# KCSE 2012 Physics Paper 1 with Marking Scheme

SECTION A (25 marks)
Answer all the questions in this section in the spaces provided.

1. Figure 1 shows a measuring cylinder containing some water.

Determine the reading on the measuring cylinder, after three drops of water each volume 0.6 cm3 are added. (2 marks)

2. A student pulls a block of wood along a horizontal surface by applying a constant force. State the reason why the block moves at a constant velocity. (1 mark)

3. A solid weighs 16.5 N on the surface of the moon. The force of gravity on the moon is 1.7Nkg-1. Determine the mass of the solid. (3 marks)

4. A bottle containing a smelling gas is opened at the front of a classroom. State the reason why the gas is detected throughout the room. (1 mark)

5. Figure 2 shows a flat bottomed flask containing some water. It is heated directly with a very hot flame.

Explain why the flask is likely to crack. (2 marks)

6. State six environmental hazards that may occur when oil spills over a large surface area of the sea. (2 marks)

7. A balloon is filled with a gas which is lighter than air. It is observed to rise in air upto a certain height. State a reason why the balloon stops rising. (1 mark)

8. In verifying the pressure law of gases, the temperature and pressure of a gas are varied at constant volume. State the conditions necessary for the law to hold. (1 mark)

9. State the reason why a steel sphere resting on a horizontal surface said to be in neutral equilibrium. (1 mark)

10. Table 1 shows the results of an experiment carried out to study the properties of a spring.
Table 1
 Force (N) 0 10 20 30 40 Extension (cm) 0 2 4 6 8
State with a reason whether the experiment was done within the elastic limit of the spring. (2 marks)
11. Figure 3 shows a graph of velocity against time for a moving body.

Describe the motion of the body during the 10 seconds. (2 marks)
12. State two reasons why the efficiency of a pulley system is always less than 100%. (2 marks)
13. Figure 4 shows a graph of temperature against time when melting pure ice at 00C is heated uniformly.

Explain what happens between parts;
1. OA: ………………………………………………….(1 mark)
2. AB. …………………………………………………(1 mark)
14.
1. An aeroplane is moving horizontally through still air at a uniform speed. It is observed that when the speed of the plane is increased, its height above the ground increases. State the reason for this observation. (1 mark)

2. Figure 5 shows parts A, B and C of a glass tube.

State with a reason the part of the tube in which the pressure will be lowest when air is blown through the tube from A towards C. (2 marks)

SECTION B (55 marks)
Answer all the questions in this section in the spaces provided.

15.
1. Figure 6 shows a graph of volume against temperature for a given gas.

Use the graph to determine the absolute zero temperature in 0C. (2 marks)

2. Figure 7 shows a horizontal tube containing air trapped by a mercury thread of length 24 cm. The length of the closed air column is 15 cm. The atmospheric pressure is 76 cm Hg.

1. State the pressure of the enclosed air. (1 mark)
2. The tube is now held in a vertical position with the open end facing upwards as shown in figure 8.

Determine:
1. The pressure of the enclosed air. (1 mark)
2. The length l of the enclosed air column. (3 marks)

3. In an experiment to demonstrate atmospheric pressure, a plastic bottle is partially filled with hot water and the bottle is then tightly corked. After sometime the bottle starts to get deformed.
1. State the purpose of the hot water. (1 mark)
2. State the reason why the bottle gets deformed. (1 mark)
16.
1. Figure 9 shows a trolley on a smooth surface being pulled by a constant force F.

1. On the axis provided, sketch the velocity-time graph for the motion of the trolley. (2 marks)

2. A parachute falling through the air attains terminal velocity after a short-time.
State the reason why it attains terminal velocity. (1 mark)
2. A ball of mass 200 g is thrown vertically upwards with velocity of 5 ms-1. The air resistance is 0.4 N.
Determine:
1. The net force of the ball as it moves up;
(take acceleration due to gravity g =10 ms-2) (2 marks)
2. The acceleration of the ball. (3 marks)
3. The maximum height reached by the ball. (3 marks)
3. Figure 10 shows the path of an object of mass m attached to a string of a length r when whirled in a vertical circle at a constant speed V. A is at the highest point on its path.

1. State the forces that provide the centripetal force on the object when it is at point A. (2 marks)
2. Indicate with an arrow on the diagram the direction of the net force F acting on the object when it is at A. (1 mark)
17.
1. Figure 11 shows how an underground room was ventilated. It had two vents, one at A and the other at B. A fire was lit at point C.

Explain what happened to the ventilation when the fire was lit. (3 marks)
2. Explain how a vaccum flask minimizes loss of heat through radiation. (1 mark)
3. In an experiment to determine the unusual expansion of water, a fixed mass of water at 00C was heated until its temperature reached 200C. On the axis provided, sketch a graph of density against temperature of the water. From 00C to 200C. (2 marks)
4. An immersion heater rated 2.5 kW is immersed into a plastic jug containing 2 kg of water and switched on for four minutes. Determine:
1. The quantity of heat gained by the water; (2 marks)
2. The temperature change of the water. (3 marks) (take specific heat capacity of water as 4.2 × 103 Jkg-1 K-1)
18.
1. Figure 12 shows a set up used to determine the mass of a solid S. The rod is pivoted at its centre of gravity C.

1. Name two measurements that need to be made to determine the mass of solid S. (1 mark)
2. Write an expression to show how the measurement in (i) above are used to obtain the mass of S. (2 marks)

2. Figure 13 shows a log of wood of mass 20 kg submerged in water in a pond and held in position by a string fixed to the bottom of the pond.

Given that the density of water is 1000 kgm-3 and that of wood is 800 kgm-3, determine the:
1. The volume of the log. (3 marks)
2. Upthrust of the log. (2 marks)
3. Tension in the string. (2 marks)

19.
1. Figure 14 shows a lift pump.

Explain why, when the piston is:
1. Pulled upwards, valve A opens while valve B closes. (2 marks)
2. Pushed downwards, valve A closes while valve B opens. (2 marks)
2. After several strokes, water rises above the piston as shown in figure 15.

State how water is removed from the cylinder through the spout. (1 mark)
3. A lift pump can lift water to a maximum height of 10 m. Determine the maximum height to which the pump can raise paraffin.
(take density of paraffin 800 kgm-3 and density of water as 1000 kgm-3).
4. State one factor that determines the height to which a force pump can lift water. (1 mark)

## MARKING SCHEME

1. 7.6+(0.6x3);
7.6+1.8
9.4ml;    (2 marks)
2. Frictional force is equal to the applied force but in the opposite direction, hence the net applied force is zero;     (1 mark)
3. m= w/g;
=16.5
1.7
=9.71kg;             (3 marks)
4. The gas diffuses; from the region of higher concentration to a region of low concentration.  (1 mark)
5. Glass is a poor conductor, unequal expansion leads to cracking;  (2 marks)
6. Oil film spreads over a large surface of the sea reducing inflow of air needed by the aquatic life;
• Reduces the light entering
• Beaches become dirty;
• Poisons marine animals when taken in; (any two correct)     (2 marks)
7. Stop rising when upthrust is equal to the weight of the balloon and its contents;  (1 mark)
8. Mass of gas must be constant;   (1 mark)
9.
• The height of it's centre of gravity above the surface is constant;
• Position of its center of gravity does not change.    (1 mark)
10. It is within the elastic limit; because
• the values of F/e = constant in all the cases F/e = 5
OR
• a graph of force against extension is straight line through the origin;
• conclusion from graph;
(2 marks)
11. The body's velocity decreases uniformly from 20m/s and becomes zero after 5 seconds: the velocity then starts increasing in the opposite direction to a maximum value of 20m/s.    (2 marks)
12.
• Friction between the moving parts of the pulley system;
• Work done against friction;
• Work done lifting the moving parts of the pulley system;
(2 marks)
13.
1. OA - heat gained is breaking intermolecular forces of the molecules/melt the ice without change in temperature;
2. AB-temperature of the water formed starts to rise until it starts to boil;
(2 marks)
14.
1. Air above the plane moves faster than air below it (because of it's shape) creating a region of low pressure above the place hence plane experiences a lift, due to the pressure difference.  (1 mark)
2. At B; because the cross-sectional area is smaller hence the air moves faster in that region;
(2 marks)

SECTION B

1.
1. Extrapolation of graph to cut the temperature axis;
absolute zero = 278 ± 2°c; (-272 ± 2°c to -280° ± 2°c;     (2 marks)
2.
1. When tube is horizontal pressure of air is equal to atmospheric pressure; i.e. 76cmHg.
(1 mark)
2.
1. When vertical,
pressure of air = pressure due to mercury column+atmospheric pressure
= (24+76)cmHg
= 100cmHg;                       (1 mark)
2. PV = a constant;
3. 76x15=(76+24)l;
l = 76 x 15
100
= 11.4cm;       (3 marks)
3.
1. To expel air;    (1 mark)
2. Pressure of air outside the bottle is greater than the pressure of air inside;   (1 mark)
3. Cooling causes condensation of vapour;
Creating a partial vacuum;   (2 marks)
2.
1.
1.

(straight line not necessarily through the origin but with positive gradient)
acceleration;
constant acceleration;
(2 marks)
2. Net force on the parachute becomes zero. (Sum of downward forces on it should be equal to sum of upward forces)  (1 mark)
2.
1. Net force = 2+0.4; = 2.4N;  (2 marks)
2. F=ma; 2.4 = 0.2a+;
a = −12ms2;        (3 marks)
3. V2 = u2 + 2as;
s =   0−52
−2x12
≃ 1.04m;          (3 marks)
3.
1.
• Weight of object;
• Tension in the string;
(2 marks)
2.

The force should be from point A to the center but not beyond    (1 mark)
3.
1.
• Fire heats air around region C which expands and becomes less dense;
• The less dense air rises up the vent and emerges at A;
• Cool (more dense) air moves down the vent at B introducing fresh air into the mine
(3 marks)
2. The flask has double walls which are silverly on both sides the shiny surface is a good reflector of heat;   (1 mark)
3.
(1 mark)
4.
1. Heat gained by water = power x time;
= 2.5x103 x 4x60;
= 6.0x105J;                                 (3 marks)
2. E=mc∆θ;
∆θ = 2.5 x 4 x 60 x 103
2 x 4.2 x 103
= 71.43°C;
(3 marks)
4.
1.
1. Lengths BC and CD;
2. 100 x BC = S x CD;
S = 100BC
CD            (2 marks)
2.
1. Volume of 10g = mass
density
=20/800
= 2.5x 10−2m3;          (3 marks)
2. Upthrust = weight of water displaced
= 20  x 100 x 10;
800
= 2.5 x 102N;      (2 marks)
3. Tension = U−mg;
= 250 − 200
= 50N;
5.
1.
1. Valve B rests under its own weight;
• pressure in the cylinder decreases and water rises into the cylinder pushing the valve open;     (2 marks)
2. Valve A rests under its own weight and the weight of the water; high pressure is created in the region between valve A and valve B forcing valve B to open;      (1 mark)
2. The water is lifted up by the piston ad comes out through the spout;     (1 mark)
3. Pwghw = Ppghp;
hp = 1000 x 10.
800
=12.5m;      (3 marks)
4.
• Force applied on piston (during downstroke);
• Ability of the parts of the pump to withstand the pressure of the liquid column;
(2 marks)

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