EDEXCEL INTERNATIONAL GCSE (9 –1)

BUSINESSEDEXCEL INTERNATIONAL GCSE (9 –1)

PHYSICSSA

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TEACHER RESOURCE PACK

Chapter 4: Momentum

1 Calculate the momentum of the following.

a a car of 2500 kg travelling at a speed of 20 m/s

b a motorbike of mass 300 kg travelling at a speed of 45 m/s

c a train of mass 30 000 kg travelling at a speed of 105 m/s

2 A 1 kg trolley A travels at a speed of 1 m/s into a stationary trolley B of mass 1 kg. The two

trolleys stick to each other after the impact.

a Calculate the momentum of trolley A before the impact.

b Calculate the velocity of the two trolleys straight after the impact.

3 A truck of mass 1500 kg moving at 5 m/s to the right collides with another truck of mass 1000 kg

moving at 2.5 m/s to the left.

a What is the total momentum before the collision?

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b What is the total momentum after the collision?

c Calculate the common velocity of the trolleys after the collision.

4 A children’s playground is fitted with a new safety floor. The material contains a large number of air pockets and appears spongey in texture. The marketing leaflet describes the flooring as: ‘A revolutionary new flooring that absorbs the impact, reducing the potential for injuries on your children should they fall. Now with increased numbers of air pockets!’

a Describe, in physics terms, why this type of flooring might reduce the risk of injuries to children.

b A child falls from a swing at a speed of 6 m/s and comes to rest as he collides with the flooring in a time of 0.2 s. Calculate the deceleration of the child.

c Calculate the impact force on the child.

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Chapter 4

1 Which of the following statements about momentum is correct?

A Momentum depends on the weight of the object.

B Momentum is a scalar quantity.

C Momentum is measured in m/s2.

D Momentum is a vector quantity.

2 Which are the units of momentum?

A Kg/m

B Kg/m/s2

C Kg m/s

D N/m

3 Which of the following equations link momentum, velocity and mass correctly?

A mass × momentum = velocity

B momentum = mass/velocity

C momentum = velocity/mass

D momentum = mass × velocity

4 Which of the following statements about momentum is not correct?

A As the velocity of a moving object increases its momentum decreases.

B As the mass of a moving object decreases its momentum decreases.

C Momentum is a vector quantity.

D Momentum is directly proportional to velocity.

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5 Complete the following statement:

The conservation of momentum principle states that the momentum before a collision is …

A more than the momentum after.

B less than the momentum after.

C the same as the momentum after.

D zero.

6 A ball of mass 2 kg travels with a constant speed of 3 m/s. What is the momentum of the ball?

A 0 kg m/s

B 0.66 kg m/s

C 6 N

D 6 kg m/s

7 A puppy bounds along with a momentum of 12 kg m/s. The puppy has a mass of 10 kg. What speed is the puppy travelling?

A 120 m/s

B 1.2 m/s

C 0.83 m/s

D 1.2 kg

8 What is the main reason for crumple zones in a car?

A to absorb the impact

B to increase the stopping force

C to stop the car

D to increase the impact time of the vehicle

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9 Which of the following is a safety feature designed to reduce the impact force on a passenger in a car?

A leather seat

B plastic interior

C radio

D seatbelt

10 A speed skater of mass 65 kg increases her speed from 3 m/s to 6 m/s. What is her change in momentum?

A 195 kg m/s

B 22 kg m/s

C 360 kg m/s

D 195 N/s

11 A motorbike has a mass of 500 kg. Its engine supplies a force of 1000 N for 3 seconds. What is its change in speed?

A 10 m/s

B 6 m/s2

C 3 m/s2

D 6 m/s

12 A trolley hits a stationary trolley half its mass. What is the total momentum of the trolleys after the collision?

A zero

B the same as the momentum of the large trolley before the collision

C less than the momentum of the large trolley

D there is not enough information to calculate this

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13 Which of the following statements about an elastic collision is correct?

A Kinetic energy is conserved.

B Momentum is conserved.

C Energy from the collision is lost.

D Both the kinetic energy and the momentum of the collision is conserved.

14 Which statement about collisions is correct?

A Particles undergo elastic collisions.

B A ball bouncing off the ground is an elastic collision.

C If two objects stick together after a collision the collision is inelastic.

D Momentum is not conserved in all collisions.

15 Person X pushes on person Y. Person X has a mass of 50 kg and person Y has a mass of 45 kg. Which statement is correct?

A The force on X is bigger than the force on Y because X has more mass.

B X is moving faster than Y.

C Only X is pushing so there is no force on Y.

D The force on Y will be the same size but in the opposite direction to the force on X.

16 Which of the following statements is correct in relation to Newton’s third law.

A When all the forces are balanced an object moves at constant velocity.

B F = m × a

C For every action, there is an equal and opposite reaction.

D None of the above.

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17 Two cars collide and move off together following a collision. Car A has a mass of 2000 kg and was travelling at a speed of 5 m/s. The second car has a mass of 3000 kg and was stationary before the collision. What is their speed after the collision?

A 2.5 m/s

B 2 m/s

C 5 m/s

D 0 m/s

18 The term impulse is used to represent which equation in physics?

A m × a

B m × g

C change in momentum / time

D F × t

19 A car of mass 2500 kg is travelling at 25 m/s and comes to a complete stop in 5 seconds. What is the force provided by the brakes to stop the car?

A 12 500 N

B 62 500 N

C 312 500 N

D 2500 N

20 A golf ball of mass 500 g experiences an impulse of 4 Ns. What is the ball’s change in momentum?

A 4 kg m/s

B 2 kg m/s

C 2000 kg m/s

D 0.008 kg m/s

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Answers 1 D – Momentum has a direction and magnitude. 2 C – Mass × velocity = kg × m/s. 3 D 4 A – From the equation momentum = m × v, if v increases then momentum will too. 5 C 6 D 7 B – Rearrange so 12/10 = 1.2. 8 D – The force acting on the body is reduced if the impact time is increased. 9 D 10 A – Change is 3 × 65. 11 D – Impulse is force × time = change in momentum. 12 B – Conservation of momentum principle states that the momentum before must equal the

momentum after so if the second trolley was not moving the total momentum after has to equal that of the moving trolley.

13 D – Both KE and momentum have to be conserved for an elastic collision. 14 D – It is not possible to have conservation in all collisions as some energy will be transferred to

other forms, e.g. sound, heat, etc. 15 D – This is Newton’s third law. 16 C 17 B – Momentum before = 10 000. After, the mass of the two trolleys is combined to find v. 18 D 19 A 20 A – Impulse is also equal to change in momentum so will equate to 4 kg m/s.

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Chapter 4: Momentum Alignment with Student Book: pages 40–47

Chapter overview

This chapter expands on Chapter 3 and is for those students studying physics as a separate subject. The idea of momentum is introduced and the chapter explores the conservation of momentum through looking at collision and explosion situations, though the latter is discussed only briefly. The relationship between impact force and an increase in time during collisions is presented in relation to car safety. Newton’s third law is also introduced in this chapter.

It is assumed that students will be able to rearrange equations using either the formula triangles or mathematical methods. Students should be aware of Newton’s second law from Chapter 3 before embarking on this chapter.

What to expect

1.25P know and use the relationship between momentum, mass and velocity: momentum = mass × velocity, p = m × v

1.26P use the idea of momentum to explain safety features

1.27P use the conservation of momentum to calculate the mass, velocity or momentum of objects

1.28P use the relationship between force, change in momentum and time taken: force = change in momentum/time, F = (mv − mu)/t

1.29P demonstrate an understanding of Newton’s third law

This will be a new topic for students and, as such, time should be spent on allowing students to familiarise themselves with calculations that require them to use the conservation of momentum principle. Vectors will once again be important in this chapter in relation to movement after a collision. Considering the situation in terms of before and after a collision is key for understanding.

Language will again be key in this chapter. Students will benefit from breaking down responses to safety feature questions in order to remove the term ‘absorbs the impact’ from their answers.

Teaching notes

Starters

Which would be harder to stop: Show students some pictures of objects moving with no speed indicated on them to start. Choose objects of varying mass and speed, e.g. a moving bullet, rocket, car, bike, ice skater, the Moon, etc. Ask students which would be harder to stop. Students can discuss in pairs and bring their ideas for discussion. Discussion should focus around the two key variables: mass and velocity.

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Skateboards or rolling platforms demonstration: Newton’s third law / momentum demo. Place two skateboards opposite one another, drawing a chalk line on the floor where they meet. Ask two students of notably different masses to stand on the skateboards. (If students are reluctant, you could participate in demo.) Ask students to place their hands out, at chest height, in front and connect with the other student’s hands. Ask students to push away from one another. Two discussions can take place here: 1) Newton’s third law in relation to the force felt on the hands of both students, equal and opposite reaction force; 2) How far did the skateboards travel? Students can measure the distance each skateboard travelled from the starting point. The skateboard with the larger student should travel less distance as the student will have accelerated less as their mass was greater. This should be visible to students as the skateboards moved. This can also be done with a set of explosion trolleys where the masses of the trolleys can be varied.

Crumple zone demonstration: A trolley, a ramp, marble, evaporating basin, wooden block, sticky tack, sticky tape and paper will be required for this demo. Set up the trolley with the evaporating basin fixed to the top of the trolley using sticky tack. Place the marble in the evaporating basin. Construct a tube using the paper and fix to the front of the trolley using tape. This will act as a crumple zone for the car. Tilt the ramp and place a wooden block at the end of the ramp. Release the trolley from the top of the ramp and watch how the marble behaves. Repeat for different lengths of crumple zone and do one run without the crumple zone. Students should note that with short tubes / no crumple zone the marble is accelerated out of the basin which implies that it experiences a much greater force. This could be adapted into an investigation if necessary.

Mains/practical

Air-track demonstration: An air-track with two trolleys of varying mass can be used to illustrate the principle of conservation of momentum. Students need to be aware that the demonstration is considering the momentum before and after the event. Discussion can also focus around why the air-track is used. Start the demonstration by having one stationary trolley in the middle of the track. Slide a second trolley towards the stationary one. When the two trolleys collide, the first trolley will transfer all of its momentum to the second trolley. The first trolley will remain stationary whilst the second moves off with the original speed. This can be repeated for different trolley masses so students can identify the impact on the speed. Some air-track set ups allow for collisions where the trolleys stick together after collision. This is a good visual aid to help explain what is happening in all instances of momentum.

Crumple zone practical: Students should be grouped into fours. Students should be issued with three sheets of newspaper, three straws and sticky tape. Students may be issued with any materials but each group should have the same amount allocated. The aim of the activity is to make a protective casing that will protect an egg falling from a height (i.e. a fire escape, landing or window). Students have 10 minutes to create their casing based on the principles learned from the force = change in momentum/time calculation. The winning team is the one who creates a construct that protects the egg with the least amount of materials.

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Differentiation

Provide weaker students with worked examples to support the calculations format.

Students can design an investigation to test different materials’ suitability for safety flooring in a children’s crèche / soft play area.

Homework

Example calculations such as those on the worksheet or on page 47 would be suitable.

Possible misunderstandings

There are two areas to focus on. Students will take time to recognise that the momentum before a collision is the same as after a collision when applying the conservation of momentum to questions. Consistent reiteration and practise will expose students to the process of calculating momentum after.

Students will also frequently comment on how safety features ‘absorb the impact’ rather than link to the idea that increasing time taken for an impact to happen reduces the impact force based on Newton’s second law. Demonstrations of this using hard materials contrasted with sponge, for example, will give weaker students a physical memory of this principle.

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PhysicsPaper1ExamQuestion– ForcesAcarhasaconstantdrivingforceof30000N.Itinitiallyacceleratesbuteventuallyitreachesaconstantspeed.Explainwhy,ifthedrivingforceisconstant,thecarreachesaconstantspeed.

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StudentResponse1

Thecardrivesoffandaccelerates.Itthenstopsacceleratingbecauseoftheairresistancebecomesequalandthedrivertakesherfootoffthepedalsoitslowsdowntoaconstantspeed.

Isthisagoodresponse?

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StudentResponse1:Verdict

Thecardrivesoffandaccelerates.Itthenstopsacceleratingbecauseoftheairresistancebecomesequalandthedrivertakesherfootoffthepedalsoitslowsdowntoaconstantspeed.

Thisanswerisverybasicandalittleconfused.Therightideaistherebutneedsdeveloping.Thiswouldbeunlikelytoscoreamark.

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StudentResponse1:Improvements

Thecardrivesoffandaccelerates.Thestudentneedstoexplainwhy thecaracceleratesasitstatesthisinthequestion.Whatwouldthisimplyabouttheforcesactingonthecar?

Itthenstopsacceleratingbecauseoftheairresistancebecomesequal…Thisanswerneedstobemoredetailedtoscorethemarksavailable.ThestudentneedstomentionNewton’ssecondlawandwhatishappeningtotheresultantforce.

…andthedrivertakesherfootoffthepedalsoitslowsdowntoaconstantspeed.Thisisnotavalidpoint.Thequestionstatesthattheforwardforcefromtheengineremainsconstant.

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StudentResponse2

Theforwardforceofthecarinitiallycausesanacceleration.Asthecarspeedsup,theairresistancegetsgreater.Thismeanstheresultantforcegetssmaller.Eventually,theforwardforceisequalandoppositetotheresistiveforcesotheresultantiszero.Thecarisatterminalvelocity.

Isthisagoodanswer?

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StudentResponse2:Breakdown

Theforwardforceofthecarinitiallycausesanacceleration.ThestudentcouldreferenceF =ma heretogetthefirstmark.

Asthecarspeedsup,theairresistancegetsgreater. [1]Thisshowsthattheresistiveforcesarelinkedtospeed.Thismeanstheresultantforcegetssmaller.Acommentontheimpactoftheaccelerationwouldbegoodhere.

Eventually,theforwardforceisequalandoppositetotheresistiveforcesotheresultantiszero.[1]Again,referencetoF =ma wouldberelevanthere.

Thecarisatterminalvelocity.[1]SAM

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