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VCE Physics Unit 3

Preparation Work

Transition into 2019

VCE Physics Unit 3+4

Units 3 and 4 include four core areas of study plus one detailed study.

Unit 3: How do fields explain motion and electricity?

Areas of study:

1. How do things move without contact (Fields and Forces) 2. How are fields used to move electrical energy

(Electromagnetism) 3. How fast can things go? (Mechanics)

Unit 4: How can two contradictory models explain both light and matter?

Areas of study:

1. How can waves explain the behaviour of light? (light as a waves) 2. How are light and matter similar? (Nature of light and matter) 3. Practical investigation

Assessment for units 3 and 4

In the study of VCE Physics students’ level of achievement will be determined by School-assessed Coursework and an end-of-year examination. Percentage contributions to the study score in VCE Physics are as follows: • Unit 3 School-assessed Coursework: 21 % • Unit 4 School-assessed Coursework: 19 % • End-of-year examination: 60 %.

Mechanics –Revision (formulae from year 11)

Average speed = distance travelled = v = ts

Time taken

Average velocity = displacement = v = ts

Time taken

Acceleration = change in velocity = a = ∆v∆t

Time taken

Equations of motion:

s = ½ (u + v) t s = Displacement

v = u + at t = time

s = ut + ½ a t 2 a = acceleration

s = vt - ½ a t 2 u = initial velocity

v 2 = u2 + 2as v = final velocity

Newtons Laws:

1. Every object remains at rest or with a constant velocity unless acted on by an unbalanced force.

2. Acceleration of a body is directly proportional to the net force and inversely proportional to mass.

ΣF = ma

3. For every action force there is an equal and opposite reaction force.

Momentum:

Momentum is ALWAYS conserved during a collision within an isolated system. When it appears that momentum is not conserved it can be explained by the fact that the system is not closed/isolated. pi = pf

Momentum = mass × velocity p = mv

Impulse = mass × change in velocity ∆p = m∆v

= net force × time interval ∆p = ΣF∆t

Work, energy and power:

Kinetic energy Ek = ½mv2

Gravitational potential energy Ep = mgh

Elastic Potential energy Es = ½kx2

Work done W = ∆E = F s cosθ

Power is the rate of doing work

P = W = F× s = Fv t t

Vector addition of forces Instructions:

1. Set up a central unknown mass hanging from two pullies as shown in the diagram below.

2. Place two equal masses on either side of the pulleys so that it is balanced (check that none of the masses are caught on anything.

3. Measure the two angles as shown below:

4. Now change the mass on the side pulleys so that they are no longer equal.

5. Record the size of each mass and the angles for each.

Use your results to determine the size of the unknown mass.

Mass 1 Angle 1 Vertical force due to mass 1

Mass 2 Angle 2 Vertical force due to mass 2

Resolving forces when pulling at an angle

Instructions

Measure the weight of a wooden block by dangling it from a spring balance.

Use a spring balance to pull a wooden block along the surface of the desk at a constant speed. Keep the spring balance horizontal.

Repeat the process, but this time hold the spring balance at a series of different angles as you pull the block along (see diagram).

Record your results in the table below:

Weight of Block:

Angle of spring balance to the horizontal

Force on spring balance (N) Horizontal component of the force (N)

Trial 1 Trial 2 Trial 3 Average

0o

Questions.

1. According to Newton’s 1st law what can you say about the net force on the block when it is travelling at a constant speed?

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2. Use your results when you kept the angle of the spring balance at 0o to determine the friction which must have been acting on the block.

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3. Consider the weight of the wooden block. Use one of your results to work out the effect of the vertical component of the spring balance on the weight of the block.

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4. Look at the Horizontal components for each of your measurements. What would you deduce about the effect of the angle on the amount of friction?

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5. Suggest other measurements you could make to check your answer to question 4.

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Force Diagrams

For each diagram, a rock is being acted upon by one or more forces. All diagrams are in the vertical plane and effect of friction can be ignored.

On the right is each diagram is dot with the dashed outline of the rock. Draw one or more arrows from the dot to represent all the forces acting on the rock. Show the relative size of forces by drawing different sized arrows.

Label each arrow with the name of the force and its full description, as in the first example.

Tension:

Force by string

Tension:

Force by string

Weight:

Force by Earth on

The inclined plane and Acceleration down a slope

Figure 1

Figure 1

THE NORMAL FORCE When an object exerts a force on a surface, the surface exerts a reaction force on the object that is normal (at right angles) to the surface. For example, the block in Figure 1(a) exerts a force on the surface because it is attracted towards the centre of the Earth by gravity. The surface exerts a normal reaction force on the block. The weight Fg is thus balanced by FN as shown in the figure. There is no net force on the block, and so Newton’s first law applies and the object remains stationary.

On an inclined plane, FN is at an angle to Fg . There is a net force down the slope and the block accelerates as predicted by Newton’s second law. Another way of viewing the forces along the inclined plane is to resolve the weight vector into two components: one perpendicular (at right angles) to the slope, and one parallel to the slope as shown in Figure 1 b. The component perpendicular to the surface is balanced by the normal force FN . The component of the weight directed along the slope is the force that actually causes the acceleration.

Therefore, for inclined planes: ΣF = mg sinθ

and a = g sinθ

Eg. A skier of mass 50 kg is skiing down an icy slope that is inclined at 20° to the horizontal. Assume that friction is negligible and that the acceleration due to gravity is 9.8 m s–2.

Determine the normal force that acts on the skier

Determine the acceleration of the Skier a = g sinθ

= 9.8 x sin 20

= 3.36 m/s2

Past exam questions

Question 2

Question 5

Jack and Jill are racing their toboggans down an icy hill. Jack and Jill are of similar mass and are using the same type of toboggan. When Jack is a certain distance from the end of the race they are travelling with the same velocity. Jack is behind Jill and decides that if he is going to win the race he must lighten his toboggan, so he pushes a box containing their ice-skating gear off the side of his toboggan.

Explain, giving clear reasons, whether this will be a successful way for Jack to catch up to Jill and help him win the race. ………………………………………………………………………………………………………………………………………………………………………

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Questions from your textbook (5.1)

Momentum and energy.

Conservation of momentum

How high will my red ball fly?

The Multiple collision accelerator (or Astroblaster) can be used to illustrate some of the important physics formulae that you already know.

The mass of the balls are as follows: Blue: 65.2g, Green: 27.7g, Yellow: 9.5g and Red: 4.0g. If they collide with the floor or each other, you can assume that tno momentum will be lost from the system.

1. If you drop the Astroblaster from a height of 1m, calculate the speed with which it will hit the ground.

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2. If the blue ball hits the ground and ‘tries to move upwards, but immediately collides with the green ball, calculate the speed of the green ball.

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3. If the green ball ‘tries to move upwards, it immediately collides with the yellow ball. Now calculate the speed of the yellow ball.

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4. If the yellow ball ‘tries to move upwards, it immediately collides with the red ball. Now calculate the speed of the red ball.

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5. How high could the red ball fly?

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6. Why won’t it really fly that high?

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Now try it and see.

Momentum questions:

1. A 60.0 g tennis ball was moving directly at a wall with velocity 20.0 m s-1 south. It bounced straight back, leaving the wall at 12.0 m s-1. The ball was in contact with the wall for 0.0080 sec.

i. What was the change in speed of the ball?

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ii. What was the magnitude and direction of the ball's

change in velocity?

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iii. What was the magnitude and direction of the impulse applied to the ball by the wall?

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iv. What was the magnitude and direction of the impulse applied to the wall by the ball?

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v. Calculate the average force applied to the ball by the wall.

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vi. The change in momentum of the ball is obvious. The change in momentum of the wall should be equal in magnitude and opposite in direction to the change in momentum of the ball. Why is the change in momentum of the wall not obvious?

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20 m s

12 m s

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Conservation of energy.

Experiment: Investigating the behaviour of a bungee cord

Background

You have recently been employed by the company Bungee Bonanza. A key part of your role is to adjust the height of the bungee jumping platform based on the mass of each participant. You want to maximise the thrill of the bungee jump and ensure safety at all times. If you set the platform too high, the participant won’t end up close enough to the ground at the bottom of their fall and it may be boring. If you set the platform too low, then the participant (and you!) will be in big trouble as they will hit the ground.

The investigation

To help you explore this problem you will create a scale model of the bungee jump apparatus using linked rubber bands as your bungee rope. You will use figurines and masses to investigate the relationship between mass and the distance of the drop. The method you use is up to you.

In your scale model the minimum height of the platform above the ground is 100 centimetres and the maximum height is the top of a retort stand placed on a laboratory bench. The minimum and maximum masses of a participant are 100 grams and 400 grams respectively. You will need to select a length of bungee rope that is appropriate based on these parameters. You will also be provided with the following equipment to help you:

Retort stand, boss head, clamp, masking tape, rubber bands, metre ruler, tape measure, scales.

The competition

After your investigation a competition will be held to provide the most thrilling, but safe, bungee jump for a new participant. Based on the mass of your new participant you will set your jump platform to an appropriate height. The participant can safely come within 2 centimetres of the ground. The group who gets their participant the closest to 2 centimetres from the ground at the bottom of their drop, but no closer to the ground, wins.

Record your results and plot a suitable graph in the space below. The objective is for you to use these results next lesson to determine the drop height for an unexpected customer.

Results:

Questions:

1. Explain in terms of key physics principles why a person with a large mass falls further than a person with a smaller mass.

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2. By calculating the area under the graph, determine the energy stored in the rubber band at maximum extension. Note this area will have unit N × m = joule = J

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3. Draw and label the forces on the bungee jumper in each diagram. Label the forces using the convention ‘Force on A by B’ . Show the relative size of the force on the small and big mass.

Small mass bungee jumper BIG mass bungee jumper

a. Just starting to fall, accelerating downwards (rope not stretched)

b. Part way through fall, cord just beginning to stretch, but still moving downwards

Small jumper slows down

Big jumper still accelerating downwards

c. As cord begins to stretch more

Small jumper at maximum stretch

Big jumper slows down

d. Maximum stretch for the biggest jumper.

Big jumper at maximum stretch

4. The following is a past exam question relating to extension of springs:

PTO

Questions from your textbook (5.7)

Holiday Checklist

You must make sure you do the following before you come back to school: Everything that you read should be very familiar to you. There are only a few new ideas in some sections

Task Tick when complete

1. Make sure you have completed all questions and practical work on your transition booklet and have this with you during your first lesson.

2. Read 5.1: Newtonian theories of motion (p 148-154). Answer 5.1, p154-155, ALL QUESTIONS (A copy of these are included in this booklet).

3. Read 5.7: Conservation of energy and momentum (p 191-195). Answer 5.7 p196, ALL QUESTIONS (A copy of these are included in this booklet).

4. Read 7.1: Impulse (p228-233). Answer 7.1 p234, ALL QUESTIONS.

5. Read 7.2: Work done (p235-239). Answer 7.2 p239, ALL QUESTIONS

6. Read 7.3: Strain potential energy (p240-243). Answer 7.3 p243, ALL QUESTIONS

7. Read 7.4: Kinetic and Potential Energy (p244-251). Answer 7.4 p251, ALL QUESTIONS

8. Recap over the motion section in EDROLO. This is called Area of Study 3, “How fast can things go?”. I would recommend you listen to Edrolo video’s and try answering the online questions.