Force Questions and Answers - Physics Form 1 Topical Revision

Questions

1. A student was heard saying “the mass of a ball on the moon is one sixth its  mass on earth”. Give a reason why this statement is wrong.
2. In the study of a free fall, it is assumed that the force f acting on a given  body of mass m is gravitational, given by F= mg. State two other forces that act on the same body.
3. State how a lubricant reduces friction in the bearings of moving part of a  machine.
4. Distinguish between mass and weight of a body stating the units for each.
5. State with reason the purpose of the oil that circulates in a motorcar engine.
6. Name two types of forces which can act between objects without contact.
7. A house in which a cylinder containing cooking gas is kept unfortunately catches fire. The cylinder explodes. Give a reason for the explosion.
8. Give a reason why the weight of a body varies from place to place
9. State why a pin floating on water sinks when a detergent is added.
10. The figure below represents a rock balanced at point O.  G is the center of  gravity of the rock. Use this information to answer the following questions:
    
    
    1. Draw and label on the figure the forces acting on the rock
    2. If the portion of the rock represented by the shaded part is chopped off, explain why the rock may topple to the right.
11. The figure shows a non- uniform log of mass 100kg balanced on a pivot by a 2 kg mass placed as shown.
    
    Determine the distance of the centre of gravity of the log from the pivot
12. The figure below shows two identical trolleys with loads A and B. The  loads are identical in shape and size.
    
    Given that the density of A is greater than that of B, explain why the trolley in figure 3 (ii) is more stable.
13. The figure below shows a beaker placed on a bench. A block of ice is placed in the  beaker as shown. State and explain the change in the stability of the beaker when the ice melts.
    
14. The system the figure below is in equilibrium
    
    Determine the weight of the bar.
15.  
    1. Give a reason why water is not a suitable liquid for use in a barometer
    2. The figure below is a simplified diagram of a hydraulic jack. The cross selection  area A₂ of the load piston is 25 times the A₁ of the effort piston, A₂ = 25N. F₁ is the force applied (Effort) while F₂ represents A₁ the load.
       
       
       1. Write an expression for the pressure exerted on the liquid by the effort piston.
          A mechanic applies a force of 100N on the effort piston while raising the rear part of a car.
       2. Determine the maximum load that can be raised (2 marks)
       3. Give a reason why gas is not suitable for use in place of the liquid in the jack
16. The figure below shows a solid cylinder standing on a horizontal surface. The cylinder is in stable equilibrium
    On the horizontal space provided, sketch the cylinder in neutral equilibrium.
    
17. The figure below shows water drops on two surfaces. In 8 (a), the glass surface is  smeared with wax while in 8 (b) the glass surface is clean.
    
    Explain the difference in the shapes of the drops.
18. A see – saw of length 5 m is pivoted at the centre. A student of mass 50kg sits at one end and is balanced by another student of mass ‘m’ sitting at a distance of 1m from the other end. Calculate the value of ‘m’
19. An astronaut is on the moon. He drops a hammer from a height of 3.2m and it takes 2.0s to hit the lunar landscape. What is the acceleration due to gravity of the moon?
20. An unloaded spring has a length of 15cm and when under a load of 24N it has a length of 12cm.  What will be the load on the spring when length is 10cm?
21. Two copper spheres M and N are joined by a light rod such that their center of mass are 30cm apart: if the radius of M is 2 times the radius of N, find the position of the COG from mass M.
22. In the diagram below the system is in equilibrium. Determine the value of F₁ in N.
    
23. The figure below shows a device for closing a steam outlet. The area of the piston is 4.0 x 10^-4m² and the pressure of the steam in the boiler is 2.0 x 10⁵Nm².
    
    Determine the weight W that will just hold the bar in the horizontal position shown.
24. The total weight of a car with passengers is 25,000N. The area of contact of each of the four tyres with the ground is 0.025m2. Determine the minimum car tyre pressure.
25. A drum which is 2m high contains water to a depth of 0.5 m and oil of density 0.5g/cm3 extends to the top. Find the pressure exerted at the bottom of drum by the two liquids.
26. The figure below shows a non- uniform rod, lying in a horizontal position. Vertical force of 5N and 4 N can just lift the rod when applied at the end A and B respectively.
    
    If the rod is 1.8m long find
    1. The position of the centre of gravity
    2. The weight of the rod

Answers

1. Mass is constant everywhere
2. Up thrust and frictional force.
3. By going between two moving parts so that the parts slid on oil instead of each other.
4. - Weight is a vector quantity while mass is a scalar quantity.
   - Weight varies from place to place while mass is constant.
   - Weight is measured using a spring balance while mass is measured using beam balance.
5. To lubricate the engine/ reduce frictional force.
6. Magnetic, electrostatic and gravitational.
7. k.e of molecules increase hence the pressure increases.
8. Because gravitational force varies with distance from the centre of the earth. Since weight depends on the gravitational pull, then it also varies.
9. The soap reduces the surface tension and hence the weight of pin becomes greater the surface tension.
10. Draw two arrowed lines one from C₁ downwards and the other from O upwards.
    Anticlockwise moment is reduced hence the clockwise moment becomes relatively more.
11. let c.o. g be X cm from pivot.
    1000N x X = 20N x 2m
    X= 0.04m
12. Over all c.o g lower since the heavier the base the lower the C.O.G.
13. When the melts the c.o.g. is lowered hence it becomes more stable
    -Anticlockwise moment is reduced hence the clockwise moment becomes relatively more.
14. c.o.g is at 50cm mark which is 20cm from pivot.
    0.2 x W = 0.3 x 5
    W = 7.5N
15.  
    1. Very low density hence, a very big column required (=11m)
    2.  
       1. p = ^F₁/_F2
       2. ^F₂/_A2=^F₁/_A1
          ∴F₂ = ^A₂/_A1
          F₁ = 25 x 100 = 2500N
       3. Gas is compressible hence it does not transmit pressure equally throughout the system.
16. For neutral equilibrium the cylinder will be horizontal.
17. In (a) adhesive forces between glass and wax are weaker than cohesive  forces between water & water.  The opposite is true (b)
18. -Taking moments and equating
    - 0.6N x 70cm = mgN x 30cm
    - w = mg = 1.4N
19. 83.3kg
20. 1.6m/s₂
21. 40N
22. 3.33cm
23. 25N
24. 20N
25. 250k Pa
26.  
    1. 9N   
    2. 0.8m