PPP Mistakes

Question 1

A massive ball is released from rest above the ground.
According to a student, the principle of conservation of momentum is violated because the ball gains momentum as it falls.
Explain why the students observation is incomplete and discuss how momentum is conserved in this situation.

Answer 1

Earth is mentioned as an integral part of the system
The earth has equal and opposite momentum to the falling ball so momentum is conserved

Question 2

Explain why the intensity of the wave changes as the distance x increases

Answer 2

As the wave spreads out the amplitude decreases
Intensity is proportional to amplitude ^2 and therefore intensity decreases

Question 3

Describe how the wavelength of the progressive wave on the stretched wire depends on frequency of vibration of the wire

Answer 3

Speed / v of the progressive wave is the same
Wavelength decreases as frequency increases
Wavelength is halved when f is doubled

Question 4

Explain without doing any calculations how you can determine the percentage uncertainty

Answer 4

Draw a worst-fit line and determine gradient h
% uncertainty = (1/2 range divide average h) x 100

Question 5

The brakes of the train are applied with the same average braking force.
State and explain the distance travelled from when the driver applies the brakes to when the car stops

Answer 5

KE is transferred to GPE
Smaller distance because larger opposing forces

Question 6

Describe how a metre rule and a stopwatch can be used to determine a final velocity v of the trolley at the bottom of the ramp

Answer 6

Distance/displacement/length measured using a metre rule and time measured using a stopwatch
Use v=root2gh

Question 7

By considering the forces acting on the tube on fig 23.2 2018 breadth

Answer 7

Upthrust is constant up to 60s
Constant gradient before 60s because upthrust increases at a constant rate
After t=60s eventually the upthust is constant because tube is fully submerged.
F=upthrust - weight

Question 8

Explain why the receiver now detects microwaves

Answer 8

This is because diffraction occurs at the narrow slits
The is because the wavelength is similar

Question 9

Compare the speed, frequency and wavelength of light in air and in water

Answer 9

Speed of light is less in water
Frequency is the same in both
Wavelength is smaller in water

Question 10

Describe with the aid of a ray diagram how an experiment can be conducted to accurately determine the critical angle for light within the glass block and hence the refractive index of the glass.

Answer 10

Use a protractor to measure angle
Refractive index determined using n = 1/sinC
Draw diagram

Question 11

Explain why the detected signals behave like this way

Answer 11

Electric field perpendicular to the direction of motion
At 0 to 180 the grille blocks all the polarised waves and at 90 the grille allows all the microwaves to pass

Question 12

Describe how you can use a ray-box together with other equipment available in laboratory to accurately determine refractive index of the plastic block

Answer 12

Measure angle using protractor
Plot sini/sinr graph
Use narrow beam of light for ray box
Refractive index = sini/sinr

Question 13

Explain how the graph now appears from the time the ball is released to the time it hits the surface

Answer 13

Line will curve and line will end with lower maximum final velocity

Question 14

What is threshold wavelength

Answer 14

The maximum wavelength for electrons to be emitted

Question 15

Describe how you would carry out an experiment to accurately determine the threshold voltage of an LED in brightly lit laboratory

Answer 15

Increase the pd until the led is just lit
To observe the light from the led use a tube

Question 16

Describe and explain how the angles p and q will change. In your answer, include the terms critical angle.

Answer 16

As p increases q increases
If light at angles greater than the critical angle then total IR occurs
SinC=1/n
When i is 60 there is no angle q

Question 17

Describe how the addition of two velocities is different to the addition of two masses

Answer 17

Mass is a scalar quantity and velocity is a vector quantity
Addition of velocities depends on direction

Question 18

Describe how you can use instruments available in a physics laboratory to determine the pressure exerted by the cylinder on the table. State how you would make your results as precise as possible

Answer 18

Pressure = Weight of cylinder/area
Weight of cylinder measured using a newton meter
Area is determined by measuring the diameter using a ruler then do piR^2
Use micrometer or vernier calliper to reduce % uncertainty

Question 19

What is the similarity and difference between emf and Pd

Answer 19

Same units
Emf is when work is being done on the electrons. PD is when work is being done by the electrons.

Question 20

Describe how the wavelength of the transverse wave on the stretched cord can be determined

Answer 20

The wavelength is twice the length of the cord

Question 21

What is the area under a force - extension graph

Answer 21

Elastic potential energy

Question 22

Describe how the student can use a graph and additional measuring instrument to accurately determine p

Answer 22

Measure diameter using micrometer
Gradient of line multiplied by CSA gives p
A=pid^2/4

Question 23

What is the wavelength and use for radiowave

Answer 23

10^-1 - 10^4
Television

Question 24

What is the wavelength and use for microwave

Answer 24

10^-4 - 10^-1
Mobile Phones

Question 25

What is the wavelength and use for infrared

Answer 25

7.4x10^-7 - 10^-3
Heaters

Question 26

What is the wavelength and use for visible light

Answer 26

3.7x10^-7 - 7.4x10^-7
Communication

Question 27

What is the wavelength and use for ultraviolet

Answer 27

10^-9 - 3.7x10^-7
Disco lights

Question 28

What is the wavelength and use for x ray

Answer 28

10^-12 - 10^-7
Computer aided tomography scans

Question 29

What is the wavelength and frequency for gamma ray

Answer 29

10^-16 - 10^-9
Cancer treatment