Forces
One popular urban myth tells a story of a man who
makes a wish from the top of the Empire State
Building, then tosses a penny off the roof. Over
1,000 feet below, a pedestrian is hit by the speeding
penny and killed. …Good luck, huh?
What causes the penny, an object we may see and
handle every day, to become a weapon?
Killer Pennies!
• Is it heavy?
– Pennies aren’t very heavy, but neither is a bullet?
• Is it going too fast?
– Dropped from that height, an object has a long way to fall
– plenty of space to speed up.
• So how does it speed up?
– Gravity. …But gravity is a single FORCE. One of several
forces, in fact, that are working on that penny.
– In fact, this myth is false. A penny is simply not heavy
enough and wouldn’t have enough speed to kill a person
walking below.
• But a roll of pennies on the other hand….
• http://science.howstuffworks.com/science-vs-myth/everydaymyths/threw-penny-off-the-empire-state-building.htm
• A FORCE is a push or a pull.
– Gravity pulls cats down toward the Earth.
– Space shuttles use explosive fuel to push
off from the ground
– Your weight pushes down on your chair.
• All forces have size and direction.
• Forces can transfer energy.
• Forces are measured in Newtons (N).
– 1 Newton is about the weight of a stick of
butter.
• There are many different kinds of forces
• Some only work when two objects are
directly touching each other.
• Others can work “at long range.”
• The forces that need direct contact are only
working while the objects are touching. Once
they are no longer touching, the force is gone.
– Direct contact – friction, applied force
• Forces that work at a distance are ALWAYS
present, but if you’re REALLY far away, the
force is nearly 0 (ineffective).
– Distant forces – gravity, electrical, magnetic
Applied Force
• An APPLIED FORCE is a push or pull from
one object put onto another object.
– It covers a lot of things.
• Pushing your siblings
• Pulling a wagon
• Chewing food
• Flying a kite
• Etc., etc., etc.
– It’s basically every force directly between
two objects, as long as it’s not…
Friction
• FRICTION is the force between two
surfaces as they slide past each
other.
–Friction always resists motion.
• If you are trying to move forward, friction
is pushing backward.
• If a sled is moving downhill, friction pulls
uphill.
• If a penny is falling from the Empire State
Building, friction pulls up.
Gravity
• Gravity is the first of the distant forces.
• GRAVITY is the pull on everything from
the center of the Earth’s mass.
– Really, everything has some small gravity.
The larger the mass, the larger the pull.
• What keeps the Earth in orbit around the Sun?
• What keeps the moon in orbit around the
Earth?
• What keeps you on the floor?
• Does Mr. Hibner have gravity?
Electrical
• ELECTRICAL FORCE is the push or pull from
electrical charges.
– What happens if you rub a balloon on your
head and pull it away slowly?
– “Opposites attract” – there are positive and
negative charges.
• “-” and “+” are pulled TOWARD each other.
• “-” and “-” are pushed AWAY FROM each other.
• “+” and “+” are also pushed AWAY FROM each
other.
Magnetic
• MAGNETIC FORCE is a push or pull based on
magnetic poles.
– What direction does a compass always
point?
• Does it ALWAYS point that way?
– Again, “opposites attract”
• “N” and “S” are pulled TOWARD each other.
• “N” and “N” are pushed AWAY FROM each
other.
• “S” and “S” are also pushed AWAY FROM each
other.
• There may be many different forces acting on
objects at the same time.
• When you look at all of the forces acting on an
object, some things cancel each other out.
– When you play tug-o-war, two sides are
each applying a force on the flag in the
middle. Two things can happen:
• One team is stronger than the other and
pulls the flag their way
• Both teams are equally strong, and the
flag stays right in the middle.
Force diagram
• When you add up all of the forces that
are working on an object at once, the
result is called a NET FORCE.
• The net force determines the motion of
an object.
• If the net force = 0, then the object’s
motion doesn’t change.
– If it’s moving, it stays moving.
– If it’s stopped, it stays stopped.
• If the net force > 0, the result is
ACCELERATION.
Doing the Math
• So, how do I find out if two forces are
balanced or unbalanced?
• If they are pointing in the same direction, ADD
THEM TOGETHER
• If they are pointing in different directions,
SUBTRACT THEM.
20 N
10 N
50 N
20 N
This picture tells me
that Batman is
standing,…
…waiting. Justice
never sleeps.
How can I tell that
Batman isn’t trying
to move?
How can I tell that
Batman isn’t falling?
What is the net force on the cheetah?
What direction
will this Angry Bird
accelerate in?
• If two forces cancel each other out, then they
are called BALANCED FORCES.
• If two forces DO NOT cancel each other out,
then they are called UNBALANCED FORCES.
• The result is acceleration.
– This skydiver will accelerate toward the ground.
• Good luck with that! 
Where do you see forces at
work?
• Individually, come up with 2 examples of
forces at work in life situations.
• With a partner, share your examples. Try
to identify if they are balanced or
unbalanced forces.
– Where is the net force pointing?
• As a class, we will discuss a few
examples.
Practice
Draw a force diagram for a bird that is flying
forward through a windstorm. Show that some
forces are more powerful than others by their
length or width.
Are the forces balanced or unbalanced?
What is the net force on the bird?
Will the bird accelerate? If so, in which direction?
• Draw a force diagram for a baseball being hit
by a player. Show that some forces are more
powerful than others by their length or width.
Are the forces balanced or unbalanced?
What is the net force on the ball?
Will the ball accelerate? If so, in which direction?
• Draw a force diagram for a car traveling at
EXACTLY 60 mph down the highway. Show
that some forces are more powerful than
others by their length or width.
Are the forces balanced or unbalanced?
What is the net force on the car?
Will the car accelerate? If so, in which direction?
• Draw a force diagram for a steel beam being
suspended by a crane. Show that some forces
are more powerful than others by their length
or width.
Are the forces balanced or unbalanced?
What is the net force on the beam?
Will the beam accelerate? If so, in which direction?
• Draw a force diagram for a sled being pushed
down a hill. Show that some forces are more
powerful than others by their length or width.
Are the forces balanced or unbalanced?
What is the net force on the sled?
Will the sled accelerate? If so, in which direction?
• Draw a force diagram for a box being pushed
across a desk. The box has a weight of 50 N.
The force of friction is 45 N, and the force of
the push is 60 N. Show that some forces are
more powerful than others by their length or
width.
In case you forgot what a
box looks like.
Are the forces balanced or unbalanced?
What is the net force on the box?
Will the box accelerate? If so, in which direction?
• Draw a force diagram for an apple hanging
from a branch on a tree. The weight of the
apple is 0.1N. Show that some forces are
more powerful than others by their length or
width.
Are the forces balanced or unbalanced?
What is the net force on the apple?
Will the apple accelerate? If so, in which direction?
Constant Velocity
• If a non-zero NET FORCE causes an acceleration,
then what does a net force of 0 cause?
• A car drives down the highway with a CONSTANT
SPEED. Draw a force diagram for this car.
• When driving, you always need to press the gas
(applied force) just a little to keep up your speed.
• A NET FORCE of 0 means that the object’s speed
isn’t changing – THIS DOES NOT MEAN THE
SPEED IS ZERO!!!
Newton’s Laws
• Isaac Newton
• 1st law – An object at rest stays at rest (an
object in motion stays in motion) until an
outside force acts on it.
– Called “INERTIA”
• 2nd law – Force = mass*acceleration
– F = m*a
• 3rd law – For every action, there is an equal
and opposite reaction.
A basketball is sitting on a table. Jake walks by and
taps it on the side. Draw a force diagram for the
ball when Jake’s hand hits it.
What will happen to the ball immediately after the hit?
– 5 seconds later?
– 5 minutes later?
– Which law does this demonstrate?
Ally likes fishing. After seeing the bobber dip under
the water, she pulls back on the rod and starts to
reel in this fish, who is struggling to get away. Draw
a diagram for the fish, assuming the line is straight
forward.
What happens if the fish pulls back too hard?
Which law does this demonstrate?
Look at Ally’s fish again. The weight of the fish is 20 N.
The fish is pulling back with a force of 60 N, and Ally is
pulling forward with a force of 70 N.
What is the value of the applied force on the fish that
helps it swim?
If the fishing line is unbreakable, which direction does
the fish go?
• What if Ally only pulls with 60 N? 50 N?
What happens if the fishing line can only hold 150 N of
force? 100 N?
Driving down the highway, you are looking forward
and a bug splats on the windshield. Draw a diagram
for the bug as it hit, assuming it was not moving.
What happens when the bug hits the windshield?
Does the bug slow down?
Does the car slow down?
What law do these things demonstrate?
Identifying Newton’s Laws
• Where do you see Newton’s Laws acting in
these pictures?
1) Object in motion stays in motion until an outside
force acts on it
2) F =m*a
3) For every force, there is an equal and opposite
force.
We will begin measuring forces – comparing balanced
and unbalanced forces using numbers.
This makes it easier for us to tell for sure whether one
force is stronger than another.
For force, the units we measure in are called
“NEWTONS,” named after Sir Isaac Newton.
Center of Mass
• When we draw a force diagram, do we
ALWAYS have to draw the picture?
• I mean…my art skillz aren’t so good….
• And sometimes the stuff is hard to draw….
• When we draw a force diagram, we should
always draw the force acting on the CENTER
OF MASS – the point where the mass is all
balanced out.
• For this reason, you can draw every object as a
box or a sphere, if you’d like! 