General Physics

Chapter 1 - Measurements

1.

Describe an experiment to show how the period of pendulum can be determined

  • The simple pendulum is made to oscillate at a small angle.
  • When the oscillation is steady, the time taken for 20 oscillations is measured using a stopwatch.
  • The measurement is repeated and the average is calculated.
  • The period of the pendulum is given by average time for 20 oscillations divided by 20.

State the precautions you would take for the experiment.

  • Ensure that the oscillation is steady before starting to time.
  • Ensure that the angular displacement of the bob is less than 10 degrees.

2.

Describe how you would obtain experimentally accurate values for internal (external) diameter of a thick metal pipe.

  • Use internal (external) jaws of vernier calipers to measure the internal (external) diameters of the pipe at three different positions.
  • Take the average values of the measurements.

Why is there a need to obtain the average of several readings?

  • To reduce random errors (the positive errors and negative errors will cancel out) 

Chapter 2 - Kinematics

1.

A car travels round a circular track at a constant speed of 40 km/h. Explain why the car’s velocity is not constant

  • Velocity is a vector quantity that depends on both direction and magnitude.
  • Since the car’s direction changes continuously as it travels around a circular track, its velocity changes all the time.

 Is the car accelerating? Explain your answer.

  • Acceleration is defined as the rate of change of velocity with time.
  • Since velocity is changing, the car is accelerating.

 2.

An athlete initially at rest, runs a 100 m race and completed it with an average speed of 13.2 m/s. Explain why at some point in time his instantaneous speed must have exceeded the average value.

  • Since the athlete starts from rest, he must have had a speed of less than 13.2 m/s during his acceleration.
  • Therefore, his speed at other parts of the race must exceed 13.2 m/sin order for the average to be obtained as 13.2 m/s

Chapter 3 - Forces

1.

Describe ways in which a force might change the motion of a body.

  • A force is may make

o   stationary bodies move.

o   moving bodies speed up, slow down or stop.

o   moving bodies change direction.

 2.

Describe how an object released from rest (subjected to air resistance) will eventually fall with constant velocity.

  • Immediately upon release, gravitational force acting on the object causes it to accelerate at 10 m/s2.
  • As the object gains speed, the upward air resistance increases. This will cancel out the downward gravitational force and decrease the acceleration.
  • Eventually the upward air resistance increases to a point where it equals the downward gravitational force.
  • The resultant force acting on the object is now zero. Therefore the acceleration is zero and it continues falling with constant velocity (known as terminal velocity).

 3.

Suggest and explain why a car moves at constant velocity after some time even though the car engine continues providing the same amount of forward driving force (thrust).

  • During acceleration, the car gains speed and the backward resistive forces (air resistance and contact friction with road) increases.
  • It comes to a point where backward resistive forces equals the forward driving force (thrust)
  • The resultant force acting on the object is now zero. Therefore the acceleration is zero and it continues moving with constant velocity (known as terminal velocity).

Chapter 4 - Mass, Weight and Density

1.

Explain why a beam balance will give the same value of mass at different places for the same object.

  • Bean balance measures the mass of the object and mass is constant and does not change from place to place.

 2.

Explain why a spring balance may give different readings at different places when weighing the same object.

  • Spring balance measures the weight of an object which depends on the different gravitational field strength at different places (usual referring to different planets).

 3.

Distinguish between mass and inertia. State how they are related to each other.

  • Mass is the measure of amount of matter in a body.
  • Inertia refers to the reluctance of a body to change its state of motion.
  • The inertia of an object increases with its mass.

4.    Describe how you would find the density of a small stone with irregular shape.

  • Place a certain volume of water in a measuring cylinder and record the volume, V1.
  • Lower the stone into the measuring cylinder of water with a string until it is fully submerged and record the new volume, V2.
  • The volume of the stone, V is given by V2 – V1.
  • Measure the mass of the stone, m, using an electronic balance.
  • The density of the stone can be calculated using the formula, density = mass / volume.

 5.

Describe how you would measure the volume of a piece of floating object by means of a measuring cylinder, a thread, a sinker, and water.

  • Lower a sinker into a measuring cylinder containing some water until it is fully submerged and records the volume, V1.
  • Attach the floating object to the sinker and lower both into the cylinder again and note the volume, V2.
  • The volume V of the floating object is given by V2 – V1.

State two precautions that should be taken when using the measuring cylinder.

  • The measuring cylinder should be placed on a flat surface when reading the volume.
  • The volume of the liquid should be read from the base of the meniscus.

 6.

Describe an experiment to find the density of a liquid. State how the measurements are taken, and show how the final results are calculated.

  • Find the mass of a clean dry beaker, m1, using an electronic balance.
  • Pour a volume, V, of the liquid from a burette (or pipette) into the beaker.
  • Find the mass of the beaker and liquid, m2, using an electronic balance.
  • The mass of the liquid m is given by m1 - m2.
  • The density of the liquid can be calculated using the formula, density = mass / volume.

Chapter 5 - Moments

1.

Explain briefly how the use of a screwdriver enables the lid of a paint can to be opened with a relative small force.

  • Moment is the product of force and the perpendicular distance between the force and the pivot.
  • When a force is applied at the far end of the screw driver, the perpendicular distance between the pivot and line of action of applied force is large. This will then produce a large moment to open the paint can.

 2.

Describe an experiment to locate the centre of gravity of an irregular shaped lamina.

  • Make 3 small holes, as far apart as possible, near the edge of the card.
  • Suspend the card through one of the holes using a pin.
  • Hang a plumbline from the pin in front of the card.
  • When the plumbline is steady, draw the vertical line on the cardboard as indicated by the plumbline.
  • Repeat the above for two other holes.
  • The point of intersection of the three lines is the position of the centre of gravity of the cardboard (lamina).

3.

For any free hanging object, the centre of gravity must be vertically below the pivot. Explain.

  • When the CG is vertically below the pivot, the perpendicular distance between the line of action of the weight to the pivot is zero.
  • Since the perpendicular distance between the force and the pivot is zero, the resultant moment will also be zero and the object does not turn.

 4.

How can an object be made more stable?

  • Lower its centre of gravity by making the base heavy.
  • Increase its base area.

Chapter 6 - Work, Energy and Power

1.

Describe the energy changes that take place when a ball is being thrown upwards by a hand. (Neglecting air resistance)

  • As the ball just leaves the hand, it possesses maximum amount of KE. Its GPE is zero due to the zero height from the hand.
  • As it moves upwards, it loses speed. The KE is converted to GPE as the ball looses speed and gain height.
  • At the highest point from the ground, the stone is momentarily at rest. The KE is zero while the GPE reaches a maximum. All the KE have been converted to GPE. The GPE at the top equals the KE as the ball just left the hand (at the bottom).
  • As the object moves downwards, its speed increases. GPE is converted back to KE.
  • Just before hitting the hand, the KE reaches maximum and GPE is zero.
  • According the Principal of Conservation of Energy, the sum of KE and GPE is always constant at any point in time.

 

2.

Explain why in practical, the actual useful work done by a motor is less than the calculated value.

  • Some mechanical energy is converted to thermal energy as it is used to overcome frictional force.
  • Some energy is also converted to sound energy.
  • The thermal energy and sound energy is lost to the surroundings.
  • Therefore, the actual useful work done is always less than the calculated value.

Chapter 7 - Pressure

1.

Explain why a sharp knife cuts better than a blunt one.

  • A sharp knife has a smaller area of contact with the object to be cut.
  • For the same force exerted, a smaller area gives greater pressure as indicated by Pressure = Force / Area

2.

*Explain why the height of the mercury in a barometer remains unchanged when the diameter of the inverted tube is changed.

  • Since Pressure = (height of liquid column) x (density of liquid) x (gravitational field strength), then the height = pressure / [(density of liquid) x (gravitational field strength)]. It means that the height of the mercury in the barometer depends only on the pressure and gravitational field strength.

3.

*Explain why the atmospheric pressure decreases with height

  • The pressure at any given altitude in the atmosphere is due to the weight of the air above.
  • As altitude increases, there will be less air above. The density of air also decreases.
  • Since Pressure = (height of liquid column) x (density of liquid) x (gravitational field strength), a decrease in height and density of the air column (or any other fluid) will lead to a decrease in pressure.