[Physics Form 4] Quantities Defined

A physical quantity represents any property that can be measured by a scientific device and technique, and given a numerical value.



Classification of quantities



Physical quantities

1. Base quantities
   
   
   • Base quantities cannot be expressed in terms of other physical quantities while derived quantities are combination of two or more base quantities.
   • The five base quantities are current, length, mass, temperature and time.
     
     
     
     
2. Derived quantities
   
   
   • Derived quantities result from the combination of base quantities through multiplication and divisive operations only.
   • Mathematical operations like subtraction and addition are not involved.
     
     Eg: Work is a derived quantity because it is obtained through the combination of three base quantities - mass, length and time.
     
     
     
     Work = Force x Displacement
     
     Work = Mass x Acceleration x Displacement
     
     Work = (Mass x Velocity x Displacement) / Time
     
     Work = [Mass x (Displacement)²] / (Time)²
     
     
     
     Note: Displacement is measured in terms of length.
     
     
     
     
3. Scalar quantities
   
   
   • Scalar quantities describe only the magnitude of physical quantities.
     
     
     
     
4. Vector quantities
   
   
   • Vector quantities describe both the magnitude and direction of physical quantities.

[Physics Form 4] In Locomotion

Acceleration, velocity and displacement are the physical quantities often used to describe the motion of objects. These quantities are also known as the quantities of kinematics.

What are acceleration? Acceleration is a derived vector quantity that describes the motion of any object in terms of change in velocity per unit time. To be specific, it describes the increase in velocity of an object in one second.



Ex: The car is moving with a constant acceleration of 4ms^-2. This means that the velocity of the car increases 4ms^-1 every 1s. This occurs constantly throughout the motion of the car.



What is meant by velocity? Velocity is a derived vector quantity that describes the motion of any object in terms of change in displacement per unit time.



Constant velocity means the velocity is fixed throughout the motion. It also means the displacement covered by object is constant every one second.



The trolley above is moving down an inclined plane at the constant velocity of 1ms^-1. This means that as the trolley is moving down the inclined plane, it covers a displacement of 1m every 1s. If the inclined plane is 3m long, it will take the trolley 3s to reach the bottom.



What is displacement? Displacement is a straight line distance measured from the initial point (reference point) to the final point.



Constant displacement is another way of saying that the object is stationary. When a displacement is constant, it means the final position is not changed. Hence, the object is resting at the final position.



Zero displacement is achieved when the final position meets the initial position. Hence, there is no distance between the final and initial positions.

[Physics Form 4] Curved Mirrors

1. The two types of curved mirrors are the concave mirror and the convex mirror.
   
   
2. Terminologies associated with concave and convex mirrors:
   
   • The pole is the centre point of the surface of the concave or convex mirror. It is symbolised as P.
     
     
   • The centre of curvature is the centre of sphere in which the concave and convex mirrors are part of. It is symbolised as C.
     
     
   • The radius of curvature is the distance from the pole to the centre of curvature. It is symbolsed as R.
     
     
   • The principal focus point is the point in which all rays parallel to the principal axis pass through (for concave mirror) or appear to emerge from (for convex mirror), after being reflected from the respective mirrors. It is located on the principal axis and is symbolised as F.
     
     
   • The focus point of a concave mirror is real, while the focus point of a convex mirror is virtual.
     
     
   • The focal length is the distance between the pole and the focus point. It is symbolised as f. The focal length is half the radius of curvature; f = R/2.
     
     
   • The focal length f a concave mirror is real, and denoted as +f, while that of a convex mirror is virtual, and denoted as -f.

[Physics Form 4] Lighting Up!

1. How are we able to see an object?
   Answer: When enough light falls on the object, incident light rays are reflected from its surface. The reflected light rays then enter our eyes and form an image on the retina. We are able to see the object when the brain interprets the image.
   
   
2. Must light be incident on an object? If objects cannot be seen in a dark room, how are we able to see a lighted bulb in it?
   Answer: A lighted bulb is a primary light source that emits its own light. Hence, its incident light rays directly enter our eyes and form an image. On the other hand, objects that do not emit light rely on the reflected incident light rays to form an image in our eyes. These objects act as secondary sources of light.
   
   
3. Is the moon seen through the reflection of the sun's light that is incident on it?
   Answer: Yes.

[Physics Form 4] Refraction

• Refraction is the phenomenon exhibited by light when it undergoes a change in speed.
  
  
• Normally, when light travels through two different media of different optical density, it undergoes a change in speed and exhibits refraction.
  
  
• In most cases, refraction is seen as the bending of light or deflection of light from its original direction of propagation.
  
  
• There are two types of refraction:
  
  1. Bending of light towards the normal, which occurs when light moves from a medium of lower optical density to higher optical density.
  2. Bending of light away from the normal, which occurs when light moves from a medium of higher optical density to lower optical density.

[Physics Form 4] Impulsive Influence

Why does a stationary car have inertia but no momentum?

An object is said to have inertia when it resists any changes to the status of its motion. Inertia is proportional to mass. Since a stationary car is a matter with mass, naturally, it has inertia.

Momentum is the product of mass and velocity. A stationary car has no velocity. Hence, it has no momentum.

In order to set a stationary car in motion, a force is needed to overcome its inertia. Or you may also say that a force is needed to overcome the limitating frictional force exerted on the car.

Unlike a stationary car, a car in motion possesses both inertia and momentum. The momentum of a car is greater when it travels at a higher velocity.

Eg: A car has a greater momentum when it travels at 20m/s instead of 10m/s.

A moving car undergoes a change of momentum when it is stopped. Since the rate of momentum change is proportional to the impulsive force, a larger force is needed to stop the car traveling at a higher velocity.

Eg: A larger impulsive force is needed to stop the car moving at 20m/s instead of 10m/s.

[Physics Form 4] Measurements Matter

When selecting a device to measure a quantity, factors like the

• type of quantity to be measured
• estimated size of the quantity
• sensitivity of the device
• accuracy of the device

need to be taken into account because sensitivity, precision and accuracy are the three important properties of a measurement.

Sensitivity is the ability of a measuring device to detect small changes in the physical quantity measured.

Eg: A miliammeter is more sensitive than an ammeter. This is because a ammeter is able to measure a smaller magnitude of electric current in the order of mA, in contrast to an ammeter, which measures a larger current in the order of A.

Precision is the ability of a measuring device to give consistent reading after several repeated measurements. The smaller the relative deviation of a set of readings, the higher is the precision level of the measurements.

Accuracy is the ability of a measuring device to provide readings that are exactly the same as, or close to, the actual value of measurement. the closer a measurement is to its actual value, the higher is its accuracy level.

[Physics Form 4] Useful Units

1. Why are units important in the measurement of a quantity?

They provide the exact size of the quantity measured; they provide information about the type of quantity measured; they allow for comparisons between two measurements of the same quantity.



2. State the SI units of the following quantities: electric current, velocity, acceleration, mass, time, energy, pressure, work, momentum and weight.

Electric current: A

Velocity: ms^-1

Acceleration: ms^-2

Mass: kg

Time: s

Energy: J

Pressure: Pa

Work: J

Momentum: kgms^-1

Weight: N



3. Differentiate between the SI unit and the SI base unit of force.

The SI unit of force is N (Newton) while the SI base unit of force is kgms^-2



4. Are cm and m base unit? Differentiate them.

Yes, both are base units of length, a base quantity. However cm is not the SI unit of length while m is.



5. State the value equivalent of 1m² and 1cm³ , respectively.

1m² = 1 x 10⁴ cm²

1cm³ = 1 x 10^-6 m³



6. How fast is a car moving in ms^-1 if its speed is 72kmh^-1 ?

72kmh^-1 = 72km / 1h

= (72 x 10³ m) / (3.6 x 10³ s)

= 20ms^-1