Download Force Diagram

Newton’s Laws Notes:
Index:
Lesson
Number
Topic
Page
(s)
1
Intro to Forces and Force Diagrams
- one force
2
1
Concept Development: What causes an object to change its motion?
3
2
Equilibrium and two forces.
4
2
Concept Development: What causes an object to stand still?
5
4
Calculating Net Force with Two forces
6
4
Concept Development: What is the net push on an object when
the forces do not cancel out?
7
5
Three Forces
8
5
Concept Development: What happens when there are more
than 2 forces?
9
7
7
8
8
10
Equilibrium and Newton’s First Law:
Newton’s First Law (The law of inertia):
Newton’s 2nd Law
Newton’s 2nd Law and Mathematics
Newton’s 3rd Law
10
11
12
13
14
1 Newton’s Laws Notes | Eibling 2014
Check
when
complete.
Intro to Forces and Force Diagrams
Observations: What is needed to make an object move or change its motion?
 At least 2 objects.
 One object must push or pull on the other.
Definition of Force: An interaction between two objects in which one pushes or pulls
on the other object.
Units: Newtons (N),
Pounds (lb):
Naming Forces:
F
object pushing
on
object being pushed
Examples:
Object of interest: Falling bomb
Object of interest: Falling
astronaut
Object of interest: Falling safe
Gravitational Force: The pull of the earth, moon, sun, etc. on an object.
Force Diagrams/ Free Body Diagrams (FBD)
How can forces be represented pictorially?
Using arrows to represent force:
1. A longer arrow represents a greater push.
2. A shorter arrow represents a
smaller
push.
3. The direction of the arrow represents
direction of the force
4. The arrow name tells you who is pushing what.
a) A toy safe is dropped. The weight of the safe
is 2 N.
Picture:
Force Diagram:
b) A 200N Astronaut is falling towards the
moon. (On Earth His weight is 1200N).
Picture:
Force Diagram:
Math Statement:
Math Statement:
2 Newton’s Laws Notes | Eibling 2014
Concept Development: What causes an object to change its motion?
Introduction to Newton’s First and Second Law
Watch the short clip from the movie Gravity.
1. Can the astronaut stop herself from spinning?
2. Can the astronaut make herself move towards
the space station?
3. Why do some objects float in space?
4. Does a tennis ball float on the Earth when it
is dropped?
5. What direction is the pull exerted on the ball?
7. Compare the tennis ball and the bowling ball.
a) A tennis ball is dropped before it is hit by
b) A bowling ball is falling towards the ground.
racket. The weight of the tennis ball is 2 N
The weight of the bowling ball is 60 N.
Picture:
Force Diagram:
Math Statement:
Picture:
Force Diagram:
Math Statement:
c) Which is pulled harder the bowling ball or a tennis ball? How does this affect the length of your arrow?
* Questions on this page modified from file “nl1-Newton’s First and Second Law Introduction-v2_JMJG15”
3 Newton’s Laws Notes | Eibling 2014
Two Forces and Equilibrium
Support Force: is an upward push that keeps an object for falling. For example, a
table exerts an upward force on a book to keep it from falling to the ground.
Equilibrium:
An object is in equilibrium when the forces cancel out. This means that the sum of the
forces is zero (The net force equals zero.)
The examples below are in equilibrium because the force up equals the force down.
Examples: The object of interest is bolded.
A 0.5 N paperweight on a table.
0.2 N Paper clips held in the air with a magnet
Picture:
Force Diagram:
Math Statement:
Picture:
Force Diagram:
Math Statement:
Equilibrium in Motion: An object can be in equilibrium and move, but it will only
move in a straight line at a constant speed.
A 40 N bowling ball rolls at a constant speed.
Picture:
Math Statement:
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Force Diagram:
Concept Development: What causes an object to stand still?
1. Observe and Represent: A girl holds a tennis ball in her hands. The tennis ball
does not fall to the ground. The weight of the tennis ball is 0.5 N.
a. List the objects pushing and pulling on the tennis ball:
Picture:
Force Diagram:
Math Statement:
2. Observe and Represent: Sheldon holds a bowling ball in his hands. The bowling
ball does not fall to the ground. The weight of the bowling ball is 60 N.
a. List the objects pushing and pulling on the bowling ball:
Picture:
Force Diagram:
Math Statement:
3. In each case, the Earth pulled down on the object. What prevented the object
from falling?
Examples from “nl3-gP1.3.1L2-multipleForces_JG15”
5 Newton’s Laws Notes | Eibling 2014
Net Force: Two forces not in Equilibrium
Force Diagram
Net Force
60 N
right
When forces are in the same direction
add.
(20N) + (40N) = 60N
-10 N
left
When forces are in the opposite direction
subtract.
(20N) + ( - 30N) = -10N
When forces cancel out the net force is 0.
(20N) + (-20N) = 0
0N
Net Force (F net) =
Static Equilibrium:
Rules
Sum of the Forces
When an object is not moving the forces in all directions cancel out.
In
equilibrium
the F net =
A ball hits the ground and bounces. While it is
hitting the ground the floor exerts a force of
5N on the ball. The weight of the ball is 1N.
Picture:
Force Diagram:
Math Statement:
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0
A cat and dog are in a tug of war. The dog pulls to the
left with a force of 300N. The cat pulls to the right
with a force of 200N. We can ignore the force of the
earth on the rope since the mass of the rope is so small.
Picture:
Math Statement:
Force Diagram for rope:
Net Force (Unbalanced Force):
Concept Development: How can we tell how hard something is pushed when
two objects are pushing on it?
The Boyfriend Example:
When Ms. Eibling was in high school she went out driving with her boyfriend after buying
her first car for $500. The car stalled out and she pulled off the road into a ditch.
1) Ms. Eibling and her boyfriend got out of the car and tried to push the car out of
the ditch. What is the total force that they are exerting on the car? What is the
direction of the force? What did you do to get this number?
2) Ms. Eibling’ boyfriend makes an unkind comment about her hair. She gets mad
and goes around to the front of the car and pushes the car backwards. What is the
total force that they are exerting on the car? What is the direction of the force?
What did you do to get this number?
3) Ms. Eiblings boyfried is laughing so hard that he twists his ankle and can only
push with one foot. What is the total force that they are exerting on the car? What
did you do to get this number?
RULE
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Three Forces and More:
Steps to draw a FBD:
1. Circle the object of interest
2. List all object s that are pushing or pulling on the object of interest
3. Draw the object of interest as a box or a dot
4. Draw an arrow with an appropriate length representing each force exerted on the object of interest.
5. Name each arrow.
Jennifer kicking a soccer ball.
A Player being tackled.
Picture:
Force Diagram:
Math Statement:
Picture:
Force Diagram:
Math Statement:
Math Statement for forces in 3 or more directions: When there are forces exerted up / down and
left/ right, you need two math statements, one for the up and down directions and the other for the
right and left directions.
F
When the total force exerted on an object is not zero, the motion of an object changes.
The sum of all of the forces changes the motion of an object. What must one do to
a. Make an object at rest
b. Make an object speed
move?
up?
c. Make an object slow
down?
8 Newton’s Laws Notes | Eibling 2014
d. Turn an object?
In both cases below there are three forces. Identify the three forces and draw the force
A bowling ball is sitting on the ground. Mr.
Goett pushes it with a force of 30 N. The
object of interest is the bowling ball. The
weight of the bowling ball is 60 N.
Picture:
Mother cat pushed a cart with a baby in it with a force
of 10N. The object of interest is the cart. The kitten
and the cart weigh 5N.
Force Diagram:
Math Statement:
Picture:
Force Diagram:
Math Statement:
diagram.
Examples: Calculate the net force and determine the direction. Is the object in equilibrium?
- 4N
+5N
+6N
- 10N
+7N
+ 10N
- 2N
- 5N
F net =
F net =
- 10N
+5N
+ 10N
- 4N
- 8N
F net =
9 Newton’s Laws Notes | Eibling 2014
+7N
+4N
- 7N
F net =
Equilibrium and Newton’s First Law:
Draw the force diagrams for the following situations. Circle the objects that have a zero net force. Is each
object in equilibrium? How do you know?
A bowling ball is moving at a constant speed across
A book sits a rest on a table.
the floor.
A penny is dropped of the Empire State Building.
A child is jumping on a trampoline and is
accelerating upward.
A water skier is pulled at a constant speed and is
turned by a boat.
Ms. Eibling skates into a wall.
Equilibrium: An object is in equilibrium when:
1.
(Static Equilibrium)
2.
(Dynamic
Equilibrium)
Equilibrium Examples:
Objects at rest:
Objects do not move at all.
10 Newton’s Laws Notes | Eibling 2014
Objects in Motion:
Objects move with constant speed.
Newton’s First Law (The law of inertia):
Examples of Newton’s First Law:
Objects at rest:
Objects in Motion:
Driver flying forward when he hits the breaks without a seat belt.
Inertia:


Rank: Rank the objects from greatest to least inertia.
A) A 40,000 kg rock at rest.
B) A 10,000 kg truck is moving at
80mph.
C) A 100 kg baseball player
flies through the air at a speed
of 10m/s.
D) A 0.01 kg penny falls with a
speed of 60mph.
E) A 0.5 kg stapler sitting in a
table.
11 Newton’s Laws Notes | Eibling 2014
F) A 2 kg hockey puck slides
across the ice at a speed of
3m/s.
Newton’s 2nd Law
Find a pattern: Find a relationship between:
 Force and acceleration
 Mass and acceleration
Experiment:
Pattern:
1. A bowling ball is
pushed lightly.
2. A bowling ball is
pushed harder.
1.
is pushed
in a rolling chair.
2.
is holding
an arm full of books
and is pushed just as
hard as before.
Rank the acceleration from greatest (1) to least (3).
A 2kg ball is pushed
A 10N net force is exerted on
with a force of 3N.
a 2kg backpack to slide it.
A 2kg cake is pushed to the
side with a net force of 1N.
Rank: the acceleration from greatest (1) to least (3).
A man exerts a force of
Two boys exert a net force of 30N
30N on a 2000kg car.
on a 120kg soldier on a cart.
A person exerts a force
30N on a 300kg cart.
Falling Objects: Why do a hammer and a feather fall with the same acceleration on the moon?

(Inertia / mass) :
.
.
.
(net Force) :

.
.

(both mass and net force) :
12 Newton’s Laws Notes | Eibling 2014
.
Newton’s 2nd Law and Mathematics
Find a pattern: A 2 kg toy car is pushed with a force of
2N, 4N and 6N while the acceleration is measured. The
three cars each with a different mass are pushed with a
force of 12N and the acceleration is measured. How can
one calculate the acceleration from the net force and
mass?
Net force
2N
4N
6N
12N
12N
12N
Mass
2kg
2kg
2kg
1kg
2kg
3kg
Acceleration
1 N/kg
2 N/kg
3 N/kg
12 N/kg
6 N/kg
4 N/kg
Rule:
Equation:
=
.
Problem Solving:
1.
+X
direction
F1 = 40 N
Fnet =
5 kg
a=
F2 = 30 N
2.
F1 = 40 N
2 kg
F2 = 20 N
3.
F2 = 25 N
F4 = 5 N
Fnet =
a=
4 kg
F1 = 20 N
Fnet =
F3 = 20 N
a=
Rank the above from greatest to least acceleration.
13 Newton’s Laws Notes | Eibling 2014
Newton’s 3rd Law
Observe and explain: Balloon Rockets
1) What happens when there is less air in the balloon?
2)
What happens when there is more air in the balloon?
3) What pushes on the balloon to make it move forward?
4)
Where does the air go? What pushes on it?
Newton’s 3rd Law:
Equation:
F a on b = - F b on a
Examples: Circle all of the 3rd law interactions in each picture. Write the 3rd Law equations.
1) A Skate boarder stands on a
skateboard.
2) A girl kicks a soccer ball.
3) A book on a table
4) Child leaning on a wall.
5) A fire extinguisher pushes a
bike forwards.
6) Balloon full of air.
Challenge: Circle all third law interactions. Explain why one team can win the tug of war when the
third law forces are equal and opposite. Hint: Draw a force diagram for the cat and one for the dog.
14 Newton’s Laws Notes | Eibling 2014