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Balanced/Unbalanced Forces (asset code : M-Force-1)

Applicable Levels : GCSE, Year 9

Applicable Examination Boards : AQA, EDEXCEL, OCR21C , OCRG

AIM

To demonstrate what happens to objects with balanced and unbalanced forces.

APPARATUS

1. Runway or bench, horizontal if possible
2. Two bench pulleys and string
3. Data logger and 2 light gates
4. Trolley
5. Two 100g mass hangers and 100g slotted masses

METHOD

1. Set up the runway or bench so that it is level. Connect up the light gates to the data logger to measure velocity at each light gate,as shown here.
2. Push the trolley from each end to check if the runway or bench is level; make adjustments as necessary.
3. Fix the bench pulleys to the runway or the bench and connect string to the trolley and both the mass hangers.
4. With no slotted masses on the hangers release the trolley and the mass hangers. Let pupils make any comment at this stage. There will be no movement if the trolley has not been pushed.
5. Repeat step 4 with 100g on each mass hanger and then 200g.
6. Start with trolley at one end and give it a push. Measure the velocity as it passes the first light gate and then when it passes the second. Apply a push at the other end.
7. Repeat step 6 after adding equal masses to the mass hangers.

Have your pupils come to the conclusion that with equal masses hanging at each end;
if the trolley is initially stationary then it remains so, and
if it is given a push then its velocity remains the same.

You should mention that the force of gravity is pulling the masses downwards and the pulleys change the direction of these forces to be horizontal and parallel to the runway or bench. Strings can ONLY PULL so the two forces acting on the trolley are in opposite directions.
Now your pupils can amend their conclusions so that with equal forces acting in opposite directions on the trolley;
if the trolley is initially stationary then it remains so, and
if it is given a push then its velocity remains the same.

You could introduce the idea of Resultant Force now or wait until after the experiments with the Resultant Force not equal to zero have been completed.

8. Now add 100g to one of the mass hangers, with no extra masses on the other, i.e the total mass is 200g one end and 100g the other. Release the trolley from the end with the smaller moving mass, measuring its velocity as it passes the light gates.
9. Add one100g mass to each mass hanger, making the hanging masses 300g and 200g. Release the trolley from the end with the smaller moving mass, again measuring its velocity as it passes the light gates.
Repeat with total masses of 400g and 300g.

It is important that pupils realise that the trolley speeds up in the same direction as that of the larger mass, i.e. in the direction of the larger force acting on the trolley,the driving force. To confirm this repeat the experiments again with the heavier mass falling at the other end.

Introduce the names “driving force” and “counter forces”, discuss what causes the two forces.
Introduce the term “resultant force”.

Show pictures of vehicles with different forces labelled. Students hold up mini whiteboards saying “speed up”, “slow down” or “constant speed” to describe the motion, plus an arrow with the direction and size of the resultant force.

EXAM SPECIFICATION REFERENCES

AQA 4.5.1.2
EDEXCEL 2.14
OCR21C P4.3.6
WJEC 4.2(a)