Today's objective
• I can explain what a force is,
how forces are measured,
and how to calculate net
force.
First we need to define the
word FORCE:
• The cause of motion (what
causes objects to move)
• Two types of forces
– Pushes
– Pulls
Two things that every force
must have…
• Magnitude – how big (or small) is the
force
– A number in Newtons
• Direction – which way is the force
moving
– Left, right, up, down
Forces are measured in
Newtons
• SI unit of force
• Symbol: N
• Measured by using a spring scale
Forces may be balanced or
unbalanced
• Balanced forces – all forces acting
on an object are equal
– There is NO MOTION
– Combined forces equal ZERO
• Unbalanced forces – one or more
forces acting on an object are
stronger than others
– There is MOTION
• A NET FORCE
Balanced and Unbalanced
Forces
Net Force
• The combination of all the forces
acting on an object is the net force.
Combining Forces in the Same
Direction
Combining Forces in Opposite
Directions
Learning Objective
Today I will be able to
investigate and describe
application of Newton’s
Laws of Motion.
Newton’s Laws
• First Law – Inertia
• Second Law – Acceleration,
Force & Mass
• Third Law – Action-Reaction
First Law
• Inertia
– An object at rest
[not moving]
remains at rest
unless acted on by
a force [push or
pull]
– An object in
motion remains in
motion unless
acted on by a force
[push or pull]
First Law
• Inertia & Mass
– The more MASS an object has,
the more INERTIA the object
has.
– INERTIA is an objects tendency
to resist a change in motion
– Bigger objects are harder to start
& stop because of INERTIA
Second Law
• The ACCELERATION of an object is
equal to the net force acting on the
object divided by the object’s mass.
• The direction of acceleration is the
same as the direction of the net
force.
Second Law
• Acceleration & Force
– The more force placed on an object,
the more it will accelerate [change its
motion]
• Acceleration & Mass
– The more mass [or inertia] an object
has, the more force it takes to
accelerate the object
Third Law
• Action – Reaction
– Forces are always produced in pairs
with opposite directions & equal
strengths
– For every force there is an equal and
opposite force
Momentum
• Momentum is a measure of how
hard it is to stop a moving object.
• Because action and reaction forces
do not cancel each other, they can
change the motion of objects.
The truck is in motion. What is the force
that causes it to stop?
The push of the stopped car.
The car is at rest. What is the force that
causes it to move?
The push of the truck.
What about the ladder on top of the truck?
The ladder is in motion because the truck is in
motion.
When the truck stops, the ladder stays in motion.
NEWTONS LAW OF INERTIA (1ST)
The truck is stopped by the force of the car, but the
ladder is not.
What force stops the ladder?
Gravity.
The truck is in motion, the car is at rest. How do
each of these vehicles accelerate (change in
velocity)?
The truck stops moving. The car starts moving.
Which one will be the hardest to accelerate?
The truck because it has the most mass (INERTIA).
Why does the car move [accelerate] when it is hit
by the truck?
The heavy and moving truck has more force than
the small, at rest car.
Why does the truck stop moving when it hits the
car?
The force of the car pushing back on the truck,
plus the force of friction between the massive truck
and the road slow down, that stop the truck.
The truck hits the car. An action force stops the
truck.
What is the equal and opposite reaction force?
The force car pushing back on the truck.
According to Newton’s second law, when the
reaction force results in an unbalanced force, there
is a net force, and the object accelerates.