PHYSICS PAPER 1 Marking Scheme - 2019 KCSE Prediction Answers Set 1

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SECTION A (25 Marks)
Answer all the questions in this section 

  1. Draw a section of the main scale and full Vernier scale that would give a reading of 3.07 cm (1 mark)
    p1 ans 1
  2. Figure 1 shows a burette partly filled with a liquid. The liquid was initially at the level shown. If the 400 drops of the liquid each volume 0.015cm3 was removed from the burette, mark the new level of liquid in the burette (1 mark)
    p1 ans 2
  3. A block of metal of mass 150g at 100oC is dropped into a lagged calorimeter of heat capacity 45 JK-1 containing 100g of water at 25oC. The temperature of the resulting mixture is 34oC. Determine the specific heat capacity of the metal block. (Take the specific heat capacity of water as 4,200 J/kgK) (3 marks)
    p1 ans 3
  4. An object is placed on water and it floats. Write an equation connecting all the forces acting on the body. (1 mark)
    • Weight of object = Upthrust of the object
  5. Figure 2 shows a 150 g mass tied on a string and whirled in a vertical circle of radius 30 cm with a uniform speed. At the lowest position of the circle the tension is 9.5 N. Calculate the speed v of the mass(3 marks)
    p1 fig 2
    p1 ans 5
  6. Figure 3 shows two systems of spring arragements A and B. The springs are identical and have a spring constant K
    p1 fig 3
    1. State with a reason which system of springs is stiffer (2 marks)
      • System B is stiffer
      • System B has a higher spring constant
    2. On the axes provided, sketch the graphs of systems A and B assuming that the springs obey Hooke's law
      p1 ans 6b
    3. Determine the value K if the extension in system B was 25 cm (2 marks)
      eT = f/2K
      2.5 = 50/2K

      K = 10 N/cm or 1,000 N/m
  7. A piece of paper is wrapped round the joint of a rod partly made of iron and partly wood so that some of the paper is over the iron rod and the other part over the wooden rod as shown in Figure 4
    p1 fig 4
    When a bunsen flame is passed over the paper several times it is observed that the paper gets charred or blacked on the region covering the wooden rod while the one covering the iron does not. Explain this observation made (2 marks)
    • Iron rod is a better thermal conductor than woodenn rod; hence wood does not conduct heat rapidly away from the flame as iron making the paper covering wood to reach ignition temperature or point
  8. A meter rule of mass 50 g is balanced by masses 35 g and 15 g suspened from its ends. Find the position of its pivot from 25g mass (3 marks)
    p1 ans 8
    Figure 5 shows a bunsen burner which is used for heating substances in a laboratory. Use the information given to answer Questions 9 and 10 below
    p1 fig 5
  9. Explain the principle of worklikng of a bunsnen burner (2 marks)
    • Gas exits out of the nozzle at high velocity causes a low pressure region above the nozzle
    • Atmosperic pressure outside is now higher hence the pressure difference draws air into the (low pressure region) Bunsen burner through the air hole
  10. Give a reason why the Bunsen burner in the Figure 5 has a wide base (1 mark)
    • Wide base creates a large angle of tilt before toppling hence increasing the stability of the burner
  11. Water rises up in capillary tubes but mercury which is also liquid, falls in capillary tubes to a level below the outside surafce as shown in Figure 6. Explain this observation. (2 marks)
    p1 fig 6
    • Water has weaker cohesive forces between its molecules than adhesive forces between water and glass molecules while mercury has stronger cohesive forces between mercury molecules than adhesive forces between mercury and glass molecules
  12. State the molecular difference between a real gas and an ideal gas (1 mark)
    • Real gas has molecules with definite volume and intermolecular forces present while ideal gas has molecules with no definite volume and zero intermolecular forces

SECTION B (55 Marks)
Answer all the questions in this section

    1. Figure 7 shows a car braking system. The brake fluid is an oily liquid
      p1 fig 7
      1. State the principle by which a car braking system works (1 mark)
        • Pascal's principle which states that pressure exerted at one part of an enclosed liquid(incompressible fluid) is transmitted equally to all the parts
      2. State one property of the brake fluid used in this system (1 mark)
        • Should be incompressible
        • Should be non-corrosive
        • Should have very low freezing point and a high boiling point
      3. The cross-sectioanl area of the master piston is 5.0cm2. A force of 1540 N is applied to the master psiton. Calculate the force exerted on each slave piston by the brake fluid given that the cross-sectional area of each slave psiton is 7.2 cm2 (3 marks)
        1540/5 = F2/7.2
        F2 = 2217.6 N
    2. A diver is 15 m below the surface of water in a dam as shown in Figure 8
      p1 fig 8

      If the density of water is 1000 kg/m3 and gravity is 10N/kg, determine
      1. The total pressure acting on the diver given that the atmospheric pressure at this place is 1.0 × 105 N/m2 (3 marks)
        p1 ans 13 bi
      2. Dam walls are made wider at the bottom than at the top. Explain (1 mark)
        • In order to withstand higher pressure at the bottom; since pressure increases with depth
    3. Water in a measuring cylinder is placed in a refrigerator and allowed to cool from about 15oC to 0oC. Assuming the water does not freeze, sketch on the axes provided the graph of Volume of water against temperature (1 mark)
      p1 ans 13c
    4. Figure 9 shows a column of air trapped by mercury thread 5 cm long. If the atmospheric pressure is 750 mmHg, determine the length of the air column when the tube is horizontal (3 marks)
      p1 fig 9
      p1 ans 13d
    1. Figure 10 shows a car of mass m moving along a curved part of the road with a constant speed
      p1 fig 10
      Explain why the car is more likely to skid at point B of the road if the speed is not changed (2 marks)
      • Radius at B is shorter than A hence greater centripetal force is needed at B; but frictional force between the tyres and road is constant hence cannot provide sufficient centripetal force needed
    2. A particle P tied to a string is moving in a horizontal circle about O as shown in Figure 11. Particle P moves with a constant speed v
      p1 fig 11
      1. State what provides the centripetal force. (1 mark)
        • Tension in the string
      2. Use an arrow to indicate the direction in which the net force F acting on P will act (1 mark)
        p1 ans 14bii
      3. Give a reason why particle P above experiences centripetal acceleration even thopugh it is moving with a constant speed v (1 mark)
        • The direction of the body changes every time as it rotates; meaning velocity changes even though its magnitude (speed) remains constant and the body accelerates 
    3. Figure 12 shows two masses 0.2 kg and 0.4 kg connected by a string through a hole on a smooth horizontal surface
      p1 fig 12
      The 0.2 kg mass rotates in a horizontal circle of radius 4 cm. Calculate the angular velocity of the mass when the system is in equilibrium. Take acceleration due to gravity, g= 10 m/s2 (3 marks)
      p1 ans 14c
    4.  A jet starts from rest with a uniform acceleration of 500 m/s2. how long does it take to cover a distance of 40 km? (3 marks)
      s = ut + ½gt2 or s = ½gt since u = 0
      40,000 = ½ × 500 × t2
      40,000 = t2

      t = 12.649 s
    1. A horizontal force of 50 N is applied on a wooden block of mass 2.5 kg placed on a horizontal surface. Given that the coefficient of kinetic friction between the surface and the block is 0.5, determine the acceleration of the block (3 marks)
      p1 ans 15a
    2. Figure 13 shows a graph of velocity aginst time for a ball bearing released at the surface of a viscous liquid
      p1 ans 15c
      1. Write an expression/equation connecting all the forces acting on the ball bearing parts
        1. AB (1 mark)
          • Weight > (Upthrust + Viscous drag)
        2. BC (1 mark)
          • Weight = (Upthrust + Viscous drag)
      2. Indicate on the graph the terminal velcoty Vt of the ball bearing (1 mark)

    3. Figure 14 shows a pulley system being used to lift a load of 150 N by applying an effort of 60N
      p1 fig 14
      1. State the velocity ratio of the pulley system (1 mark)
        V.R = 3
      2. Calculate the efficiency of the pulley system (4 marks)
        p1 ans 15cii
    4. Show that the velocity ratio of a wheel and axle machine whose cross-section is shown in Figure 15  is given by V.R. =R/r where R is the radius of the wheel while r is the radius of the axle (2 marks)
      p1 fig 15
      • In one complete rotation the wheel effort moves a distance of one circumference (2πR) while load moves a distance of (2πr)
      • VR= Effort Distance/ Load Distance
      • VR = 2πR/2πr hence VR = R/r
    1. Define the term "specific latent heat of fusion" of a material (1 mark)
      • The amount of heat required to convert a unit mass of a solid to liquid without change in temperature
    2. A metal bar of mass 30 g and specific heat capacity 880 J/kgK is placed in a small furnace. Figure 16 shows how the temperature of the metal bar varies with time t in seconds
      p1 fig 16
      1. State what happens to the bar between t = 600s and 1000s (1 mark)
        • Metal bar melts (Metal bar changes from solid to liquid)
      2. Calculate the energy supplied to the bar between t = 0s and 600s (3 marks)
        Q = mcΔθ
        Q = 0.03 × 880 × 650
        Q = 17, 160 J
      3. Between t = 600s and 1000s the furnace supplies 30 joules of energy per secodn to the bar. Calculate the specific latent heat of fusion oof the metal bar (3 marks)
        Pt = mLf
        30 × 400 = 0.03Lf
        Lf = 400,000 J/kg
    1. State Archimedes' principle (1 mark)
      • A body totally(fully/completely/wholly) or partially imersed in a fluid experiences upthrust which is equal to the weight of fluid displaced
    2. The system in figure 17 below is in equilibrium
      p1 fig 17
      1. When the temperature of the water is raised the system is observed to tilt to the right, state the reason for this observation (2 marks)
        • Increase in temperature casues water to expand reducing its density and hence the upthrust on block N reduces; Apparent weight of block N increases hence increasing the clockwise moment on the system
      2. Calculate the apparent loss of weight of the metal block N given the dimensions of the block are 2cm × 2cm × 6cm (3 marks)
        Apparent loss of weight = upthrust = weight of water displaced by block N
        U = pvg
        U = 1000 × (0.02 × 0.02 × 0.06) × 10
        U = 0.24 N
      3. Determine the apparent weight of the metal block N (2 marks)
        Apparent weight of block N
        W= Wa − U = 0.99 − 0.24
        W= 0.75 N
      4. How far is the metal block N from the pivot? (2 marks)
        From the principle of moments
        0.55 × 0.45 = 0.75 × d
        d = 0.33 m

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