KCSE 2018 Physics Paper 1 with Marking Scheme

QUESTIONS

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

1. State the reason why an object on earth has a higher weight than on the moon.(1 mark)
2. Figure 1 shows the position of a students eye while measuring the length of a wooden block using a metre rule.
   
   Figure 1
   Determine the length of the block as viewed by the student.(1 mark)
3. Describe how the knowledge of the oil drop experiment may be used to estimate the area of oil spillage from a ship in the sea assuming the surface water is not disturbed.(3 marks)
4. Figure 2 shows an instrument used to measure atmospheric pressure.
   
   Figure 2
   State with a reason the modification that would be required in a similar set up if mercury were to be replaced with water.(2 marks)
5. It is observed that a drop of milk carefully put into a cup of water turns the water white after some time. State the reason for this observation.(1 mark)
6. Figure 3 shows the shape of a bimetallic strip after it was cooled below room temperature,
   
   Figure 3
   Explain why the strip curved as shown.(2 marks)
7. A wooden cube of side 1.5 m floats in water fully submerged. Determine the weight of the cube. (density of Water = 1 gcm^-3 ).(2 marks)
8. Figure 4 shows a stone whirled in a vertical circle.
   
   Figure 4
   On the axes provided, sketch a graph of tension against time as the stone moves through point A, B, C and D. (3 marks)
   
9. Figure 5 shows a ball spinning as it moves.
   
   Figure 5
   1. On the diagram, sketch the path followed by the ball as it moves.(1 mark)
   2. Explain why the ball takes that path.(3 marks)
10. Figure 6 shows the relationship between volume and pressure for a certain gas.
    
    Figure 6
    Name the law that the gas obeys.(1 mark)
11. Figure 7 shows an L-shaped wooden structure.
    
    Figure 7
    On the diagram. construct appropriate lines to show the position of the centre of gravity for the structure.(2 marks)
12. Figure 8 shows the graph of extension against force for a certain helical spring.
    
    Figure 8
    On the same diagram sketch the graph of extension against force for a spring with a lower value of spring constant.(1 mark)
13. State two ways in which a mercury based thermometer can be modified to read very small temperature changes.(2 marks)

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

14.                    
    1. State two differences between boiling and evaporation(2 marks)
    2. State three ways in which loss of heat by conduction is minimised in a vacuum fiask.(3 marks)
    3. In a certain experiment, 50 g of dry steam at 100°C was directed into some crushed ice at 0°C. (latent heat of vsporisation of water is 2.26 x 10⁶Jkg^-1, latent heat of fusion for ice is 3.34 x 10⁶Jkg^-1 and specific heat capacity of water is 4.2 x 10⁶Jkg^-1 )
       Determine the:
       1. quantity of heat lost by steam to change to water at 100°C.(2 marks)
       2. quantity of heat lost by water to cool to 0°C.12 marks)
       3. mass of ice melted at 0°C.(3 marks)
15.              
    1. State Newton's first law of motion.(1 mark)
    2. A wooden block resting on a horizontal bench is given an initial velocity u so that it slides on the bench for a distance x before it stops. Various values of x are measured for different values of the initial velocity. Figure 9 shows a graph of u' against X.
       
       Figure 9
       1. Determine the slope of the graph.(3 marks)
       2. Determine the value of k given that u? =20kd where k is a frictional constant for the surface.(2 marks)
       3. State with a reason what happens to the value of k when the roughness of the bench surface is reduced.(2 marks)
    3. An object is thrown vertically upwards with an initial velocity of 30 ms^-1. Determine its maximum height (acceleration due to gravity g is 10 ms^-2).(3 marks)
16.                          
    1. An electric crane uses 8.0 x10'N of energy to lift a load of 2.0 x 10⁴N in 4 s.
       1. Determine the
          1. power developed by the crane,(2 marks)
          2. height to which the load is lifted,(2 marks)
          3. efficiency of the crane whose motor is rated 2.5 x 10⁴W.(2 marks)
       2. State two forms of energy transformation that lead to the crane's inefficiency(2 marks)
    2. A stone is dropped from the top of a building to the ground. On the axes provided, sketch il graph of potential energy against time for the stone.(1 mark)
       
17.                      
    1. State Pascal's principle of transmission of pressure in liquids.(1 mark)
    2. Figure 10 shows heights of two immiscible liquids X and Y in a U-tube (drawn to scale).
       
       Figure 10
       1. State with a reason which of the two liquids X and Y has a higher density.(2 marks)
       2. Determine the value of h.(2 marks)
       3. Given that the density of liquid Y is p, write down an expression for the density d of liquid X in terms of p.(2 marks)
    3.                      
       1. With the aid of a diagram, describe how a liquid may be siphoned from one container to another lising a flexible tube.(3 marks)
       2. State one application of the siphon.(1 mark)
18.                        
    1. State two quantities that must be kept constant in order to verify Boyle's law. (2 marks)
    2. An air bubble at the bottom of a beaker full of water becomes larger as it rises to the surface. State the reason why:
       1. the bubble rises to the surface,(1 mark)
       2. it becomes larger as it rises.(1 mark)
    3. State two assumptions made in explaining the gas laws using the kinetic theory of gases.(2 marks)
    4. Figure 11 shows an incomplete experimental set up that was prepared by a student to verify one of the gas laws.
       
       Figure 11
       1. State with a reason which one of the laws may be verified using the set up.(2 marks)
       2. State what the student left out in the diagram of the set up.(1 mark)
    5. The volume of a fixed mass of a gas reduced from 500cm³ to 300³ cm at constant pressure. The initial temperature was 90°K. Determine the final temperature. (3 marks)

MARKING SCHEME

1. Earth's gravity is higher than the moon's gravity.
2. 7.3-6.0=1.3cm
3. - Using the same oil spilled, to measure a known volume.
   - Determine the area of spread of the measured volume (on the same water (sample)
   - Estimate the area of spread on the sea.
4. The tube will be very long since water has much lower density than mercury.
5. Milk particles move to occupy the spaces between the water particles through diffusion.
6. Brass contracts more than invar hence contracts leaving the side with invar longer hence the curve.
7. Volume of water displaced =
   0.5 x 0.5 x 0.5
   = 0.125m³
   W of cube.
   = weight of water displaced
   (a floating body)
   = 1 x 125000g
   = 125kg 
   = 1,25 x 10³N
8.                              
   
   - not touching O on y axis and x axis
   - maximum tension at B
   - Equal tension at A & C
   - Allow straight line
9.                
   1.                                    
      
   2. Because of Bernoullis effect caused by diference in air pressure due to the streamlines. As the ball moves to the right airstreams move to the left. Spin is anti-clockwise, a region of low pressure forms above lifting the ball.
10. Boyle's law.rks)
11.                         
    
12.                               
    
13.  - Making the bulb thinner.
     - Making the bore narrower
14.                                
    
    1. - Boiling takes place at fixed temperature while evaporation takes at all temperatures,
       - Boiling takes place in the entire liquid while evaporation takes place at the surface.
    2. - Presence of a vacuum.
       - Poor conductor material used to make stopper.
       - Use of glass (poor conductor).
    3.                                
       1. Heat lost by steam at 100°C.
          ML_v = -5.0 x 10^-2 x 2.26 x 10⁶
          = 11.3  x  10⁴
          1.13 x 10⁵J
       2. Heat lost by hot water to cool to 0°C.
          MCΔθ = 5.0 x 10^-2 x 4.2 x 10³ x 10²
          = 2.1 x 10⁴J
       3. Amount of ice melted at 0°C.
          M_iceL_r = Mlv+MCΔθ
          M = 2.1 x 10⁴ +1.13 x 10⁵
                          3.34x10⁵
          = 0.401kg.
          = 401g
15.                        
    1. A body remains in its state of rest or uniform motion in a straight line unless acted upon by an external force.
    2.                   
       1. Slope = Δu²
                      Δx
          = 50-0
              8-0
          = 6.25
       2. u² =  20but u² =  slope
          x                x
          k = slope
                 20
          = 6.25
               20
          k = 0.3125
       3.  K would reduce since friction has reduced
    3. Hmax = u²
                   2g
       = 30 x 30
           2 x 10
       = 90 = 45
           2
16.                        
    1.                          
       1.                            
          1. power = Work done
                             time
             = 80000
                    4
             = 20000W
          2. d= Work
                  force
             = 80000
                20000
             = 4 m
          3. Efficiency = Workoutput x 100%
                               Workinput
             = 20000 x 100%
                25000
             = 80.00%
       2. Mechanical energy - heat and sound.
    2.                        
       
       Since it accelerates towards the ground
17.                          
    1. Pressure applied at one part in a liquid is transmitted equally to all other parts of the enclosed liquid
    2.                        
       1. Liquid y is denser since it rises to a smaller height i.e. the atmospheric pressure supports a lower height of y than x.
       2. h = 2.2 x 20
                3.6
          12.22 cm
       3. d = 13.6 = 1.636 
          p     2.2
          d=1.6p
    3.                          
       1.                          
          
          - Two containers placed such that the levels are at different heights
          - Expel air from the tube after placing in container with lievid
          - Diagram
       2. - The flushing of a toilet
          or
          - Drinking using a straw.
18.                                
    1. - Mass
       - Temperature
    2.                    
       1. The gas is less dense than the water.
       2. As it rises the pressre around the bubble reduces and since the temperature is the same, the volume increases.
    3. - The size of the molecules is assumed to be neglibible,
       - Intermolecular forces are also assumed to be negligible.
       - Real gases can never have zero volume yet the gas laws assume presence of zero volume.
       (Any two correct)
    4.                                    
       1. - The pressure law.
          - Has ability to measure the temperature and the pressure while keeping the volume constant.
       2. - Source of heating for the temperature to be changed.
    5. V₁ = V₂
       T₁     T₂
       T₂ = 300 x 90
                  500
       = 54k