- Created by: mallika
- Created on: 01-12-08 09:55
Physical Quantities are the building blocks in terms of which the laws of physics are stated.
All physical quantities are measured by comparison with a standard quantity. This standard quantity is called a unit. All measurements in physics are made using SI units.
Some Physical Quantities you need to know with units:
Physical Quantity-Symbol for PQ-Unit-Symbol for Unit
Mass - M,m - kilogram - kg
Length - l,w,t,b,h - metre - m
Time - T,t - second - s
Temperatue - T,θ - kelvin - K
Area - A - metre2 - m2
Volume - V - metre3 - m3
Density - D - kilogram/metre3 - kg/m3
Weight - W - newton - N
Force - F - newton - N
Momentum - p - kilogram metre/second - kgm/s
Rules for writing units:
1. When writing an answer ALWAYS use the correct symbol for the unit. eg. 5s not 5 secs or 5 seconds
2. Always write units in lower case unless it's named after a scientist. eg. newton - N
Standard Form and Significant Figures
How do you write an answer in standard form?
Place the decimal point after the first digit (excluding 0) & write your answer to 3 significant figures multiplied to the appropriate power of 10.
12753066 = 1.28 x 107
Rules for writing answers to significant figures:
1. All non zero digits are significant. eg. 22.2 has 3s.f.
2. All zeroes between 2 non zero digits are significant. eg. 2008 has 4 s.f.
3. For numbers less than one, zeroes directly after the decimal point are not significant. eg. 0.056 has 2s.f.
4. A zero to the right of a decimal point & following a non zero digit is significant. eg. 0.0500 has 3s.f.
5. All other zeroes are NOT significant. eg. 1300 has 2s.f.
6. In scientific notation zeroes have signicance. eg. 1.00 x 102 has 3s.f.
Scalars and Vectors - What are they?
Scalar quantities are physical quantities that are completely specified by a number with a unit.
Examples of scalar quantities are mass, length, time, temperature, area, volume etc.
Vectorquantities are physical quantities that are completely specified by a number with a unit and a direction.
Examples of scalar quantities are displacement, velocity, acceleration, force etc.
Mass and Weight - What's the difference??
1. Mass is a measure of the amount of substance in an object
2. SI unit - kilogram (kg)
3. Is constant everywhere in the universe
4. Measured using a beam balance
1. Weight measures the gravitational pull of an object
2. SI unit - newton (N)
3. Changes with the position of an object
4. Measured using a spring balance or newton meter
What is density?
Density - How Packed the molecules are within an object
-How compact an object is
Density = Mass / Volume
In order to findthe density of an objectyou must:
1.Find the mass of the object;Mass can be measured using a beambalance.
2. Find the volume of the object; this canbe done in twoways depending on the shape of the object.If it isa regular shape(eg. cube) then the corresponding formula can be used. If not we can use the method of displacement.
What is linear motion?
Linear motion is a study of the movement of objects in a straight line.
Some important physical quantities to remember when doing linear motion:
Distance Travelled - It is the actual length of the path that an object travels, it is measured in metres (m) and is a scalar quantity.
Displacement- It is defined as the distance travelled in a given direction, it is the shortest distance, it is also measured in metres (m)but is avector quantity.
Speed- Average speed = distance travelled/time
- It is measured in metres persecond(m/s) and is a scalar quantity.
Velocity- Averagevelocity = displacement/time
- It is also measured in metres persecond(m/s)but is avector quantity.
Equations of Motion
u - initial velocity
V - final velocity
s - displacement
a - acceleration
t - time
1. V= u + at
2. s= (u + V / 2) x t
3. s= ut + 1/2at2
4. V2 = u2 + 2as
Newton's 1st Law of Motion
Before stating the laws, the defination of the term force should be stated:
A foce is an external factor that can cause a change in either the state of rest or the state of uniform motion of an object. It's SI unit is newton (N).
Newton's first law:- Every object continues in its state of rest or of uniform motion unless acted upon by an unbalanced external force
Inertia - Inertia is an inherent property of all matter due to which an object at rest will remain at rest and an object in uniform motion will remain in uniform motion. Mass is a measure of inertia.
So basically a force can move a stationary object, stop a moving object and change the direction of motion of an object.
If an object is at rest or moving with constant velocity then the sum of the forces acting on the object must be zero. ie. the forces must be balanced.
Terminal Velocity is the velocity with which a freely falling object moves when the downward force of it 's weight is balanced with hthe upward force of air resistance.
Newton's 2nd Law of Motion
While the first law defines a force, it is the second law that gives us an equation to measure force.
Newton's second law:- The rate of change of momentum is proportional to the applied force this takes place in the direction of the force
Now in order to derive this equation, the rate of change in momentum must be known.
But first what is momentum?
Momentum = mass x velocity
SI unit - kgm/s
Change in momentum = Final Momentum - Initial Momentum
Suppose that the rate of chnge in momentum happens in time (t) then:
Rate of change in momentum = Change in momentum/Time
Somathematically Newton's Second law would be: