VCE Physics - Random Exam Revision 4

Two identical bar magnets of the same magnetic field strength are arranged at right angles to each other and at the same distance from point P, as shown.

At point P on the figure, draw an arrow indicating the direction of the combined magnetic field of the two bar magnets.

_{VCAA 2021}

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Answer explanation

The direction of the field is given by the sum of the fields from the horizontal magnet (to the right) and from the vertical magnet (down). The two components have the same size so the angle needed to be 45⁰. Make sure that you arrow touches point P.

The figure below shows the source of a magnetic field.

Which magnetic field pattern it produce?

_{VCAA 2005 Modified}

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The figure below shows the source of a magnetic field.

Which magnetic field pattern it produce?

_{VCAA 2005 Modified}

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The figure below shows the source of a magnetic field.

Which magnetic field pattern it produce?

_{VCAA 2005 Modified}

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The figure below shows a small ball of mass 1.8 kg travelling in a horizontal circular path at a constant speed while suspended from the ceiling by a 0.75 m long string.

Calculate the speed of the ball.

_{VCAA 2020}

• 1.2 ms^-1

• 1.4 ms^-1

• 1.8 ms^-1

• 3.1 ms^-1

Answer explanation

In the vertical direction

T cos 25⁰ = mg (1)

In the horizontal

T sin 25⁰ =   mv²/r       (2)

 

Dividing (2) by (1) gives

tan 25⁰ = v²/rg

       v² = rgtan 25⁰

 

You need to find ‘r’.

r = 0.75 × sin 25⁰

r = 0.317 m

v = sqrt (0.317 x 9.8 x tan 25⁰)

v = 1.2 m s^-1

A rubber stopper of mass m is whirled in a horizontal circle of radius r.

An experiment is performed to investigate the relationship between m, r, the speed of the stopper v, and the centripetal force, F, acting on the stopper.

The centripetal force acting on the stopper can be measured during the experiment by:

_{VCAA 1976}

• finding the mass of the stopper and multiplying by g, the acceleration due to gravity.

• finding the mass of the stopper and multiplying by v²/r

• multiplying the weight, mg, of the stopper by v²/r

• counting the number of iron washers on the end of the string.

Answer explanation

The net force is always given as F_net.

       F_net = mv²/r

Which one of the following statements about systematic and random errors is correct?

• Random errors can be reduced by repeated readings.

• Both random and systematic errors can be reduced by repeated readings

• Systematic errors can be reduced by repeated readings.

• Neither systematic nor random errors can be reduced by repeated readings

Answer explanation

The official answer was "...repeated readings", although it hs been contested that this may not reduce the random errors, but provide a greater range of random errors.

increasing the number of trials can reduce the effect of the random errors.

A series of measurements were taken during an experiment to determine the acceleration experienced by a body. The equation used to determine acceleration is a=(v-u)/t, where, in this case:

acceleration (a) = ?

final velocity (v) = 9.35 m s^–1

initial velocity (u) = 2.71 m s^–1

time (t) = 3.0 × 10^–2 s

 

How many significant figures should be present in the answer?

_{QLD 2016}

• 1

• 2

• 3

• 4

Answer explanation

Time, t, is only measured to two significant figures. So you can only quote two in your answer.

Matter is converted to energy by nuclear fusion in stars.

If the star Alpha Centauri converts mass to energy at the rate of 6.6 × 10⁹ kg s⁻¹, then the power generated is closest to:

_{VCAA 2020}

• 2.0 × 10¹⁸ W

• 2.0 × 10¹⁸ J

• 6.0 × 10²⁶ W

• 6.0 × 10²⁶ J

Answer explanation

The power is the rate of using energy, therefore the units in the answer must be Watts.

Use ∆E = mc² to find the energy being created. The energy created every second is the power:

6.6 × 10⁹ × (3.0 × 10⁸)²

= 5.94 × 10²⁶ J s^-1

A linear accelerator (LINAC) is a device that is used to accelerate electrons using electromagnetic fields. As the electron energy increases, the speed of the electron approaches c (the speed of light).

Which of the following is true with regard to the electron’s speed, and its mass, as the speed of the electron approaches c?

_{VCAA 2006}

• Its speed increases slightly while its mass remains fixed at 9.11 × 10^-31 kg.

• Its speed increases slightly while its mass increases substantially

• Its speed increases substantially while its mass increases slightly

• Both its speed and its mass continue to increase at a steady rate

Answer explanation

As the speed approaches c, there is little gain in speed when energy is added, instead  the energy goes into mass.