Science 8 Reviewer

GRADE 8 SCIENCE
Force- is a vector quantity that may change or not change in the motion of an object.
- A push or pull upon an object
Net Force- vector sum of forces acting on an object.

When an object is in a state of rest it has zero net force.
Friction- is a force that opposes the motion of an object.
Three Laws of Motion
Newton’s first law of motion or Law of Inertia- states that an object at rest remains at rest
and an object in motion continues to move at a constant velocity unless acted upon by an
external net force.
Inertia – is a tendency of an object to resist change in its state of motion.
Mass – also determines the object’s inertia.
More mass means more inertia.
Example:
The bigger punching bag has more mass and thus more inertia.
Pushing a bicycle is easier since it has lesser mass and thus lesser resistance to a change in
state of motion.
Newton’s second law of motion or Law of Acceleration- states that “the acceleration of an
object is directly proportional to the net force applied on it and inversely proportional to its
mass”. F=ma
Force acting upon an object is increased, the acceleration is increased.
Mass of an object is increased; the acceleration is decreased.
Newton’s second law of motion is expressed through the equation:
𝐹 = 𝑚𝑎
Wherein:
𝐹 = net force acting on the object (N or kg m/s 2)

m = mass of the object (kg)
𝑎 = acceleration (m/s2)
1 N = 1 kg m/s2
Law of Motion in Uniform Circular Motion

This centripetal motion or acceleration indicates that there is a centripetal force.
It is used to describe the net force acting upon an object in uniform circular motion.
Example: - a car turning around a circle path
- the moon orbiting Earth
Newton’s third law of motion or Law of Interaction- states that “for every action, there is
an equal but opposite reaction”.
The law of interaction explains that these forces always come in pairs.
Action-Reaction Pairs Example
1. Action: when you punch a punching bag, it gets pushed away from you.
Reaction: The punching bag pushes your hand back.
Isaac Newton was a physicist and mathematician who developed the principles of modern
physics, including the laws of motion and is credited as one of the great minds of the 17th-
century Scientific Revolution.
Work- defined as the product of force applied on an object and its displacement.
Conditions that need to be met before one can say work is done on an object.
1. Force must be applied on the object
2. The object must have displacement; it must move from one position to another.
3. The direction of the displacement must have a component in the direction of the force
applied.
W = Fd
Wherein:
W = work
d = displacement
F = force
Power – the rate of doing work
- To express how fast work is done
The unit of power is Watt (W) named after James Watt who improved the steam engine.
P=W P = Fd
t t
Wherein:
P = power (W)
F = force (N)
t = time (s)
d = distance (m)
Energy – is defined as the ability to do work.

Comes in different forms and has two main types – potential and kinetic energy.
Potential Energy - It is a stored energy by an object due to an object’s position above
P.E. = mgh
Wherein:
m = mass of an object in kg (Kg)
g = acceleration due to earth’s gravity
h = height of an object from the ground. (m)
Acceleration of gravity 9.8 m/s
Elastic Potential Energy- stored energy in elastic materials such as rubber bands, springs,
garters and bungee cords, when either these are stretched or compressed.
P.E. = 1/2kx2
Wherein:
P.E. = elastic potential energy (J)
x = elongation or compression of the spring/rubber band in meters (m)
k = spring constant in Nm
Kinetic Energy- commonly defined as energy in motion or energy in motion.

KE=1/2mv2
Wherein:
K.E. = Kinetic energy (J)
m = mass (kg)
v = speed (m/s)
Sound – is a disturbance that carries energy and transfer among particles.

Produced when vibration causes pressure variations in the material.
Speed of Sound – refers to the rate at which the disturbance travels from one particle to
another particle of medium.
S = distance
Time
Wherein:
S = speed of sound (m/s)
d = distance (m)
t = time (s)
Example: A sound wave from an aircraft is observed to have travelled a distance of 5 000 m in 14
s. Determine the speed of sound.
Given:
Distance = 5 000 m time = 14 s
Solution:
Speed = distance = 5000 m = 357,1 m
time 14 s s
Density – refers to the ratio of the mass and the volume of a substance.
Elasticity – refers to the ability of an object to return to its original shape after being distorted.
Sound waves are transmitted faster and more efficiently in solid than in liquid and in gas.
Temperature – measure of the average kinetic energy of molecules in a medium.
With more collision per unit time, energy is transferred more efficiently resulting in sound
traveling quickly.
The speed of sound in a given temperature of air can be obtained using the following formula:
V = 331 m/s + 0.6 m/s (T)
C
Wherein:
v = the speed of sound
t = temperature
331 m/s is a constant factor
0.6 m/s is a constant factor

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GRADE 8 SCIENCE

- A push or pull upon an object

Net Force- vector sum of forces acting on an object.

𝐹 = net force acting on the object (N or kg m/s 2)

Law of Motion in Uniform Circular Motion

Power – the rate of doing work

- To express how fast work is done

Energy – is defined as the ability to do work.

Kinetic Energy- commonly defined as energy in motion or energy in motion.

Sound – is a disturbance that carries energy and transfer among particles.

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