WJEC AS Physics Unit 1.1 - Basic Physics

What is a quantity?

In S.I. a quantity is represented by a number x a unit, (e.g. mass = 3.0 kg).

What is a base unit?

A unit of measurement that all other units are derived from.

What are the 6 essential base units?

• Mass

• Length

• Time

• Amount of Substance

• Current

• Temperature

What is Mass measured in?

Kilograms (Kg)

What is Length measured in?

Metres (m)

What is Time measured in?

Seconds (s)

What is Amount of Substance measured in?

Moles (mol)

What is Current measured in?

Amperes (Amps)

What is Temperature measured in?

Kelvin (K)

What are derived units used for?

Quantities other than that of the base units such as Force, Power and Resistance.

What are the 8 essential derived units?

• Force

• Pressure

• Energy (Work Done)

• Power

• Electric Potential Difference (Voltage)

• Electric Resistance

• Electric Charge

• Frequency

How do we derive Force?

Equation (for derived quantity); F=ma (Force = mass x acceleration)

Units; Newtons (N)

Units in SI units base; kgms^-2

Deriving force; kg x ms^-1 = kgms^-2

How do we derive Pressure?

Equation (for derived quantity); P=F/A (Pressure = Force/Area)

Units; Pascals (Pa)

Units in SI units base; kgm^-1s^-2

Deriving Pressure; kgms^-2/m² = kgm^-1s^-2

How do we derive Energy (Work Done)?

Equation (for derived quantity); E=Fd (Force x displacement)

Units; Joules (J)

Units in SI units base; kgm²s^-2

Deriving Energy (Work Done); kgms^-2 x m = kgm²s^-2

How do we derive Power?

Equation (for derived quantity); P=E/t (Energy/time)

Units; Watts (W)

Units in SI units base; kgm²s^-3

Deriving Power; kgm²s^-2/s = kgm²s^-3

How do we derive Electric Potential Difference (Voltage)?

Equation (for derived quantity); V=P/I

Units; Volts (V)

Units in SI units base; kgm²s^-3A^-1

Deriving Electric Potential Difference (Voltage); kgm²s^-3/A = kgm²s^-3A^-1

How do we derive Electric Resistance?

Equation (for derived quantity); R=V/I

Units; Ohms (Ω)

Units in SI units base; kgm²s^-3A^-2

Deriving Electric Resistance; kgm²s^-3A^-1/A = kgm²s^-3A^-2

How do we derive Electric Charge?

Equation (for derived quantity); Q=It

Units; Coulombs (C)

Units in SI units base; As

Deriving Electric Charge; A x s = As

How do we derive Frequency?

Equation (for derived quantity); Rotations per second

Units; Hertz (Hz)

Units in SI units base; s^-1

Deriving Frequency; 1/s = s^-1

Checking equations for homogeneity

Breaking down the equation into its constituent base units to check whether the equation is correct. If the equation is correct it will have the same base units on either side, meaning the equation is homogeneous.

What is a scalar quantity?

A quantity that has magnitude only.

What is a vector quantity?

A quantity that has both magnitude and direction.

What are some examples of Scalar quantities?

• Mass

• Density

• Speed

• Frequency

• Time

• Area

• Volume

• Pressure

• Energy 

• Power

What are some examples of Vector quantities?

• Velocity

• Force

• Acceleration

• Weight

• Momentum

What are coplanar vectors?

Vectors with the same direction (plane) as one another.

What are some methods of adding/subtracting vectors?

• Drawing tip to tail

• Pythagoras

• Basic Trigonometry (Soh/Cah/Toa)

What is Resolving a vector in particular directions?

This means finding vectors (the so-called components) in these directions, which add together vectorially to make the original vector, and so, together, are equivalent to this vector.

How do we resolve perpendicular vectors?

Combine them to form a resultant using Pythagoras.

What is resolving a vector?

Splitting a vector into two separate components, e.g. horizontal and vertical forces.

What is the shortcut for resolving vectors?

• Component is next to the angle; Cos

• Turns away from the angle; Sin

What is Density?

ρ=m/v

Units; kgm^-3 or gcm^-3

What is the Moment of a force?

The moment of a force is called the “turning effect” of a force. If a body is pivoted at some point and a force is applied to the body it will rotate about that point.

What is the definition of a Moment?

A moment is defined as the force x the perpendicular distance between the pivot and the perpendicular distance of the line of action of a force. 

What is the Moment equation?

Moment = FD

F is the Force (N)

D is the perpendicular distance (m)

Moment is measured in Nm

What is the principle of moments?

For a system to be in equilibrium, ∑ anticlockwise moments about a point = ∑ clockwise moments about the same point.

What is a Centre of Gravity?

A single point on the body from which the entire weight of the body may be considered to act.

How do we determine the centre of gravity of a sphere?

Draw a square around it and use the intersection of diagonal lines from each corner.

How do we determine the centre of gravity of a cuboid?

Draw diagonal lines from each corner and use the intersection.

How do we determine the centre of gravity of a cylinder?

Treat it as a 2D shape and draw diagonal lines from each corner. Then use the intersection.

What is a scenario in which centre of gravity is useful?

Toppling angles.

Which conditions are needed for equilibrium?

• The resultant force must be zero.

• The resultant moment must be zero.

Torque of a couple

One of the forces x perpendicular distance between them