# Forces, FBDs, Newtons laws ```What causes acceleration?
• Acceleration – a change in velocity
1. Do all forces cause acceleration?
2. Give an example where a force does cause
acceleration and an example where it does not.
3. How is the force you apply related to the
direction of acceleration?
4. What if two people were pushing equally in
opposite directions on an object, what would
happen?
Forces and Free Body Diagrams
What is a force?
• Force
▫ any push or pull
▫ Measured in Newtons (N)
• Most forces involve contact (pushing a car) with
the exception of action at a distance forces
▫ These forces do not require the two objects putting
force on each other to touch
▫ Ex. Gravitational force, magnetic force
Types of Forces
• Using page 27, for each force describe:
▫ Description:
▫ Direction it acts:
• Leave some space below each description for
diagrams
Drawing Free Body Diagrams
Steps:
1. Draw a box on the appropriate
surface
2. Draw a dot at the centre
3. Draw and label an arrow for each
force present
Examples
1. A car sitting on a flat road
2. Car being pulled by a rope over a
flat surface
3. Car being pushed up a ramp
4. Car suspended by a string
Calculating Net Force
• Net Force – the sum of all the forces
on an object
FNET = F
FNET,X = FX
FNET,Y = FY
Calculating Net Force
• Net force
▫ the resulting force if you add all the
forces acting on a single body
▫ Not an actual force but it results from
actual forces
▫ The only force that you can see the effect
of
• Tug of war example
• For these types of problems, you must
ALWAYS define a positive and
negative direction and use these for
Example
• Miraj pulls with 200 N of force to
the left on a rope. Reuben pulls
with 300N of force to the right on
a rope. What is the net force on
the rope?
Net Force Examples
• On board
Newton’s Second Law
Newton’s Second Law
• Acceleration is related to mass and net
force
FNET = ma
FNET – Net Force (N)
m – mass (kg)
a – acceleration (m/s2)
FNET = ma
• What does this equation say?
▫ Directions are equal – direction of fnet
equals direction of a (the object will
accelerate in the direction of the net
force)
▫ If net force is equal to zero, acceleration
equals zero
▫ If force is constant,
 Mass increases, acceleration decreases
 Mass decreases, acceleration increases
If net force is equal to zero, acceleration equals
zero
• What does zero acceleration look
like?
▫ Object is not moving and stays not
moving
▫ Object is moving at a constant speed and
stays at that speed (zero acceleration
means it does not change speed)
Real Life
•If you are late for school and
stopped at a red light, do you
want to be behind a full dump
truck or an empty dump truck?
Video Example
Homework:
• Pg 44 # 7
• Pg 45 # 1-4
Newton’s First Law of
Motion
Inertia
Inertia
– a property of matter that causes a body to
resist a change in motion
- Greater the mass, greater the inertia
• Is it easier to move?
▫ A rock or a mountain?
▫ A smart car or a big rig?
• Greater mass = greater inertia = more resistant
to a change in motion
Newton’s First Law
• Objects at rest want to stay at rest
• Objects in motion want to stay in motion
• UNLESS,
▫ An unbalanced force (net force not equal to
zero) is applied to them
Summary
• No net force, no change in motion
• If the velocity is constant or zero, the net force
acting on it must be zero
• If velocity is changing (size or direction), new
force is NOT zero
Examples
•
•
•
•
•
Alien Tracker
Beaker with pennies
Air hockey table
Dirt on a shovel
Water off toothbrush
Video Example
Homework
write a paragraph which relates it to Newton’s
Second law
• Pg 36 # 5,6,8
• Pg 37 # 1,3
Newton’s Third Law
• For every action there is an equal
and opposite reaction
• For every force, there is another
force equal in magnitude but
opposite in direction
Examples
•
•
•
•
•
•
•
Rocket
Wrench in space
Getting out of a boat
Swimming
Fan boat
Helicopter
Recoil
Video
Work
• Pg 48 # 1
• Pg 50 # 5,6
• Pg 51 # 2
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