# 03Non-zero force changes speed

Mo03TA of the Motion topic
• ## 01 Tug of warMo03TAnugget01 Activity

### It's all about resultant force

What the activity is for

The purpose of the activity is to provide a memorable event that offers the opportunity to talk about whether the resultant force is zero or not. You'll be focusing on horizontal forces only. Depending on the class, you may need to make this explicit.

What to prepare

• rope with marker (possibly a red ribbon) in the middle

Safety note: Use a substantial rope, perhaps a climbing rope. Take care than any pupils who stumble do not fall back onto furniture. Arranging the activity in a playground or a corridor is a safer option.

What happens during this activity

Start with the rope and red ribbon and no pupils pulling. Emphasise the point that we are interested in the state of motion of the ribbon. Why does the marker remain initially unmoved? (no forces acting – equilibrium). Then encourage two pupils on each end of the rope to take the strain so that the marker still does not move. Lead a discussion focusing on the idea that the resultant force acting on the rope must be zero. This is another equilibrium situation.

Now add an extra two people to one side of the rope, and get the pupils to take the strain again. This time, the marker will change its motion: moving towards the side that has more people on it. Invite pupils to suggest why. Encourage the use of the terms resultant force, changing motion and equilibrium.

The marker moves because the forces acting on it do not add to zero. Four people provide a greater force than two people and so the marker, which is initially at rest, speeds up and moves in the direction of the greater force (towards the four people). The general point is that a change of motion always follows when a resultant force acts on an object.

• ## 02 Trolley forcesMo03TAnugget02 Activity

### Motion of a trolley

What the activity is for

The purpose of this activity is to involve pupils in discussion about resultant forces. How will various forces affect the motion of the trolley?

What to prepare

• dynamics trolley
• two pulleys
• two sets of 100 gram masses and hangers

What happens during this activity

With the trolley set up as in the diagram, pupils can predict and observe the effect of different combinations of forces acting on the trolley. There are two important strands to the discussion. In which direction will the trolley move? What type of motion (speeding up? slowing down? constant speed?) will result from forces which do not add to zero?

Whilst you have a situation where all forces add to zero, it is a good idea to invite someone to cut the string on one side of the trolley. The resulting discussion will allow you to assess how far the learners have appreciated the ideas (the trolley gets going in one direction due to the action of the resultant force).

You might like to use this model to support the discussion about their predictions:

• ## 03 Apply those brakesMo03TAnugget03 Activity

### Applying the brakes

What the activity is for

This is an exploration that gives reliable, valid results, which can be a good way of considering how assertions are related to the evidence.

What to prepare

Per group:

• a flexible track to run a cheap model car down
• an old pen case, with a rubber fixed in the end
• six 5 gram masses
• a small object to trigger the brakes
• a metre ruler
• this interactive object, displayed on a large screen

### Making predictions

What happens during this activity

Introduce the apparatus and show how the brakes are applied when the car runs past the small obstruction. Show how the braking force can be changed by adding masses to the top of the brakes. Ask how to measure the braking distance (from where the brakes are applied to where the car comes to a halt).

Ask how the pupils expect the braking distance to change if you double the force? Try and show them how this pattern will work out using a sketch graph – perhaps using the interactive provided. Then challenge them to find out if their expectations are correct by plotting a graph from their own data of braking distance against braking force.

Carefully done experiments very reliably give this shape of graph, confirming that the distance is inversely proportional to the force.

As an extension you can also measure how the distance depends on the energy in the kinetic store, simply by releasing the car from steadily increasing heights. Both of these results have interesting links to road safety. The first to driving in adverse conditions, the second to safety related speed limits.

You might like to illuminate the results using this model:

• ## 04 Modelling brakingMo03TAnugget04 Activity

### Building a model of braking

What the activity is for

You can build this model with pupils to show how the forces deliberately applied to the car by operating the accelerator or the brake lead to the car speeding up or slowing down. Two different models are presented. Both give the same behaviour.

What to prepare

• either one computer, connected to a large display, running the modelling program VnR
• or a collection of computers running the modelling program VnR, so that pupils can work in groups of two or three

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 after working through the appropriate story.

The first model emphasises the calculation of differences in the forces acting to find one force that has the same effect as all of the forces applied separately. This one force then changes the speed.

The second model shows how the forces contribute to the kinetic store of energy. This in turn affects the speed. The accelerator always contributes positively, the drag and the brakes always negatively.

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.

Alternatively, you could use one or a series of models built in CMR, perhaps finishing with this one, if you are focusing on energy:

Or you might finish with this one, if you are focusing on the forces and resulting accumulations:

• ## 05 Convection currentsMo03TAnugget05 Activity

### Demonstrating convection currents

What the activity is for

The purpose of this activity is to provide a clear demonstration of convection currents.

What to prepare

• a waxy paper straw to provide smoke
• matches
• a candle under one of a pair of linked chimneys

What happens during this activity

Place the apparatus in a favourable light, where it can be clearly seen. Make sure there is a plain white backdrop. Describe the apparatus, showing the linked pair of chimneys with the candle placed under one.

Ask what will happen to the air in the chimney with the candle when the candle is lit. Make sure you challenge any suggestions that heat rises. Then ask what will happen to the remainder of the air in the apparatus. Try to build a clear expectation of what will happen before carrying out the demonstration. Light the candle – no air movement is seen. Introduce the smoke as a tracer above the empty chimney by pouring it down the smouldering straw with the unlit end held just above the chimney, but lower than the lit end. The smoke will follow the air down this chimney and up the other one.

• ## 06 Hot roomsMo03TAnugget06 Activity

### Showing convection currents in warm air

What the activity is for

The purpose of this demonstration is to show the build-up of convection currents of warm air in a room.

What to prepare

• a model room
• a data logger plus three temperature probes
• a data projector plus large display

The model room can be made from a shoebox with the front cut away and covered with acetate film. The ceiling and rear wall of the box should be covered with thermochromic film. The room contains a heater mounted next to one end wall that is made from a resistor of a few ohms (sized to match the room – for a shoebox say 1 ohm, 10 watt). Finally, make small holes in the roof and both end walls to accept three temperature probes.

What happens during this activity

Turn on the heater (two to four volt across the resistor usually works well) and wait until the thermochromic film shows the spread of warmth – usually about five minutes. You'll need to practice the demonstration first, and possibly alter the value of the resistor and the power supply settings.

Turn off the heater and lift off the roof of the model room to show the pattern of the temperatures inside. It may be worth capturing this pattern by taking a photograph with a digital camera for future reference.

Then replace the now cool roof over the cool heater and repeat the warming, this time with three temperature probes installed (make sure that the class is involved in choosing where these are placed, so exploring their understanding of what they've just seen). Before the second demonstration, ask the class to predict the shapes of the temperature-time curves as well as the relative values.

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