Basic Physics

Question 1

What is meant by the term quantity?

Answer 1

A quantity is a value, multiplied by a defined unit.

Question 2

What are the 6 base quantities and their corresponding units?

Answer 2

mass - kilogram (kg)
length - metres (m)
time - seconds (s)
temperature - kelvin (K)
current - ampere (A)
amount - mole (mol)

Question 3

Express the unit of force, newtons (N) in base SI units.

Answer 3

F=ma
[force] = [mass] × [acceleration]
N = kg × ms^-2
N = kgms^-2

Question 4

Express the unit of energy, joules (J) in base SI units.

Answer 4

Ek = 1/2mv^2
[energy] = [mass] × [velocity]^2
J = kg × (ms^-1)^2
J = kg × m^2s^-2
J = kgm^2s^-2

Question 5

Express the unit of power, watts (W) in base SI units.

Answer 5

P=W/t
[power] = [energy] ÷ [ time]
W = J ÷ s
W = kgm^2s^-2 ÷ s
W = kgm^2s^-3

Question 6

What makes an equation homogenous?

Answer 6

A homogenous equation has the same units on both sides.

Question 7

Show that the equation x = ut + 1/2at^2 is homogenous.

Answer 7

LHS:
x = [displacement] = m
RHS:
ut = [velocity] × [time]
ut = ms^-1 × s
ut = m
1/2at^2 = [acceleration] × [time]^2
1/2at^2 = ms^-2 × s^2
1/2at^2 = m
so
LHS = m and RHS = m + m = m

Question 8

What is a scalar quantity? and provide an example.

Answer 8

A scalar quantity has only magnitude - speed.

Question 9

What is a vector quantity? provide an example.

Answer 9

A vector quantity has a magnitude with a direction - force.

Question 10

What are the 2 mathematic formulas we use for resolving a vector?

Answer 10

The 2 mathematic formulas we use are pythagoras and trigonometry.

Question 11

What is the equation for mean speed?

Answer 11

mean speed = distance/time

Question 12

What is the equation for mean velocity ?

Answer 12

mean velocity = displacement/time

Question 13

What is the difference between calculating the mean speed and the mean velocity? for example if a pilot travels 50km north and 30km east in 2 hours.

Answer 13

Mean speed is the distance travelled (50+30) / time ( 2 hours) so speed would be 40kmh^-1.

Mean velocity is the displacement / time so you would need to resolve a vector and work out displacement before calculating. You would also need to specify a direction at which the pilot travelled.

Question 14

How we do convert a single force quantity into 2 quantities, vertical and horizontal?

Answer 14

We use trigonometry, sin(x) × hyp for vertical and cos(x) × hyp for horizontal.

Question 15

What is the equation for density?

Answer 15

density = mass ÷ volume
P = m / V

Question 16

Estimate the density of air at 0°C.

Answer 16

1.29kgm^-3

Question 17

Estimate the density of water.

Answer 17

1000kgm^-3

Question 18

In density questions you are required to convert units. What do you do if volume is given in cm^3 and the density in kgm^-3?

Answer 18

Convert the volume using 1cm^3 = 1×10^-6m^-3
or
Convert the density using 1000kgm^-3 = 1gcm^-3

Question 19

What is meant by the term moment?

Answer 19

The moment of a force is the “turning affect”. If a body is pivoted at a point and a force is applied, the force will rotate around that point.

Question 20

What is the equation for moments?

Answer 20

moment = force × perpendicular distance
P = Fd
or
P = Fxcos∅ which is P = (Fcos∅)x

Question 21

What is the principle of moments?

Answer 21

For a body to be in equilibrium, the sum of the clockwise moment has to be equal to the sum of the anticlockwise moment.

Also for a body to be in equilibrium, the net moment must be zero and the resultant force must be zero.

Question 22

What is meant by an object’s centre of gravity?

Answer 22

Centre of gravity is the point of an object at which all of its weight is acting.

Question 23

What is meant by the toppling angle?

Answer 23

The toppling angle of an object is the the point at which the object’s centre of gravity is directly above the balance point and the object can topple either direction.

Question 24

What are the 2 conditions for equilibrium to take place?

Answer 24

1) The resultant force is zero

2) The resultant moment is zero