Transcript Describing Motion - Science

Describing Motion
Newton’s Laws
First we need to define the
word FORCE:
• The cause of motion (what
causes objects to move)
• Two types of forces
– Pushes
– Pulls
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
• Unbalanced forces – one or more
forces acting on an object are
stronger than others
– There is MOTION
• A NET FORCE
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
– Mass is the amount of matter in an
object
– The more MASS an object has, the
more INERTIA the object has.
– Bigger objects are harder to start &
stop
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Second Law
• Acceleration & Mass Definitions
– Acceleration is a change in velocity
[speed or direction]
– Mass is the amount of matter in an
object
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
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.
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?
The truck stops moving. The car starts moving.
Which one will be the hardest to accelerate?
The truck because it has the most mass.
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, the stop the truck.
The truck hits the car. An action force stops the
truck.
What is the equal and opposite reaction force?
The force that pushes the car forward.