# 02Contact forces

Fo02TA of the Forces topic
• ## 01 Newtonmeter questionFo02TAnugget01 Activity

### The newtonmeter question

What the activity is for

The purpose of this activity is to get pupils thinking and talking about forces. The question focuses on thinking about the force to support a load.

What to prepare

• copies of the questions, numbers to suit the style of activity chosen.

Support sheet

What happens during this activity

You might use a vertically hanging slinky as a teaser or starter:

Teacher: Why are the coils of my hanging slinky farther apart than those at the bottom?… No, don't answer straight off. We're going to have a look at a question together that'll help us to construct a good explanation.

Use the question to set up a discussion, using groups of 3 or 4.

The question can lead naturally into discussions about the tension in a rope, whether hanging or horizontal. Some useful ideas can be explored, for more confident pupils, by discussing the tension in the rope during a tug of war.

• ## 02 Measuring support forcesFo02TAnugget02 Activity

### Compression

What the activity is for

This is a variation on the extending a spring experiment. Here the measurements are of compression and the support sheet Compression directs learners towards thinking about support forces.

What to prepare

• some top pan force scales (Bathroom scales will do, as will kitchen scales. If the only ones you can find are calibrated in grams and kilograms then use these but add a note to learners to remember that when a scale reads 1 kilogram it is because there is a force acting which is about 10 newton. Hence your pupils will have a conversion to do. If possible, have one top pan scale per pair of pupils or incorporate this activity into a forces circus. If you have no top pan scales then an equivalent experiment can be done by top loading the material with 1 kilogram masses.)
• you will also need a compressible material which matches the scale of the force meter (Weak compression springs, pieces of soft foam or car-wash sponge will all be fine.)
• rulers to measure compression
• copies of support sheet Compression

Support sheet

Safety note: Make sure that pupils don't get into situations where 1 kilogram (or larger) masses could fall on hands or feet.

What happens during this activity

The idea is that pupils will record the additional force required to deform the foam, spring or sponge. A piece of cardboard placed on top of the object will ensure an even force distribution.

Pupils measure the compression distance with a ruler. A table of results should be recorded and a suitable graph plotted to show the compression pattern.

• ## 03 Experiencing buoyancy forcesFo02TAnugget03 Activity

### Floating balloons

What the activity is for

The purpose of this activity is to give pupils first hand experience of a buoyancy force and to encourage them to talk about their experiences and make links to their previous work on the directions of forces.

What to prepare

• some party balloons
• a bowl of water or water in a sink – or outside

What happens during this activity

Invite pupils to float the inflated balloon on the water, then to gently push the balloon into the water. The volume of the balloon is such that a considerable force upwards will be experienced by the pupils. This is such a simple exploration, yet one that few pupils will have experienced first hand.

• ## 04 Sinking a puntFo02TAnugget04 Activity

What the activity is for

In this activity pupils measure how the load the punt supports first hand experience of an buoyancy force and to encourage them to talk about their experiences and make links to their previous work on the directions of forces.

What to prepare

• a cardboard punt, from the template
• an ice-cream tub of water, or equivalent
• a set of 10 gram masses

Support sheet

What happens during this activity

Pupils make up their punts and place them carefully on the surface. Then, using the scale on the side of the punt – and loading the punt evenly – they plot a scatter graph (as the data is collected, naturally) to show how the depth varies with the load.

The depth is a rather good proxy for the volume of water displaced. It would of course be better if the punts were perfect cuboids, but aesthetic considerations were considered more important.

We think it's a good idea to be very clear that there is a clear pattern to be found, and that skilful measurement will confirm it, perhaps like this:

Teacher: There's a very clear pattern here, and I'd like you to take great care with the measurements to see if you're good enough to make a graph that shows it very cleanly.

• ## 05 Floating and sinkingFo02TAnugget05 Activity

### Sinking bottles

What the activity is for

The purpose of this activity is to guide pupils through a sequence of first hand experiences that enable them to make sense of floating and sinking. Floating and sinking are related to the resultant of gravity and buoyancy forces acting on an object, to the weight of the object, and to the volume of water displaced by the object.

What to prepare

Per group:

• some prepared bottles
• a bowl of water or water in a sink
• some coloured water in a 1 litre jug
• a few large stones of different volumes, to place in the bottles one at a time

### Doctored bottles

What happens during this activity

Invite pupils to take the sealed bottle to gently push it into the water.

Teacher: How much do you have to push down on the bottle to make a hole in the water? How does that force you exert depend on the size of the hole?

Teacher: What will happen if you try with a bottle that does not push the water out of the way, so not making a hole?

Now try with the doctored bottle, that does not make such a hole. Now gentle pushing will not result in a buoyancy force. Whatever the position of the bottle, the buoyancy force is zero.

Teacher: The first bottle is more like a boat – the second a boat that'll never float. But now let's have a boat that's partly loaded.

Try with an open topped bottle, about a quarter full of coloured water. This'll weigh more as it's carrying water – a water ballasted boat.

Teacher: How far into the water did the bottle go when you allowed it to find its own height? What force did you have to exert on the bottle to make it flow higher?… and lower?

Now try again replacing the water in the bottle with a stone that is less heavy than the water.

Teacher: Can you predict how deeply the bottle will float now? What about with a stone that is less heavy than the water?

Draw out the conclusions at a level of detail and precision appropriate to the class. It might be possible to measure the volume of water displaced (the hole made by the bottle) and relate it to the stone's mass – but it may not be useful to do so.

• ## 06 Measuring buoyancy forcesFo02TAnugget06 Activity

### Measuring buoyancy forces

What the activity is for

The purpose of this activity is to make measurements to find the size of buoyancy forces and to start to think about what the upthrust force depends upon.

What to prepare

• a top pan balance calibrated in newtons
• a spring balance calibrated in newtons (a newtonmeter)
• a bowl or large beaker of water and an object (perhaps a wooden block) to weigh
• copies of sheet to support practical work
• copies of sheet to support homework

Support sheet – Exploring

Support sheet – Thinking

What happens during this activity

As a preliminary activity, to encourage thinking, the object should be weighed on the top pan balance and pupils invited to describe the forces acting on the object (pull of gravity and a support force exerted by the balance). The balance is then replaced by a pupil's hand and pupils invited to describe the forces as the object rests on their hand (gravity force and the support exerted by the hand).

The object is then transferred to the newtonmeter and these processes repeated. The object, still suspended from the newtonmeter, is lowered into the water. The reduced reading on the meter is noted. Stop and pause before the reading reaches zero. Now invite pupils, in small groups, to describe the forces acting on the wooden block. A support sheet with a sequence of sketches is provided to help focus thinking.

• ## 07 Measuring densityFo02TAnugget07 Activity

### Measuring the density of a material

What the activity is for

This activity allows pupils to make some measurements to measure the density of a material for themselves.

What to prepare

• a simple balance or scale calibrated in grams or kilograms to find the mass
• a ruler
• a measuring jug or cylinder

What happens during this activity

Pupils find the mass of an object and then its volume. By dividing mass by the volume they can calculate the density of the material from which the object is made. The maths is simple – just a division of two numbers.

The volume can be measured using a ruler if the object is a regular shape such as a cube (length  × width  × height). For irregular shapes such as a lump of plasticine use a measuring jug or a scientific measuring cylinder. By immersing the object in water, the volume can be seen as equal to the extra volume which the water appears to reach on the scale.

A note on units

The units of density are the units of mass divided by the units of volume. If you measure the mass in grams and the volume in cubic centimetres then the density can be stated in units of gram / cubic centimetre (g / cm 3 ). For example the density of plasticine might be 2.5 g / cm 3.

For larger volumes, such as water in a swimming pool, the mass might be measured in kilogram and the volume in metre 3. The density might then be quoted as 1000 kilogram / metre 3. (Incidentally a swimming pool full of plasticine would have a very large mass. The density would be calculated from measurements in kilogram and metre 3 and would be quoted as 2500 kilogram / metre 3. However it is still plasticine. The density is the same, no matter how much of it you are measuring).

It is important to note that the density of a material is unchanged should its value be quoted in different units. It is still the same stuff. So water might be quoted in a book as having a density of 1 kilogram / centimetre 3 but also 1000 kilogram / metre 3. They amount to the same thing. Similarly, a density of 2.5 gram / centimetre 3 is the same density as 2500 kilogram / metre 3.

• ## 08 Friction investigationsFo02TAnugget08 Activity

### Measuring friction

What the activity is for

Here pupils make some measurements of frictional force, and investigate the factors affecting that force.

What to prepare

Per group:

• a newtonmeter
• several shoes – pupils might be encouraged to bring in their own
• several one kilogram masses
• a plank of wood and a means of supporting it at different angles
• copies of the support sheet

Support sheet

Safety note: Support planks properly. Avoid having planks on top of benches, where they may fall off.

What happens during this activity

There are two possible investigations:

• How much force is required to make a shoe slide?
• With the shoe on a slope, what angle of slope is needed to just make the shoe start to slide?

For each, try to motivate exploration by relating the problem back to everyday concerns.

The two investigations can be carried out with very simple apparatus, as shown.

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