# Physics Questions and Answers - Form 3 Opener Term 1 Exams 2022

PHYSICS

INSTRUCTIONS.
Answer all the questions in the spaces provided.

## Questions

1. Define the term fluid. (1mk)
2. Distinguish between streamline flow and turbulent flow. (2mks)
3. In deriving the equation of continuity, there are some assumption made. State the three assumption that the fluid must have. (3mks)
4. The figure below shows a pithball placed in a flask. When a jet of air is blown over the month of the flask as shown, the pithball is observed to rise from the bottom. (2mks)
5. Water with negligible viscosity flows steadily through a horizontal pipe of various cross-section area. At a point A of cross-section area 10cm2 the velocity is 0.2m/s. calculate:
1. The velocity at a point B, of cross-section area 2.5cm2. (3mks)
2. State two hazards of Bernoullis effect. (2mks)
6. State and explain three factors affecting velocity of sound in air. (3mks)
7. The diagram below shows sound waves passing through air. Study it and answer the questions that follow.

Label the following:
1. Compression (1mk)
2. Rarefaction (1mk)
3. Wavelength (1mk)
8.
1. Define moment of a force. (2mks)
2. State two factors affecting moment of force. (2mks)
3. A uniform metre rule pivoted at its centre is balanced by a force of 4.8N at 20cm mark and some other two forces, F and 2.0N on the 66cm and 90cm marks respectively. Calculate the force F. (2mks)
9.
1. Define magnetic field. (1mk)
2. Draw the field pattern in following. (2mks)
3. State three uses of magnets. (3mks)
10. The air pressure at the base of a mountain is 75.0cm of mercury while at the top it is 60.0cm of mercury. Given that the average density of air is 1.25kg/m3 and the density of mercury is 13600kg/m3, calculate the height of the mountain. (3mks)
11.
1. Differentiate between a transverse wave and a longitudinal wave. (2mks)
2. The figure below shows the displacement time graph for a wave.

With reference to this wave motion, determine the:
1. Period (1mk)
2. Frequency (2mks)
3. If the wave travel at a speed of 340m/s, calculate the wavelength. (3mks)
12. A small electromagnet used for lifting and then releasing a small steel ball is made in the laboratory as shown below;

1. Explain why soft iron is a better material than steel to use for the core. (1mk)
2. In order to lift a slightly larger ball, it is necessary to make a stronger electromagnet. State two ways and explain the electromagnet could be made more powerful. (4mks)
13.
1. In an experiment to estimate the size of a molecule of olive oil, a drop oil of volume 0.12mm3 was placed on a clean water surface. The oil spread into a patch of areas 6.0x104mm2. Estimate the size of a molecule of olive oil. (3mks)
2. Give two assumption made when calculating the thickness of the oil drop. (2mks)
14. The diagram below shows a vacuum flask with an enlarged view of the part in the circle.

1. What materials are items A and C made of? (1mk)
2. What types of heat energy are reduced by or prevented by the parts marked B,C and D. (3mks)
3. Explain how A is effective in reducing heat transfer. (1mk)
15. An object is placed 10cm in front of a:
1. Convex mirror of a focal length 20cm. determine the position of the image.(2mks)
2. Nature of the image. (1mk)
3. Define the following terms as used in curved surface. (2mks)
Pole:
16. Sketch the field in each of the following. (4mks)
1. Current in the same direction.
2. Current in opposite direction.

## Marking Scheme

1. Define the term fluid. (1mk)
• Fluid refers to both gases and liquid.

2. Distinguish between streamline flow and turbulent flow. (2mks)
• Streamline flow- it is a flow in which at any given point each and energy particle of the fluid travels in the same direction and with the same velocity while turbulent flow is a flow in which the speed and direction of a the fluid particle passing at any point vary with time.

3. In deriving the equation of continuity, there are some assumption made. State the three assumption that the fluid must have. (3mks)
• Incompressible i.e. changes in pressure produce insignificant change in its density.
• Non-viscous

4. The figure below shows a pithball placed in a flask. When a jet of air is blown over the month of the flask as shown, the pithball is observed to rise from the bottom. Explain the observation (2mks)
5. Water with negligible viscosity flows steadily through a horizontal pipe of various cross-section area. At a point A of cross-section area 10cm2 the velocity is 0.2m/s. calculate:
1. The velocity at a point B, of cross-section area 2.5cm2. (3mks)
• A1V1=A2V2
10 x 10-4 x 0.2 = VB X 2.5 x10-4
VB=0.8mls

2. State two hazards of Barnallis effect. (2mks)
• Blowing off of root-tops

6. State and explain three factors affecting velocity of sound in air. (3mks)
• Temperature of the air: sound travels faster in hot air than in cold air.
• Humidity of the air: velocity of sound on air increase with humidity.
• Direction of the wind: wind blowing in the same direction as sound increases the velocity of the latter.

7. The diagram below shows sound waves passing through air. Study it and answer the questions that follow.
• Label the following:
• Compression (1mk)
• Rarefaction (1mk)
• Wavelength (1mk)

8.
1. Define moment of a force. (2mks)
• Turning effect of the force or product of the force and perpendicular distance between the point of support (pivot) the line of the force.

2. State two factors affecting moment of force. (2mks)
• Amount of force
• Perpendicular distance between line of action of force and point of support.

3. A uniform metre rule pivoted at its centre is balanced by a force of 4.8N at 20cm mark and some other two forces, F and 2.0N on the 66cm and 90cm marks respectively. Calculate the force F. (2mks)
• sum of clockwise moments=sum of a anticlockwise moment.
Fx0.16 +2.0x0.40=4.8 x0.30
0.16F+0.80=1.44
0.16F=0.64
F=0.64
0.16
F=4.0N

9.
1. Define magnetic field. (1mk)
• Space around a magnet where the magnetic influence is felt.

2. Draw the field pattern in following. (2mks)
3. State three uses of magnets. (3mks)
• used in hospital for removing pieces of iron from the eyes of patients.
• Used in audio and video recorders.
• Used in loudspeakers
• In resolting six’s minimum and maximum thermometer.
• Used in industries as strainers, lifting iron scrap metals.

10. The air pressure at the base of a mountain is 75.0cm of mercury while at the top it is 60.0cm of mercury. Given that the average density of air is 1.25kg/m3 and the density of mercury is 13600kg/m3, calculate the height of the mountain. (3mks)
• Pressure difference due to column of air=pressure differences due to mercury column
Ha=(75-60) x 13600x10
100 1.25 x 10
= 15x13600x10
125x10
= 1632
The height of the mountain is 1632m

11.
1. Differentiate between a transverse wave and a longitudinal wave. (2mks)
2. The figure below shows the displacement time graph for a wave.
With reference to this wave motion, determine the:
1. Period (1mk)
• 0.025

2. Frequency (2mks)
• T=1/f, I=1/t = 1/0.02 = 50H2

3. If the wave travel at a speed of 340m/s, calculate the wavelength. (3mks)
• V=fx
340=50x
X=340/50
=6.8m

12. A small electromagnet used for lifting and then releasing a small steel ball is made in the laboratory as shown below;
1. Explain why soft iron is a better material than steel to use for the core. (1mk)
• Easy to magnetize and demagnetize.

2. In order to lift a slightly larger ball, it is necessary to make a stronger electromagnet. State two ways and explain the electromagnet could be made more powerful. (4mks)

13.
1. In an experiment to estimate the size of a molecule of olive oil, a drop oil of volume 0.12mm3 was placed on a clean water surface. The oil spread into a patch of areas 6.0x104mm2. Estimate the size of a molecule of olive oil. (3mks)
• Volume of the drop = 0.12mm3
Area of the patch = 6.0 x 104mm2
Thickness, t, of patch =V/A
0.12
6.0 x104
=2.0 x 10-6mm
This thickness, t = 2.0x10-9m

2. Give two assumption made when calculating the thickness of the oil drop. (2mks)
• Oil drop is perfect Sphere
• Oil molecule is monolayer

14. The diagram below shows a vacuum flask with an enlarged view of the part in the circle.
1. What materials are items A and C made of? (1mk)
• Silver

2. What types of heat energy are reduced by or prevented by the parts marked B,C and D. (3mks)
• B – conduction and convection
• D- Evaporation

3. Explain how A is effective in reducing heat transfer. (1mk)
• Poor emitter and a poor absorber of heat.

15. An object is placed 10cm in front of a:
1. Convex mirror of a focal length 20cm. determine the position of the image.(2mks)
• 1/f = 1/u + +1/v
=1/20=1/10+1/v
=1/v=-1/20-1/10
=3/20
V=-0.6667cm

2. Nature of the image. (1mk)
• Image is virtual ( since v-is negative)

3. Define the following terms as used in curved surface. (2mks)
• Pole ,p
1. Geometrical centre of the mirror.
2. radius of curvature.
3. Centre of the sphere of which the mirror forms a part.

16. Sketch the field in each of the following. (4mks)
1. Current in the same direction.
2. Current in opposite direction.

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