Forces and Motion
Demonstrate and calculate how
unbalanced forces change the speed
or direction of an objects motion
Force
• Any push or pull on an object
• Act in pairs
• Measured in Newtons (N)
• Every force has a magnitude and direction
• Forces can work together or against each
other
• Forces can be balanced or unbalanced
Balanced Forces
• Occurs when there is NO CHANGE in an
objects motion
• No acceleration – no change in motion
– objects are at rest or traveling at a
constant speed and direction.
Balanced forces
Unbalanced Forces
• Always cause a change in motion Acceleration
• Acceleration – change in speed or
direction of an object
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•
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Starting
Stopping
Speeding up
Slowing down
Turning
Net Force
• Combination of all forces acting on an
object.
• Add forces working in the same
direction.
• Subtract forces that are working in
opposite directions.
Vectors
• Lines with arrows that show force.
• Length of the line shows the amount of
force (magnitude)
• The arrow shows the direction of the
force.
Add forces acting in the
same direction
• 8 N + 6 N = 14 Newtons
Subtract forces acting in
opposite directions
• 20 N – 18 N = 2 Newtons
Net Force = 0
Forces are balanced
• 20 N – 20 N = 0 Newtons
• Forces are balanced - No change in motion
Net force and direction
• What is the net
force?
• What direction are
they moving?
• Why?
Newton’s 1st Law of Motion
• Law of Inertia
• An object at rest will stay at rest
unless acted on by an unbalanced force.
• An object in motion will stay in motion
at the same speed and direction unless
acted on by an unbalanced force.
In other words…
• An object will keep doing what it is
doing unless forces become unbalanced
• Also known as the “Law of Inertia”
• Inertia – An object’s tendency to resist
change in motion.
Law of Inertia (1st law)
Vehicle Restraints
• What is the purpose of vehicle
restraints such as seat belts?
• Describe how Newton’s 1st Law of
motion applies to vehicle restraints.
Newton’s 2nd Law of Motion
• Law of Acceleration
• The acceleration of
an object by a force
in inversely
proportional to the
mass of the object
and directly
proportional to the
force applied.
In other words…
• The smaller the object, the greater the
acceleration AND the larger the object,
the smaller the acceleration.
• The smaller the force applied, the
smaller the acceleration AND the larger
the force applied the larger the
acceleration.
Therefore
• Force = Mass X Acceleration
• Force = 1,000 kg X .05 m/s/s
• Force = 50 Newtons
Acceleration
• The rate of change in the velocity,
speed and/or direction, of an object.
• Acceleration = Force/Mass
• Measured in meters/sec2
Force = Mass X Acceleration
• If the amount of force stays the same
and the mass of the object increases,
what will happen to the amount of
acceleration?
• If the amount of force stays the same
and the mass of the object decreases
what will happen to the amount of
acceleration?
What if the acceleration has to
stay the same?
• What will have to happen to the amount
of force needed if the mass of an
object increases?
• What will happen to the amount of
force needed if the mass of an object
decreases?
Spring Scales
• Used to measure force.
• Use the spring scale to measure the
amount of force needed to pull the CPO
car up the ramp with different amounts
of weight/mass.
• What happens to the amount of force
when the mass increases?
Newton’s 3rd Law of Motion
• For every action, there is an equal but
opposite reaction
In other words…
• All forces act in pairs.
• When one object exerts a force on a
second object, the second object
exerts a force back that is equal in size
but opposite in direction
Newton’s 3rd Law
Space Shuttle Launch
Speed/Velocity
• How fast an object is moving
• Rate at which an object covers a
distance
• Speed = distance/time (meters/second)
Calculating Speed
• While on vacation, Lisa Carr traveled a
total distance of 440 miles. Her trip
took her 8 hours. What was her
average speed?
• Speed = 440 miles/8 hours
• Average speed = 55 miles/hour
Calculating Distance and Time
• Calculating the amount of time it takes
to travel a distance.
• Time = distance/speed
• Calculating the distance traveled in a
certain amount of time.
• Distance = speed X time
What’s the difference between
Speed and Velocity
• Velocity - The rate at which an object
changes position. It’s the measurement
of the rate and direction of motion.
Force = Mass x Acceleration
(F = MA)
Which object would have the greatest
force?
– A 500 g object accelerating at 10m/s2
or
– A 10 g object accelerating at 10m/s2
– Use your formula chart to calculate Force
• Force = Mass x acceleration
Force = Mass x Acceleration
• 500g X 10 m/s2 = 5000 Newtons force
• 10g X 10 m/s2 = 100 Newtons of force
• The 500g object exerts a greater force
What is the motion of the
object in the graph below?
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•
•
•
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No motion
Moving away fast at a constant speed
Stopped (no motion)
Moving away more slowly at a constant speed
Stopped (no motion)
What is the motion of the
object in the graph below?
• Moving away at a constant speed
• Stopped moving (no motion)
• Accelerating
Distance Time Graphs
What is the motion of the object in the
graph below?
What is the speed of the object?
Speed = distance ÷ time
• 5 meters in 1 sec.
• 25 meters in 5 sec.
• 50 meters in 10 sec.
• 5 ÷ 1 = 5 m/s
• 25 ÷ 5 = 5 m/s
• 50 ÷ 10 = 5 m/s
This object is travelling at
a constant speed of 5
m/s (meters per second)
Interpret the graph below
Kinetic vs Potential Energy
• Kinetic Energy – Energy of motion
– The faster you are travelling the more
kinetic energy you have
• Potential Energy – Stored energy
– The more stored energy you have built up
the more potential energy you have
Where does the rollercoaster have
the greatest kinetic energy vs.
potential energy