Module 3 & 5 Model Answers

Question 1

Describing the motion of an object falling in presence of drag

Answer 1

Object falling in presence of drag

• When t=0 v=0 FN = mg a=g
• When t>0 v>0 FN = mg -D a< g
• When t»0 v»0 FN = mg -D a&laquo_space;g
• Terminal Velocity reached when D=mg FN = mg -D = 0 a=0

Question 2

Acceleration due to g PAG
1. Manual Method
1. Electronic Method

Answer 2

Manual Method

1. Drop ball from different height and time taken to fall.
2. Measure height with ruler and time with stopwatch
3. Repeat to identify and remove anomalies
4. Draw graph of 2s against T2 Gradient is and hence g

Electronic Method

1. Light gates linked to data logger
2. Enter length of interrupt card - Drop and record acceleration shown by data logger - Repeat
3. Try again with Double data logger but a SINGLE light gate. Enter length of the interrupt part of card

Question 3

Generic model for an LAQ about an investigation

Answer 3

1. State what you will change and what you will measure.
2. State the equipment you will to measure both variables
3. State the equation that links them
4. Rearrange to give the quantity needed.
5. If graph if required state what is on each axis and what the gradient will represent (generally all constants left in gradient)
6. Describe how to calculate quantity from gradient
7. List some limitations to accuracy (If asked)
8. Suggest solutions to said limitations
9. Describe associated hazard (if asked)
10. Suggest precautions to said limitations

Question 4

Describing an experiment to determine an object’s centre of mass

Answer 4

• Hang shape on pin so it can freely rotate
• Hang plumb line off pin.
• Draw line along plumb line
• Rotate object and repeat
• Where lines intersect is centre of mass

Question 5

Young’s Modulus PAG

Answer 5

• Measure diameter of wire (with micrometer/callipers) in 3 places and calculate mean. Hence calculate A
• Run wire over pulley (to reduce friction)
• Clamp wire between two blocks
• Add marker on wire nearer to pulley end
• Measure original length (from blocks to marker) Add masses to hanger on loop - calculate
• Measure extension - calculate
• Draw graph of stress over strain. Draw LOBF
• YM is gradient of linear section only

Question 6

Techniques used to measure motion of objects

Answer 6

• Light gates and data logger
• To reduce effect of friction raise track at one end or use air track.

Question 7

Techniques used to measure collisions of objects.

Answer 7

• Trolleys with springs
• Interrupt card on trolleys - enter value into data logger
• Light gates and data logger
• Place trolleys between light gates - compress springs.
• Start data logger and Trigger release mechanisms

Question 8

Observing Brownian motion and conclusions

Answer 8

• Observe smoke particles lit from side under microscope.
• Random and Haphazard motion
• Movement from collision with air particles
• Net displacement zero means particles from all direction
• Cannot see particles thus their mass very small
• To impart visible momentum on smoke particles therefore velocity of air much higher

Question 9

State the Assumptions of Kinetic Mode

Answer 9

• Body of gas as large number of particles flying around colliding
• Collisions perfectly elastic
• Volume of particles negligible compared to volume of gas
• Duration of collision negligible to time between collisions
• No forces (eg g or E) acting except during collisions

Question 10

Showing Time period independent of amplitude/angle

Answer 10

• Hang mass on string on clamp and stand.
• Change angle of deflection and measure time period
• Measure angle with protractor OR using trig
• Measure time period with stopwatch.
• Plot a graph of Time period against angle - gradient is zero

Question 11

Describe the Formation of a Star

Answer 11

• Nebula (cloud of gas and dust) collapses under gravity
• Gravitational potential energy → kinetic energy → temperature increases
• Volume decreases → pressure increases (Gas Law: P∝T
• Particles move faster → more kinetic energy
• Protostar forms (hot, dense core)
• When temperature is high enough: hydrogen nuclei overcome electrostatic (EM) repulsion
• Strong nuclear force allows fusion of hydrogen into helium
• Fusion releases energy → gas and radiation pressure increase
• Gas and radiation pressure balance gravity
• Star reaches stable phase → main sequence star

Question 12

Explain why gas exerts pressure

Answer 12

• Large number of Particles flying around & colliding w container wall
• Particles has momentum During collision direction changes
• As is vector, as direction changes so does and Change =
• As wall exerts force on particle
• N’s 3rd says equal and opposite Force exerted on wall
• Small individual force but total force from many particles
• As pressure exerted

Question 13

Evolution of Universe

Answer 13

• Very hot & very dense
• All forces unified
• Expansion led to cooling
• Quarks & leptons made
• More matter than antimatter - Annihilates to leave matter
• Colling continues until Quarks combine to form hadrons
• Imbalance of protons and neutrons -helium produced
• Cooling continues until Atoms were formed
• Gravitational force responsible for the formation of stars & galaxies
• Temp becomes 2.7K
• Universe saturated with cosmic microwave background radiation

Question 14

Evidence of Expanding Universe and Hot dense start

Answer 14

• Galaxy’s recessional velocity proportional to distance
• Expansion of space as only explanation
• Microwave background is cosmologically redshifted Gamma