Learning Is FUN !!: May 2012

Physics is a branch of science concerning the study of natural phenomena, properties of matter and energy.

Based quantities: physical quantities that cannot be defines in terms of other quantities.

Example: length, mass, time

Derived quantities: physical quantities that derived from combination of base quantities.

Example: area, volume, force, velocity

Conversion of units (prefixes)

Example: 1 cm²= 10^-4 m², 13.6 g cm^-3 = 1.36 x 10⁴ kg m^-3

Scalar quantities: magnitude

Example: mass, speed, area

Vector quantities: magnitude and direction

Example: momentum, displacement, acceleration

Micrometer screw gauge is used to measure the diameter of a wire of the thickness of a thin object.

Vernier callipers are usually used to measure the internal or external diameter of an object.

Accuracy: ability of the instrument to measure the true value or close to the true value. The smaller the percentage error, the more accurate the instrument is.

Sensitivity: ability of the instrument to detect any small change in a measurement.

Consistency: ability of the instrument to produce consistent measurement.(the values are near to each other). The lower the relative deviation, the more consistent the measurement is.

How to increase accuracy?
- repeat the measurements and get the mean value.
- correcting for zero error.
- avoiding parallax error.

The sensitivity of a mercury thermometer can be increased by
-having a bulb of thinner wall.
-having a capillary tube of smaller diameter or bore.

Distance: total length of the path travelled from one location to another (scalar quantity)

Displacement: length of the straight line connecting the two locations in a specific direction. (vector quantity)

Speed: distance travelled per unit time / rate of change of distance

Velocity: rate of change of displacement

Acceleration: rate of change of velocity

Linear motion equations and graphs (refer NOTES)

Inertia: tendency of the object to remain at rest or, if moving, continues its uniform motion in a straight line.

Example:

1. If stand in a bus which starts suddenly from rest, you are likely to fall backwards.

2. If the moving bus stops suddenly, you are likely to fall forward.

3. Bigger vehicle (Truck) is more difficult to stop than a light vehicle (Motorcycle).

Mass and Inertia

It is to be put in mind that inertia is dependent upon the mass of the object. The larger the mass, the larger its inertia. Hence, it is harder to push a heavy box than to push a lighter box.

Ways to reduce inertia in vehicle:

1. Seat belts help to tighten the passenger during collision. This is to prevent the passenger from being thrown forward due to inertia.

2. Air bag is fitted inside the steering wheel. It provides a cushion to prevent the driver from hitting the steering wheel.

Momentum: product of its mass and its velocity. p = m v (unit: kg ms^-1)

Principle of conservation of momentum: total linear momentum of a closed system is constant.

I)   Elastic collision: linear momentum, kinetic energy and total energy are conserved.
                                  m₁u₁+m₂u₂ = m₁v₁+m₂v₂

II) Inelastic collision: only linear momentum and total energy are conserved and there is a loss in kinetic energy.
                                     m₁u₁+m₂u₂= (m₁+m₂)v

III) Explosion, where two objects move in opposite directions, the total linear momentum before and after the explosion is zero.
                                  m₁v₁=m₂v₂

The acceleration of a rocket leaving the earth increases because:
a) its mass is decreasing.
b) air resistance is decreasing.
c) gravitational pull is decreasing.

Impulse: change momentum, Ft = mv-mu (unit: Ns)

Impulsive force: rate of change of momentum. F = Impulse / time = mv-mu / t (unit: N)

When the time of action is prolonged, the impulsive force will decrease. If the time of action is shortened, the impulsive force will increase.

Ways to reduce impulsive force:

1. The front and the rear parts of the car are made of soft metal so that the car is easily crumpled during an accident.

During collision, the time taken for the change in speed (from a high speed to zero) is prolonged, force will decrease when the time increase.

Ways to utilize impulsive force:

Pestle and mortar
- The pestle and mortar are made of hard materials.
- During pounding or grinding, the pestle moves very fast. The mortar stops the motion of the pestle in a short time.

- A strong impulsive force is produced and the food can be broken into pieces easily.

Weight = gravitational force acting on an object. W = mg (unit: N)

F = ma (Newton's Second Law)

Forces in Equilibrium

a) two forces act along the same direction, F = F₁+ F₂ifopposite direction, F = F₁- F₂

b) triangle of forces – trigonometry (cos θ = A/H, sin θ = O/H, tan θ = O/A), theorem Pythagoras

c) parallelogram method
d) resolution of forces

Work : product of the applied force and the displacement of an object in the direction of the applied force.
W = F x S (unit: Joule (J), Nm)

Work is not done when:

a. The object is stationary aka not moving
b. No force is applied on the object in the direction of displacement.
c. The direction of motion of the object is perpendicular to that of the applied force.

When work is done to an object, energy is transferred to the object.