What the Activity is for

Forces without motion.

The purpose of this activity is to revise with pupils the idea that if an object is not moving there can be a number of forces acting on it that balance each other out.

What to Prepare

• a book on a table
• a ping pong ball balanced on a jet of air
• a mass hung from a spring
• a person standing on the floor
• a tug of war rope with two people on each side holding the rope stationary

What Happens During this Activity

Demonstrate each of the above to the class and ask the following questions:

• What forces are acting on the object?
• What can you say about the direction of the forces?
• What can you say about the size of the forces?
• What can you say about the overall effect of the forces?

Conclude by getting the class to draw up a statement about what we know about such forces which add to zero (e.g. for a stationary object, the forces acting on it add to zero). This is a review of the work covered in episode 01 of the SPT: Forces topic.

Pupils may agree that equal and opposite forces can keep an object stationary, but it is more difficult to accept that when an object moves with a constant speed the forces are also equal and opposite. Encourage pupils to talk about how the speed of a cyclist changes as they start a journey and why. (Initially the driving force is greater than the retarding forces and the bike speeds up). Ask any cyclists in the class to explain why they don't keep going faster, even though they keep pedalling. This should lead to the idea of retarding forces (slip and drag) and the fact that these oppose motion. You can feel the effects of both at quite low speeds by using rough surfaces or large surface areas. The aim here is to encourage pupils to move away from the stationary object situation towards talking about forces acting on a moving object.

What the Activity is for

The purpose of this activity is for pupils to consider examples of everyday objects travelling at a constant speed and to talk about the forces acting on them.

What to Prepare

• a set of images to start pupils thinking

What Happens During this Activity

Working in groups of two or three, pupils should come up with as many everyday examples of people or objects travelling at a constant speed as they can think of (e.g. swimmers, runners in a race, a car travelling at 30 mph, parachutist falling). In each case, the group should identify the forces acting on the object and comment on their size and direction. Alternatively, this activity could be supported by a picture resource or video clips with examples of moving objects. Pupils should come to appreciate that many objects do travel at a constant speed and that the forces acting on those objects add to zero.

What the Activity is for

Measuring constant motion.

Pupils gain experience of timing objects moving at a constant speed and recognise that the forces acting add to zero.

What to Prepare

What you need depends on the activity being undertaken – there are three sets of apparatus:

Ball bearings falling through glycerol

• 1 large measuring cylinder (2 litre) full of glycerol, with marks every 100 millimetre
• set of small ball bearings (about 5 millimetre diameter)
• set of stop clocks

All bearings can be removed from the glycerol using a strong magnet positioned on the outside of the measuring cylinder.

Styrocell beads falling through water

• 1 large measuring cylinder full of water, with marks every 100 millimetre
• box of styrocell beads
• set of stop clocks

Styrocell beads need to be projected into the water with a reasonable speed, to ensure that they don't just float on the surface of the water.

Air bubbles travelling upwards in a tube of water

• one large transparent tube of water clamped in position
• an air valve connected via rubber tubing to the bung at the bottom of the tube
• stop clocks

In this experiment, the air bubbles injected into the water will rise upwards.

What Happens During this Activity

In this demonstration the pupils time how long it takes for an object to travel successive distances of 10 centimetre. The key questions to address are:

• What forces are acting on the object?
• What can you say about the size of the forces?
• Why does the object fall (or rise) at a constant rate?

Organisation

Split the class into groups of three to four and if possible give each pupil a stop watch or clock.

1. All pupils start their clocks when the ball passes the first line (the starting line).
2. Group one pupils stop their clocks when the ball passes the second line. They have measured the time for the ball to travel 10 centimetre.
3. Group two stop timing when the ball has crossed the third line – they have timed for a total distance of 20 centimetre.
4. Groups continue making timings until the ball bearing has reached the bottom of the measuring cylinder.

The results can be used in at least two ways:

• An average time can be found from the individual readings of each pupil in a group. Using this average time, the average speed of the ball bearing can be calculated for the distance travelled (e.g. average speed for the first 10 centimetre, average speed for the first 20 centimetre, 30 centimetre, 40 centimetre etc.).
• Alternatively, by subtracting the time to travel 10 centimetre from the time to travel 20 centimetre, the time to travel the second 10 centimetre can be calculated. All the times for successive 10 cm distances can be compared, and should be very similar. In this case, speed does not need to be calculated since all the distances are the same.

Get pupils to identify the forces acting on the objects in each of the above experiments. Pupils may struggle to understand that the forces acting on the object add to zero. While initially there must be a slightly larger force acting down in order to start the object moving in that direction, the two forces quickly balance each other and so the object travels at a constant speed.

What the Activity is for

Forces on a moving object.

The purpose of this activity is to revise with pupils the idea that even when an object is moving there can be a number of forces acting on it which add to zero.

What to Prepare

• runway
• trolley
• motion sensor light gates and software
• books or clamps to adjust height of runway

What Happens During this Activity

This is a demonstration activity in which the slope of a runway is adjusted to make a trolley move down it at a constant speed.

With the runway horizontal, push the trolley and note that it slows down. The final speed is less than the initial speed. Why does it slow down? The slowing down indicates that frictional forces are acting on the trolley in a horizontal plane. Raise the runway and repeat the experiment so that the trolley speeds up. Now the force of gravity pulling down the slope is greater than the frictional force.

Finally, reduce the slope so that the trolley runs down the runway at a constant rate. At this point the runway has been friction compensated.

This means that the gravitational force acting to pull the trolley forwards is opposed by the frictional force acting against the moving object. If these forces add to zero then the trolley moves at a constant speed.

This demonstration can be carried out so that pupils judge with their own eyes whether the trolley is speeding up, slowing down, or moving at a constant speed. Alternatively use a pair of light gates or other motion sensors linked to a suitable data logger to record the times taken for the trolley to pass through the gates.

What the Activity is for

Parachutes provide greater frictional forces.

This activity introduces the idea that objects do not continue to fall faster and faster but reach a final constant speed (or terminal speed). The size of the final speed depends on the frictional forces acting (with a big open parachute the final constant speed is much lower than for free-fall of the same object or person). Furthermore, a final constant speed is an indicator of zero resultant force.

What to Prepare

• toy plastic figures with parachutes (can be bought or made)

What Happens During this Activity

Use two identical figures – one with a parachute strapped to their back, the other with their parachute open and untangled. Drop the two figures from the same height at the same time and see which one hits the ground first.

Discuss result with the class. The downward force acting on both figures is the same – the same force of gravity acts on both. Why do they not reach the ground at the same time? Because the upward force acting on the two figures is not the same. The open parachute provides a greater frictional force upwards, which means that the figure with the open parachute reaches their constant terminal speed before the other one.

The figure with the open parachute therefore takes a longer time to reach the ground and lands safely. The figure with the closed parachute would also reach a terminal speed if allowed to fall a great height but would first need to speed up until the force of the wind, or air resistance, was strong enough to balance the force of gravity pulling down.

What the Activity is for

You can build this model with pupils to show how the balance between the different forces leads to different terminal speeds.

What to Prepare

• either one computer, connected to a large display, running the modelling program VnR
• a computer linked to a large display running a modern browser, to share some of the interactive diagrams below

or

• a collection of computers running the modelling program VnR, so that pupils can work in groups of 2 or 3
• a computer linked to a large display running a modern browser, to share some of the interactive diagrams below

What Happens During this Activity

Either you or the pupils build the models shown. The reconstruction given here gives the salient points, but you need not follow the sequence slavishly.

In particular, you can adapt the starting points, perhaps giving the pupils the variables and asking them to connect them together. They could then move on to a larger range of variables, with a different goal set by you. In any case, it is wise to set them mileposts along the way to the final destination and to get them to check their models as they go.

If you choose to build the model up with the class, you may find it a good strategy to have a volunteer to do the keyboarding and pointer driving for you, allowing you to concentrate on running the discussion with the class.

A good way to work is for you to build up parts of the model then allow the pupils to build, or at least be in the driving seat for, the remaining significant linking steps.

You can explore the different facets of the argument using these partial models.

Discuss the evolution of the forces on the skydiver using this interactive diagram:

Using this tool to draw four diagrams to show snapshots in the evolution of forces acting on the skydiver:

Exploring how the drag force varies with velocity: