IGCSE Physics
Chapter 1-4: Definitions, equations, and key concepts
Mass
Density
Weight
Period
Speed
Average speed
Scalar
Vector
Velocity
Acceleration
Newton (N)
Resultant force
Friction
Air resistance
Drag
Gravity
Gravitational
field strength
Terminal velocity
Momentum
Impulse
Conservation of
momentum
Pivot
Moment
Equilibrium
Principle of
moments
Centre of gravity
Stable
Unstable
Lamina
Centripetal
Force
The quantity of matter an object is composed of. Measures an object’s resistance to change in motion. SI Unit: kg.
Mass per unit volume. SI Unit: kg/m3.
The downward force of gravity that acts on an object because of its mass. Unit: N.
The time taken for one complete oscillation.
The distance travelled by an object per unit time. SI Unit: m/s
The total distance travelled divided by the total time taken. SI Unit: m/s
A quantity that has magnitude but not direction.
A quantity with both magnitude and direction.
the speed of an object in a given direction. SI Unit: m/s.
The rate of change of velocity. SI Unit: m/s2.
Force required to give a 1kg mass an acceleration of 1m/s2.
A single force that has the same combined effect on a body as two or more forces.
The force when two surfaces rub together. It acts in the opposite direction to the motion.
Friction acting on an object moving through the air. It acts in the opposite direction to the motion.
Friction acting on an object moving through a fluid (a liquid or a gas). It acts in the opposite direction to the motion.
The force that exists between any two objects with mass.
The force exerted per unit mass placed at that point. SI Unit: N/kg.
The greatest speed reached by an object moving through a fluid.
(At terminal velocity, the resultant force acting on the object is zero and its acceleration is zero).
An object’s mass multiplied by its velocity. SI Unit: kgm/s
A change in an object’s momentum, or the force acting on an object multiplied by the time for which the force acts.
SI Unit: Ns or kgm/s
The total momentum in all directions is constant and does not change because of interactions between bodies.
A fixed point about which a lever turns.
The turning effect of a force about a pivot.
When the resultant force and resultant moment on a body is zero.
When an object is in equilibrium, the sum of anticlockwise moments about any point equals the sum of the
clockwise moments about the same point.
A point from which the mass of a body or system may be considered to act.
Unlikely to fall over, often because it has a low centre of gravity and a wide base.
Likely to fall over, often because it has a high centre of gravity and a narrow base.
A flat, two-dimensional shape.
The force required to keep an object in circular motion.
General measurements
To improve accuracy, measure
multiple times and take an average
of the measurements.
Keep eye parallel with scale when
measuring to avoid parallax.
Measuring oscillations
Start and end the measurement at a
notable place (e.g., one end of the
swing).
Measure at least 20 oscillations and
divide the total time by the number
of oscillations measured.
Distance vs time graph
- Gradient = speed
o Constant gradient  constant speed
o Increasing gradient  increasing speed
o Decreasing gradient  decreasing speed
Speed vs time graph
- Gradient = acceleration
o Constant gradient  constant acceleration
o Increasing gradient  increasing acceleration
o Decreasing gradient  decreasing acceleration
- Area under graph = distance travelled
Using a tangent to find value of a changing gradient
 A tangent only touches the curve at one point.
 Draw a line which is a tangent to the curve at the point you want to know the
gradient of.
 Find the gradient of the tangent.
Measuring thickness
Measure 20 or more pages/sheets
and dived the total measurement by
the number of pages/sheets
measured to find the average
thickness of one sheet.
Vector and Scalar
Scalar quantities only have magnitude (e.g., mass, temperature, speed, time)
Vector quantities have move magnitude and direction (e.g., velocity, acceleration, force, momentum).
Newton’s first law
If no external force is acting on it, an
object will if stationary, remain
stationary if moving, keep moving at a
steady speed in a straight line.
Newton’s Second Law
F = ma
Unbalanced forces (Fnet  0) acting on
an object causes acceleration.
Acceleration can mean a change in
either an object’s speed or direction.
Newton’s Third Law
For every action (force), there is an
equal but opposite reaction (force).
Vector Triangle
A graphical representation of vectors so that the resultant vector can be calculated.
If vectors A and B are drawn to scale, then the resultant vector, R, can be measured and
its magnitude can be found using the same scale. The direction can be found by
measuring the angle, θ, using a protractor.
The values of A, B, R, and θ can also be found using trigonometry.
Free fall
In the absence of air resistance, all objects fall with the same acceleration downward, g, because of the weight of the object.
Air resistance reduces the acceleration of the object as its force opposes the object’s motion.
Air resistance  speed
Eventually air resistance = weight of the object
Fnet = 0, a = 0, terminal velocity reached
Circular motion
Objects moving in a circle at a constant speed are accelerating because the
direction of their velocity is changing.
The velocity of objects moving in a circle is tangential to the circle.
Objects moving in a circle have acceleration acting on them directed towards the
centre of the circle they’re moving in.
Objects moving in a circle have a resultant force acting on them directed towards
the centre of the circle they’re moving in.
Resultant force
An object moving in a circle requires a certain amount of force to continue this
motion. This force is called centripetal force.
A force provides centripetal force. (e.g., friction provides centripetal force when
a car turns a corner, gravitational force for planets around the sun etc.)
If the amount of force that is provided (by gravity, friction etc.) is less than the
centripetal force, then the circular motion won’t continue.
Moment & Equilibrium
The principle of moments states that when in equilibrium
the total sum of the anti-clockwise moment is equal to
the total sum of the clockwise moment.
When a system is stable or balanced it is said to be in
equilibrium as all the forces in one direction are equal to
the forces in the opposite direction.
Stability
An object will topple (fall over) when the centre of gravity
moves outside the base.
A stable object has a low centre of gravity and/or a wide
base.
An unstable object has a high centre gravity and/or a
narrow base
Equations
Area = height x width
F = ma
Volume = height x width x length
ρ = m⁄V
W = mg
v = ∆s⁄∆t
Δv = vfinal – vinitial
a = ∆v⁄∆t
p = mv
Impulse = Δp = mΔv = FΔt
moment = perpendicular distance to pivot x Force
In equilibrium:
Resultant moment = 0
Resultant force = 0
Marking schemes are provided for the following questions in your SQ book.
We haven’t done any of them in class or for homework.
1.1
Measuring Length
P7
1.2
Q2
Measuring Density
P10-2 Q3-2
1.3
Measuring Time
P13
2.3
Speed Time Graph
P30
3.2
3.3
3.4
4.1
Q3
Q5
Newton’s Second Law of Motion
P47
Q4
P51
Q7
P53
Q9
P56
Q10
Objects falling in the air
P72
Q5
P74
Q6
P76
Q7
Scalars and Vectors
P77
Q1
P78
Q2
I have included all the marking schemes
P80
Q3
for the vector diagram questions because
P81
Q4
different classes did different questions
P82
Q5
Moments & Equilibrium
P87
Q4
P90
Q6
P91
Q7
1.1
Measuring Length
P7
1.2
Q2
Measuring Density
P10-2 Q3-2
(a)
Space 1
Space 2
Space 3
Space 4
15.80
15.87
1.00
0.85
[1]
[1]
[1]
[1]
(b)
Volume ratio = height ratio
Total Liquid volume = 96.67 cm3
Clear Liquid Height = (20/96.67)x10
Top
Bottom
1.3
Clear
Blue
Green
Red
Measuring Time
P13
Q3
Height (cm)
2.06
1.64
2.66
3.64
2.3
Speed Time Graph
P30
3.2
Q5
Newton’s Second Law of Motion
P47
Q4
P51
Q7
P53
Q9
P56
Q10
3.3
Objects falling in the air
P72
Q5
P74
Q6
P76
Q7
3.4
Scalars and Vectors
P77
Q1
P78
Q2
P80
Q3
P81
Q4
P82
Q5
4.1
Moments & Equilibrium
P87
Q4
P90
Q6
P91
Q7