+
Day 3 – Free body diagrams
Mr. Lambert
Integrated Science 1A
Trimester 1, 2014
+
Bell Ringer
I am pushing my car
while it is in neutral.
What are two ways that
I can increase the
acceleration of car?
+
Agenda
1.
Bell Ringer
2.
Learning Targets
3.
Push or Pull LAB
4.
Newton’s second law
5.
Exit Slip
+
Learning Targets
On your Bell Ringer
Sheet, write the
following:
We will determine
when the forces on an
object are either
balanced or
unbalanced.
+
Lab Review
 Look
at reflection questions at end
+
Videos!

Drag Racing


Bungee jumping (Start 3:00)


http://www.youtube.com/watch?v=fdvN
MfYxNyo
http://www.youtube.com/watch?v=kJslNvmFYA
Parachute off tower (0:40-1:40, 5:20-5:50)

http://www.youtube.com/watch?v=b1A
n1MFwXxQ
+
Newton’s Second Law
 Force
F
= Mass x Acceleration
= ma
+
Newton’s Second Law
Newton’s
Second Law states that an object
will only accelerate if there is
a net or unbalanced force acting upon it
+
Balanced vs. Unbalanced Forces
Balanced Forces

Objects DO NOT accelerate

Objects remain at constant
speed or at rest

NET FORCE EQUALS ZERO

Ex: Tug of War, no sides winning
WHO’S WINNING HERE?
Unbalanced Forces

Cause objects to accelerate (slow
down, speed up, change
direction) OR stop

Represented by arrows that ARE
NOT equal

NET FORCE DOES NOT EQUAL
ZERO

Ex: Tug of War with one side
winning
WHO’S WINNING HERE?
+
Net Force
Net
force– sum of
all forces acting on
an object
Forces
add and
subtract
+
Free Body Diagrams
A
free-body diagram is a simple
diagram that shows the size and
direction of all the forces acting on an
object
+
Free body diagram reading
After finishing the reading, answer the following
questions in your notebook.
1. Name
three different forces. Explain each force.
2. Explain
the free body diagram of the two rugby
players pushing each other.
3. Draw
a free body diagram of a book sitting at a
desk (it is not moving).
+
Example
FNormal
FApplied
FFriction
p
FGravity
+
Strategies for Constructing a Free-Body Diagram
Read the Problem
Determine all the forces
that are acting on the
object
Determine the direction
that each force is acting
+
Steps for Constructing a Free-Body Diagram
1. Draw the object as a box.
2. Draw arrows on the box, each
pointing in the direction the force is
acting.
3. The size of the arrow represents the
size of the force.
3. 5. Label each arrow.
FNormal FApplied
FGravity FFriction
+
Net or Unbalanced Forces
When
a ball is falling, which is greater, the
gravitational force, or the air resistance?
Net Force is
downwards!
Fair resistance
Fair resistance
Fgravity
Fgravity
Therefore the
baseball
accelerates
downwards!
+
Net or unbalanced forces
 What
 The
about a book on a table? Does it accelerate?
Fgravity and Fnormal are the same in opposite directions!
Net Force is
zero!
Fnormal
Fnormal
Fgravity
Fgravity
Therefore the
book does not
accelerate!
+
Net or Unbalanced Force
 When
you push a box at a constant speed, is
the box accelerating?
 Therefore, which
is greater, your applied
force, or the force of friction?
Push/Applied Force
Push/Applied Force
Friction Force
Friction Force
Net Force is
zero!
Therefore the
box does not
accelerate, but
it can move at a
constant speed
+
Whiteboards!
Under
your desk, there should be a
whiteboard and a pouch containing a
marker, eraser, and calculator.
When
you are done, put everything back in
the pouch!
+
Problem #1
A radio sits on a bookshelf. Draw a free body
diagram of the forces acting on the radio.
+
FNormal
FGravity
+
Practice #2
An apple if falling through the air. It is
accelerating downwards.
+
Practice #3
A ball is hanging from a length of
string.
+
Draw a free body diagram as the girl starts to
decelerate

Bungee jumping (Start 3:00)

http://www.youtube.com/watch?v=kJslNvmFYA
+
Practice #3
A book is left on the table.
+
Practice #4
A stickman is pulling a box. The
box is moving at a constant velocity
to the right.
+
Draw a free body diagram before and after
they deploy their parachutes

Parachute off tower (0:40-1:40, 5:20-5:50)

http://www.youtube.com/watch?v=b1A
n1MFwXxQ
+
Practice #5
A
force is applied to the right to drag a sled
across loosely packed snow with a rightward
acceleration. Neglect air resistance.
+
Problem #6
A
car is coasting to the right and slowing
down. Neglect air resistance.
+
Put whiteboards away!
+
On a half-sheet of paper answer the questions
A group of engineers is doing research on how to maximize
the efficiency of hybrid cars. They decide to do some test
runs first.
1.
Draw a free body diagram of the hybrid car before it
begins to move.
2.
Draw a free body diagram of the hybrid car as it begins
to accelerate.
Compare and contrast the forces in the free body diagrams
above. How do the differences in the forces from the first
FBD to the second FBD explain the change in motion of the
hybrid car?
+
Question continued….
The engineers decide to modify some components of
the hybrid car.
1.
On hybrid car A, the engineers made the car bigger
and used a stronger, heavier metal. Using Newton’s
Second Law, explain why hybrid car A will have a
slower acceleration.
2.
On hybrid car B, the engineers added a more
powerful engine, increasing the force of the car. Using
Newton’s Second Law, explain why hybrid car B will
have a faster acceleration.
+
 The
first FBD shows only gravitational force pushing
down and normal force pushing up. Both of these
forces are the same and therefore cancel out. There
are no forces pushing it along the road. Since there
is no net force, and since the forces are balanced,
the car does not move and does not accelerate.
 The
second FBD shows an engine force (thrust) that
moves the car. There is also frictional force pushing
in the opposite direction. Since the car is
accelerating, the engine force is bigger than the
frictional force. The forces are therefore
unbalanced and there is a net force, causing the
car to accelerate.
+
1. The bigger car and heavier metal increase
the mass of the car. Since newton’s second law
states the F = ma, if the mass of the car
increases, and the force stays the same, the
acceleration has to decrease.
2. The more powerful engine will increase the
force that moves the car. Since Newton’s Second
Law states that F = ma, if the force on the car
increases, and the mass stays the same, the
acceleration has to increase.
+
Calculations with Newton’s Second Law
Force = Mass x Acceleration
Step 1: Identify the mass
Step 2: Identify the acceleration
Step 3: Plug it into the equation!
+
Units
Mass = kilograms (kg) or grams (g)
m
Velocity (Speed) =
s
m/s
Acceleration =
s
Force = Newtons (N)
m
= 2
s
+
Practice #1
 An
object has a mass of 10 kg and accelerates at 5
m/s2. What is the applied Force?
+
Practice #2
 Mike’s
car, which weighs 1000kg, is out of gas. Mike is
trying to push the car to a gas station, and he can
make the car go 0.05 m/s/s (m/s2). What is the force
Mike is applying to his car?
+
Draw a cartoon strip
 In
your notebook, draw a cartoon strip of a soccer ball being
kicked.
 Include
the following scenes:
 Ball at rest
 The moment it is kicked
 As the ball flies through the air.
 The ball rolling on the ground slowing down.
 The ball at rest again.
+
Exit Slip
A book placed on the
table and is pushed
towards the left with
acceleration. Draw a free
body diagram. Do not
take air resistance into
account.