads close together. / electrostatic repulsion (single unbalanced force on each balloon)
Attach a hanger and masses (20–50 g) to a toy car using at least 1 m of strong thread. Let the masses fall over the edge of the bench. / tension and friction

l Answers

The forces should be correctly identified and labelled (using the table above for guidance).

The force arrows should be in opposite directions correctly labelled.

If forces are balanced, the force arrows are equal in size.

If forces are not balanced, arrows should be unequal in size, the larger one pointing in the direction of the object’s change in motion.

The length of arrows should represent the size of force (e.g. 1 cm per N) – if you asked students to measure the forces involved.

Examples of correctly drawn force diagrams are shown below.

Physics • Topic 2 • Forces and effects

l Hazard assessment

There are no significant hazards.

Minor risks are:

· Slip hazard from spilt water (or from balls left on the floor). Place equipment in a tray.

· Falling weights if the string snaps or slips. Do not use heavy objects and select string for suitable strength.

· Misuse of equipment, e.g. swinging objects on string, throwing balls. Give clear instructions and expectations beforehand, and remove equipment if used incorrectly. Make groups responsible for the use of their equipment.

· Stumbling over surrounding hazards when pulling the magazines apart. Check for surrounding hazards and remove.

l Technician notes

Three to four sets of equipment should be used from each of the following two tables, ideally for each student group. The actual objects used are not critical in most cases and substitutes can be used.

Each group also needs:

· laminated instruction sheet to go with each set of equipment

· two pairs of arrows made from cardboard: one pair about 4 cm long; one pair about 8 cm long

· Blu-tack to stick the arrows onto objects.

Equipment / Laminated instruction sheet
Beaker of water
Ping-pong ball, squash ball
Alternatively: cubes of plastic/wood or pieces of fruit/vegetables / 1 Put the ping-pong ball in the water.
Write down two forces that are balanced.
Draw the force diagram.
2 Swap the ping-pong ball with the squash ball.
How are the forces different on the squash ball?
Draw the force diagram.
A 20 g mass suspended by string about 20 cm long from a clamp stand
A 50 g mass attached to string about 20 cm long / Write down two forces that act on the 20 g mass hanging from the clamp stand.
Draw the force diagram.
How are the forces different when a 50 g mass is hanging?
Draw the force diagram.
Balloon filled with helium tethered to a small weight / Write down the two forces acting on the balloon.
Draw the force diagram.
Hairdryer clamped so the air flow is upwards
Ping-pong ball / Place the ping-pong ball so it balances in the air from the hairdryer.
Write down the two forces acting on the ball.
Draw the force diagram.
Interleave the pages of two magazines (at least
20 pages overlapping about 80%) / With your partner, try to pull the magazines apart, pulling on the spines only.
Write down the forces acting on the magazines.
Draw the force diagram.

Equipment and instructions: Balanced forces experiments

Physics • Topic 2 • Forces and effects

Equipment / Laminated instruction sheet
Helium balloon / Release the balloon.
Write down two forces on the balloon as it floats up.
Draw the force diagram.
Is the direction of the balloon the same as the direction of the
resultant force?
Beaker of water
Ping-pong ball or
squash ball / Push the ball under the water, then let go.
Write down two forces on the ball as it moves up.
Draw the force diagram.
Is the direction of the ball the same as the direction of the
resultant force?
Lava lamp / Watch the wax in the lava lamp.
Write down two forces acting on the wax.
Draw the force diagram.
Explain why the warm wax moves up and the cool wax moves down.
Piece of bathroom sponge or access to a soft surface, e.g. mud (suitable for an object to sink slowly into)
Object that sinks slowly / Place the object on top of the soft surface or sponge.
Write down two forces acting on the object when it is sinking.
Draw the force diagram.
Is the direction of the object the same as the direction of the resultant force?
Sheet of paper / 1 Drop a sheet of paper flat.
Write down two forces acting on the falling paper.
Draw the force diagram.
Is the direction of the paper the same as the direction of the resultant force?
2 Crumple the paper then drop it.
Write down two forces acting on the falling paper.
Draw the force diagram.
Explain why the paper falls faster.

Equipment and instructions: Unbalanced forces experiments