Mass, Force and
Motion
th
8
Grade
Physical
Science
S8P3. Students will investigate
the relationship between force,
mass, and the motion of objects.
b.Demonstrate the effect of
balanced and unbalanced forces
on an object in terms of gravity,
inertia, and friction.
What is a force?
Calculating Net Forces
A net force is the total amount of
all forces acting on an object.
Add forces moving in the same
direction.
Subtract forces moving in opposite
directions.
BALANCED FORCES vs.
UNBALANCED FORCES
• Balanced forces are
always equal to
ZERO and DO NOT
cause motion.
• Unbalanced forces
ALWAYS cause
motion and NEVER
equal to zero.
FRICTION is a force that works in
the opposite direction to motion.
Static Friction vs. Kinetic Friction
• Static friction is found
where there is NO
motion.
• Kinetic friction is
found where MOTION
occurs.
Newton’s
st
1
Law of Motion
Newton’s 1st Law of Motion or The
Law of Inertia states…..
An object at rest tends to stay at
rest and an object in motion tends to
stay in motion unless an unbalanced
force acts upon it.
Newton’s 2nd Law of Motion
More on the Law of Acceleration
Newton’s 3nd Law of Motion
According to Newton’s 3rd Law of Motion,
Forces Always Come In Pairs (or Twos)
SI (International System) Units of
Measurement
We measure forces in NEWTONS.
We measure weight in NEWTONS.
Weight is measurement of the
FORCE of GRAVITY.
Gravity is a major force in the universe!
S8P5. Students will recognize the
characteristics of gravity,
electricity, and magnetism as major
kinds of forces acting in nature.
a. Recognize that every object
exerts gravitational force on every
other object and that the force
exerted depends on how much mass
the objects have and how far apart
they are.
Gravitational Force
The Force of Gravity and Mass
The Force of Gravity and Distance
S8P3. Students will investigate
the relationship between force,
mass, and the motion of objects.
a. Determine the relationship
between velocity and acceleration.
Speed and Velocity
Both use the same equation:
Speed or Velocity = distance
time
Velocity has a
direction…..Speed
does not go in a
particular
direction.
Velocity and Acceleration
SI (International System)
Units of Measurement
We measure speed in m/s.
We measure velocity in m/s.
We measure acceleration in m/s2.
(Or…meters per second per second.)
S8P3. Students will investigate
the relationship between force,
mass, and the motion of objects.
c. Demonstrate the effect of simple
machines (lever, inclined plane,
pulley, wedge, screw, and wheel
and axle) on work.
What Is Work?
Work is the transfer of energy to an
object.
In science, work is done when a
force causes an object to move in
the direction of the force.
Do forces always cause work?
How to Calculate Work
We use the equation W = F x D
“Work equals Force times Distance”
SI (International System)
Units of Measurement
We measure work in J.
J stands for Joule
A Joule is a Newton times a
meter.
Joule = N x m
What is Power?
Power is the rate at which energy
is transferred to an object.
How to Calculate Power
P=W
t
Power = Work
time
In order to calculate Power, you divide
the amount of work done (W) by the
time (t) it takes to do that work.
SI (International System)
Units of Measurement
We measure power in W.
W stands for Watt
A Watt is one Joule per second.
Watt = J/s
What is a Machine?
A machine is a
device that
makes work
easier by
changing the
size or
direction of the
force.
Work Input and Work Output
When you use a machine,
you do the work on the
machine and the machine
does the work on
something else.
The work that you do on
the machine is called the
“work input”.
The work done by the
machine is called the
“work output”
The “work output” can
never be more than the
“work input”
Machines Can Change The Input Force
Machines Change the Size or
Distance of an Input Force
Mechanical Advantage
Some machines
increase force
more than other
machines can.
The amount of
times that a
machine multiplies
the force is called
mechanical
advantage.
Mechanical Efficiency
Mechanical
efficiency is the
comparison of a
machine’s work
output with the
work input.
We measure it with
the following
equation:
Simple Machines
There are six basic simple machines
that help make our lives much easier.
Inclined Plane
An inclined plane
is a simple
machine with a
straight slanted
surface. A ramp
is an inclined
plane.
Lever
A lever is a simple machine that has a bar
that pivots about a fixed point called a
fulcrum.
There are 3 types of levers:
1st Class
2nd Class
3rd Class
Pulley
A pulley is a
simple
machine
that has a
grooved
wheel that
holds a
rope or
cable.
Wheel and Axle
A wheel and axle is a simple machine that
consists of two circular objects of
different sizes.
Screw
The simple machine
called a screw is an
inclined plane wrapped in
a spiral around a cylinder.
Wedge
The simple machine
called a wedge is a
pair of inclined planes
that move.
Compound Machines
Compound machines are
machines that are made
up of two or more simple
machines.