Newton’s Laws Notes:
Index:
Lesson Number / Topic / Page (s) / Check when complete.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 / Equilibrium and Newton’s First Law: / 10
7 / Newton’s First Law (The law of inertia): / 11
8 / Newton’s 2nd Law / 12
8 / Newton’s 2nd Law and Mathematics / 13
10 / Newton’s 3rd Law / 14
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. / b) A 200N Astronaut is falling towards the moon. (On Earth His weight is 1200N).Picture:
/ Force Diagram:
/ Picture:
/ Force Diagram:
Math Statement: / Math Statement:
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 racket. The weight of the tennis ball is 2 N / b) A bowling ball is falling towards the ground. The weight of the bowling ball is 60 N.
Picture:
/ Force Diagram:
/ Picture:
/ Force Diagram:
Math Statement: / 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”
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 magnetPicture:
/ Force Diagram:
/ Picture:
/ Force Diagram:
Math Statement: / 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:
/ Force Diagram:
Math Statement:
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”
Net Force: Two forces not in Equilibrium
Force Diagram / Net Force / Rules/ 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
/ 0 N / When forces cancel out the net force is 0.
(20N) + (-20N) = 0
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. / 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:
/ Force Diagram:
/ Picture: / Force Diagram for rope:
Math Statement: / Math Statement:
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?
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.
A Player being tackled. / Jennifer kicking a soccer ball.Picture:
/ Force Diagram:
/ Picture:
/ Force Diagram:
Math Statement: / 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.
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 move? / / b. Make an object speed up? /c. Make an object slow down? / / d. Turn an object? /
In both cases below there are three forces. Identify the three forces and draw the force diagram.
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. / 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.Picture:
/ Force Diagram:
/ Picture:
/ Force Diagram:
Math Statement: / Math Statement:
Examples: Calculate the net force and determine the direction. Is the object in equilibrium?
F net = /
F net =
F net = /
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 the floor./ A book sits a rest on a table.
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. / 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.
/ 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 with a force of 3N./ A 10N net force is exerted on 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 30N on a 2000kg car./ Two boys exert a net force of 30N 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) :
Newton’s 2nd Law and Mathematics
Net force / Mass / Acceleration / Rule:
2N / 2kg / 1 N/kg
4N / 2kg / 2 N/kg
6N / 2kg / 3 N/kg
12N / 1kg / 12 N/kg
12N / 2kg / 6 N/kg
12N / 3kg / 4 N/kg
Equation:
= .
Problem Solving:
1./ Fnet =
a =
2.
/ Fnet =
a =
3. / Fnet =
a =
Rank the above from greatest to least acceleration.
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.