Forces in action

Lesson 1 – The turning effect of forces and levers

You’ve just bought and IKEA desk, but hammered a nail in the wrong place. Have you ever tried pulling the nail out with your bare fingers? Is really HARD to IMPOSSIBLE!Let’s talk about turning forces: What are they?

Demo:

1. What is A?2. What is going on here?3. Where is the pivot point? 4. Where is LESS EFFORT needed to OPEN the tin?5. Would it be better to have a longer or shorter screwdriver?6. The tin lid has some dried paint around the edge that is sticking the lid to the tin.

How does this affect the amount of force needed to be applied to the screwdriver?

Levers and PivotsA lever is a simple machine consisting of a rigid object (like a bar) that will rotate around a fixed point – the pivot

7. Copy and complete the sentence belowAs the lever arm length increases the effort needed to mover the load……..

8. If we added a second bolder to the load in the diagram what two ways could we change the diagram to ensure the woman can lift it?

Defining a level and pivot.

9. Label the pivot, the lever and the direction of the force in each of these examples to work.

Moment Formula

This nail, I’ve trying to put out, just wouldn’t budge! Any ideas as to what I could do to help get this nail out? Have a think!The two factors that affect the moment are the force applied and the distance from the pivot.

Moment = Force x Perpendicular distance

(Nm)(Ncm)

(N)(N)

(m)(cm)

Calculate the turning force of a spanner that is 20cm long when a force of 13N is applied. Moment = F x dMoment = 13 x 20

Moment = 260NcmThe turning effect of a force is called a “moment”. We work it out using The force must be in Newtons (N), the distance in metres (m) and we measure moments in “Newton Metres”(Nm). Sometimes the distance may be in (cm) and then the moment is in Ncm.

10.Calculate the moments of the forces in each diagram. Make sure you show your workings correctly

11. What is the distance from the pivot that a weight of 3.6N needs to be placed at to produce a moment of 9.5 Ncm? Give your answer to 3 significant figures.

12.A tap has a handle that is 11cm long. The turning force required to turn it on is 17Ncm. Calculate the force needed to turn the tap on. Give your answer to 1 dp.

13.A man holds a fishing rod with a fish hooked. The rod length is 3.2m and the weight of the fish is 22N. He holds the rod 30cm up from the bottom. Calculate the turning effect he feels. Give your answer to 2 significant figures.

Lesson 2 Moments in balanceHow is this even possible?To understand how objects can balance we need to be able to analyse the moments on each side of the pivots separately. If the clockwise and anticlockwise moments are equal, then the objects will balance. If the moments are not equal, then the objects will topple.

So lets look at each side of the PIVOT and Its MOMENTS separately 14.A see-saw has two moments. Calculate the moments for both sides and state which

way the seesaws will tip (clockwise or anticlockwise). Show your working. (Check back to lesson 1 to see how this is done)

a.

Moment = Force x Distance (Nm) (N) (m)i

3m 2N2N 4m

b.

c.

15. The diagram shows a crane lifting a load. The counterweight and the load are balanced.

 

(a)     The load is moved away from the pivot, to the right.

(i)      What happens to the turning moment produced by the load?

(ii)     What should happen to the counterweight to keep the arm balanced?

(b)     A load of 5000 N is placed 8 m from the pivot.

(i)      What is the turning moment of the load? Give the unit.

(ii)     How far from the pivot must the 10000 N counter weight be placed in order to balance the load?

(iii)     The counterweight is placed 3 m from the pivot.What load could now be balanced 8 m from the pivot?

4m 0.5N2N 0.75m

4cm 1.5N2N 3cm

16.(a) A ladder is hung on one hook so that it balances horizontally.

Draw an arrow on the diagram below to show the position of the hook.

 

(b)     A window cleaner puts the ladder on his shoulder in the position shown.

 (i)      The weight of the ladder acts at its centre, as shown. Calculate the turning effect, or moment, about his shoulder due to the weight of the ladder. Give the unit.

         The window cleaner hooks his bucket onto the ladder as shown in the diagram.The bucket weighs 15 N and is 0.8 m from his shoulder.

 

(ii)     Calculate the turning effect, or moment, about his shoulder due to the weight of the bucket only.

(iii)     Calculate the size and direction (clockwise or anticlockwise) of the total turning effect, or moment, about his shoulder due to the weight of the ladder and the weight of the bucket.

(c)     To keep the ladder and bucket balanced on his shoulder, the window cleaner has to pull down with his hand.

(i)      How can he move the ladder on his shoulder to reduce the force his hand has to balance?

(ii)     How could he change the position of the bucket to reduce the force his hand has to balance?

17.The crane has to lift a load of 10000N at a distance of 20m from the pivot.What distance should the counterweight of 2500 N be placed to ensure balance?

18.A load of 100N is positioned 2 metres to the left hand side of a pivot. How far away would you have to place a 200N load on the right hand side of the pivot?

19.A 150N load is held 3 metres from a pivot. If the load is balanced by another load which is 1.5 metres away from the pivot, what force is needed to do this?

20.Two pupils are sat on a see-saw. If the pupil on the left has a load of 45N and is sat 2 metres away from the pivot, how far away must the other pupil sit if they have a load of 30N?

21.A crane lifts a 4000N block using an arm which is 20 metres in length. If the counter weight is 2 metres from the pivot, what force must the counter weight produce?

30N

50cm

22.What is a moment?23.Calculate the moment of the forces shown. Show your working. Give the unit. Say

whether each is clockwise or anticlockwise.

(c)

24.Harry was trying to loosen a nut on a car tyre. It does not move.(a) calculate the moment Harry

applied to the nut(b) Suggest a value for the

frictional force of the nut on the screw

(c) Suggest two ways to increase the moment

25N 10N

5mx

25.Look at this example of a balanced system.

a) Calculate the size of the clockwise momentb) State the size of the anticlockwise moment c) Calculate the distance x.

26.The picture shows a crane lifting a load. Explain why, in this instance, the counterweight needs to be heavier than the load.

Lesson 2 WORK!So what does work in physics actually mean?Work done is the amount of energy transferred when a force causes something to be displaced (move). When a force moves an object, energy is transferred, and work is done.

• When something moves, something else must provide some sort of effort to move it

• The something putting in the effort needs a supply of energy such as food, fuel, electricity.

• It does work and so moves the object – energy is transferred into other stores e.g kinetic

• We therefore say, work is done.

FormulaWithout drawing a formula triangle, rearrange the formula to show all three combinations.

Work Done (J) = Force (N) x Distance (m)Making the FORCE the subject:

Force (N) = Work Done (J) Distance (m)

Making the DISTANCE the subject: Distance = Work Done (J)

Force (N)Let me show you one being done:A car is dragged 570cm over flat ground. It is pulled by a force of 360N. What is the work done?

Work done = Force x DistanceDistance must be in m so 570/100 = 5.7m

Wd = 360 x 5.7 = 2052J

SKILLS:Significant figures

• If the answer you asks for a particular number of significant figures, make sure you do this – there will be one mark for doing this.

• Make sure you round appropriately• E.g

13265J to 3 significant figures = 13300J8.7625J to 3 significant figures = 8.76J

27. Round the following numbers to 3 significant figures (or 3 Sig Figs)a. 56,147b. 26,813c. 79,611d. 0.8006e. 0.4589

f. 57,402 g. 5288 h. 89.67 i. 0.42 j. 0.758

28.Look at the table below. Try to complete the boxes by using the values for force and distance. For example W= F x d , W= ? F=12N d=35cm = 0.35mW= 12x0.35= 4.2J

Force (___) (Force = Work done x distance)

Dis

tanc

e (…

…)

(Dis

tanc

e =

wor

k do

ne /

forc

e)

12 120,000 8

35cm 4.2 J3.85 J

100m 37200

3.62cm

1.87m

1.2 J 1.1 J

29. The diagram shows an adult and a child pushing a loaded shopping trolley.                                   

(a)     (i)      What is the total force on the trolley due to the adult and child?

(ii)     Which one of the terms in the box means the same as total force?

         Draw a ring around your answer. 

          answer force          mean force        resultant force

(1)

(iii)     The trolley is pushed at a constant speed for 80 metres.

         Calculate the work done to push the trolley 80 metres.

         Show clearly how you work out your answer.

(b)     Complete the following sentences by drawing a ring around the correct word in each of the boxes.

 

(i)       The unit of work done is the

joulenewtonwatt

.

30. The diagram shows a worker using a constant force of 60 N to push a crate across the floor.

 

(a)     The crate moves at a constant speed in a straight line

(i)      Draw an arrow on the diagram to show the direction of the friction force acting on the moving crate.

(ii)     State the size of the friction force acting on the moving crate.

Give the reason for your answer.

(b)     Calculate the work done by the worker to push the crate 28 metres.

Show clearly how you work out your answer and give the unit.

31. The diagram shows a climber part way up a cliff.

 

(a)     Complete the sentence.

When the climber moves up the cliff, the climber

gains gravitational ______________________ energy.(1)

(b)     The climber weighs 660 N.

(i)      Calculate the work the climber must do against gravity, to climb to the top of the cliff.

Lesson 4 Simple machinesMachines have truly revolutionised civilisation. Simple machines have allowed humans to build the ancient cities of Romans, Greeks and Egyptians. Machines do not have to be complex. A simple machine is a device which alters the size of a force or the direction in which a force acts.Examples include:

LeversWheels / GearsAxels

PulleysScrewsWedges

32.Label the diagrams below using the keywords above

33.Some students investigated how the radius of the cog effected the force need to make them move

Work out the averages for these results:

Radius of cog (cm)

Force Applied (N)

Test 1 Test 2 Test 3 Average

5 105.0 110.0 100.0 

10 20.0 50.0 50.0 

15 33.0 34.0 33.0 

20 25.0 25.0 60.0 

25 19.0 35.0 21.0 

34.What is the relationship between the radius of cog and the average force applied?35.What is the relationship between the radius of cog and the distance moved?36.What do you notice about the average force applied x distance moved?37.

(a) The diagram shows a builder using a plank to help load rubble into a skip.

 

The builder uses a force of 220 N to push the wheelbarrow up the plank.

Use information from the diagram to calculate the work done to push the wheelbarrow up the plank to the skip.

Show clearly how you work out your answer.

(b)     A student investigated how the force needed to pull a brick up a slope, at a steady speed, depends on the angle of the slope.The apparatus used by the student is shown in the diagram.

 

The student used the results from the investigation to plot the points for a graph of force used against the angle of the slope.

 

(i)      Draw a line of best fit for these points.

(ii)     How does the force used to pull the brick up the slope change as the angle of the slope increases?

(iii)    Consider the results from this experiment.Should the student recommend that the builder use a long plank or a short plank to help load the skip? Explain the reason for your answer.

Lesson 4 Hooke’s law.When a force acts on an object it can deform, for elastic objects this deformation can be stretching or squashing. Hooke’s law looks at the relationship between the force on an elastic object and how much it will stretch by.Hooke’s law states that: The extension of an elastic object is directly proportional to the force applied.As more force is applied the spring should extend more and this should be constant i.e doubling the force will double the extension.You are going to plan and carry out an investigation to see if Hooke’s law holds true.

Hypothesis: The extension of a spring is proportional to the force applied to the springIndependent variable:…………………………………………………………………………………………………………………………………………..…………………………………………………………………………………………………………………………………………………………………………..Dependent variable:………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..Control variables:………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..

Method:1. Hang your spring off the clamp and stand and secure a ruler so that the zero line is level with the bottom of the spring.2. Add the hanger with 100g (1N) mass onto the bottom of the spring. 3. When the spring is still, read off the extension – how much longer the spring has got. 4. Record your measurement and then continue, adding 100g masses up to 500g. Do not go beyond that. 5. Remove all masses and repeat. Diagram:

38.Design your own table for results. Include columns for repeats & means and convert g into force (100g = 1N)

39.Plot your results on the graph paper below.

40.What was the range and interval of your independent variable? Don’t forget the unit.

41.Describe the relationship between force and extension. What does a straight line through the origin tell you?

42.What happened to the shape and size of your spring when you removed the weights?

43.What word is used to describe objects that behave in this way?44.A student carried out the same experiment as you – their graph is shown to the

right. What happened after 1.5N? From the practical you should be able to prove Hooke’s law true, you should have been able to produce a straight line graph that passes through the origin, if not it’s important to think about what errors might have occurred that skewed your results.There is a formula linking the force on the elastic material and it’s extension:

Force=springconstant ×extension

F=K x

The spring constant is how much force is required to extend the spring by 1 meter and is measured in Newtons per meter (N/m), the force will always be calculated in Newtons (N) and the extension in meters (m).Example question:Calculate the force applied to a spring that stretches 0.2m and has a spring constant of 10N/m.

F = 0.2 x 10F = 2N

All elastic objects will obey Hooke’s law but only to a certain point, this is known as the limit of elasticity, after this point the elastic material becomes permanently deformed and the extension and force applied are no longer directly proportional. This would be characterised by a plateau on the force-extension graph.When the spring is stretched energy must be transferred into the spring, this energy will transfer the elastic store of the spring, when the spring is released the energy store will transfer into the kinetic energy of the spring as it moves back into its natural shape, this energy eventually transfers to the thermal store of the surroundings.

45.State Hooke’s law.46.Give the formula relating, force, spring constant and extension.47.What two things should be seen if a relationship is directly proportional.48.Sketch a graph showing Hooke’s law, label the limit of elasticity.49.A spring has a spring constant of 5 N/m and extends by 0.3 metres. Calculate the

force needed to make this happen.50.A spring has a spring constant of 7.5 N/m and extends by 0.45 metres. Calculate

the force needed to make this happen.51.A spring has a spring constant of 13 N/m and extends by 0.7 metres. Calculate the

force needed to make this happen.

52.A spring has a force of 2.1 N applied to it and a spring constant of 0.3 N / m. Calculate the extension.

53.A spring has a force of 4.6 N applied to it and a spring constant of 4.3 N / m. Calculate the extension.

54.A spring has a force of 7.3 N applied to it and a spring constant of 2 N / m. Calculate the extension.

55.A rubber ball has a force of 2 N applied to it and is compressed by 0.04 metres. Calculate the spring constant.

56.A spring has a force of 6 N applied to it and is compressed by 0.2 metres. Calculate the spring constant.

57.A spring has a force of 4.5 N applied to it and extends by 0.08 metres. Calculate the spring constant.

58.A bungee cord extends by 50 m when a force of 0.6 KN is applied to it. Calculate the spring constant.

59.An elastic band stretches has a spring constant of 10 N / m and extends by 15 cm when a force is applied to it. Calculate the force needed to cause this extension.

60.A watch spring has a spring constant of 17 N /cm. When the timing mechanism applies a force of 34 N to the watch spring, calculate the extension in metres.

61.A student investigated how the extension of a spring depends on the force applied to the spring.

The diagram shows the spring before and after a force had been applied.

 

(a)      (i)     Complete the following sentence using letters, A, B, C or D, from the diagram.

The extension of the spring is the distance between the positions labelled

___________and ___________ on the metre rule.

(ii)     What energy store increases when the spring is stretched?

(b)     The results from the investigation are plotted on the following graph.

 (i)     The graph shows that the student has made

an error throughout the investigation.

What error has the student made?

Give the reason for your answer.

(ii)     The student has loaded the spring beyond its limit of proportionality.

Mark on the graph line the limit of proportionality of the spring. Label the point P.

Give the reason for choosing your point P.

Revision 62.A student investigated how different forces affected a spring. She added masses,

100g at a time, to the end of a clamped spring, and measured the length of the spring. Her results are shown below:Force added (N) Length of spring (cm) Extension (cm)

0 3.2 01 3.5 0.32 3.8 0.63 4.14 4.45 4.76 5.57 6.1

a) Complete the table by calculating the extension of the spring for each mass. b) Describe the energy transfer as the mass is added to the spring:As the mass moves down.. As the mass slows to a stop…Some energy will always be transferred to….

63.Draw the force/extension graph on graph paper. 64.What would the extension of the spring be if: 1.5N was added ………………………………………. 8N was added …………………………………………65.What force would need to be added to make the spring length 3.7cm?

……………………66.Describe the relationship shown between force and extension in your graph

between 0 and 5N67.What has happened at 6N?68.The elastic limit of the spring may not be 6N. Suggest how it could be found more

accurately. 69.A spring has a force of 2.1 N applied to it and a spring constant of 0.3 N / m.

Calculate the extension.

70.An elastic band stretches has a spring constant of 10 N / m and extends by 15 cm when a force is applied to it. Calculate the force needed to cause this extension.

71.A watch spring has a spring constant of 17 N /cm. When the timing mechanism applies a force of 34 N to the watch spring, calculate the extension in metres.