Download Forces - Lincoln Park High School

Forces
Insert your final Star Wars joke here
General Definition
• A force is any influence that causes an object to
undergo a change in speed, a change in
direction, or a change in shape.
• The units of force are newtons, named after
Isaac Newton. Don’t hate.
• As the unit of force, the newton is equal to a
kilogram-meter per second squared.
Types of Forces
• Contact Force
– any force applied on an object through direct
contact with that object.
• Field Force
– Any force applied on an object through a
“field”, acting at a distance without direct
contact.
» Gravity
» Magnetism
Newton’s Laws of Motion
• Isaac Newton:
– Arguably the most important scientist in human
history.
– Wealthy Nobleman.
- International
Inventor
Criminal
Brew-master
Philanthropist.
Mastermind.
of the
of
ladies
spork.
fineman.
ales.
• Came up with three basic laws that governed
motion.
–
–
–
First law: The velocity of a body remains constant unless the body is acted
upon by an unbalanced external force.
Second law: The acceleration a of a body is parallel and directly proportional
to the net force F and inversely proportional to the mass m, i.e., F = ma.
Third law: The mutual forces of action and reaction between two bodies are
equal, opposite and collinear.
Quick Quiz:
• True or False:
– If an object is at rest, it will stay at rest until
something causes it to move. TRUE
– If an object is moving, it will continue to
move in the same way until sometime causes a
change in the motion of that object. TRUE
Inertia and Newton’s First Law
•First law:
–The velocity of a body remains constant unless the
body is acted upon by an unbalanced external force.
–This resistance to a change in motion is called inertia.
This is an object.
It is at rest.
The blue arrow represent forces, does the motion
of the object change?
Inertia and Newton’s First Law
•First law:
–The velocity of a body remains constant unless the
body is acted upon by an unbalanced external force.
–This resistance to a change in motion is called inertia.
This is an object.
It is at rest.
The blue arrows represent forces, does the motion
of the object change?
Inertia and Newton’s First Law
•First law:
–The velocity of a body remains constant unless the
body is acted upon by an unbalanced external force.
–This resistance to a change in motion is called inertia.
This is an object.
It is at rest.
The blue arrows represent forces, does the motion
of the object change?
Inertia and Newton’s First Law
•First law:
–The velocity of a body remains constant unless the
body is acted upon by an unbalanced external force.
–This resistance to a change in motion is called inertia.
This is an object.
It is at rest.
The blue arrows represent forces, does the motion
of the object change?
Inertia and Newton’s First Law
•First law:
–The velocity of a body remains constant unless the
body is acted upon by an unbalanced external force.
–This resistance to a change in motion is called inertia.
This is an object.
It is in motion.
The green arrow indicates the motion and blue
arrows represent forces, does the motion of the
object change?
Inertia and Newton’s First Law
•First law:
–The velocity of a body remains constant unless the
body is acted upon by an unbalanced external force.
–This resistance to a change in motion is called inertia.
This is an object.
It is in motion.
The green arrow indicates the motion and blue
arrows represent forces, does the motion of the
object change?
Inertia and Newton’s First Law
•First law:
–The velocity of a body remains constant unless the
body is acted upon by an unbalanced external force.
–This resistance to a change in motion is called inertia.
This is an object.
It is in motion.
The green arrow indicates the motion and blue
arrows represent forces, does the motion of the
object change?
Inertia and Newton’s First Law
•First law:
–The velocity of a body remains constant unless the
body is acted upon by an unbalanced external force.
–This resistance to a change in motion is called inertia.
This is an object.
It is in motion.
The green arrow indicates the motion and blue
arrows represent forces, does the motion of the
object change?
Inertia and Newton’s First Law
•First law:
–The velocity of a body remains constant unless the
body is acted upon by an unbalanced external force.
–This resistance to a change in motion is called inertia.
This is an object.
It is in motion.
The green arrow indicates the motion and blue
arrows represent forces, does the motion of the
object change?
Newton’s Second Law
•Second law:
– The acceleration a of a body is parallel and directly
proportional to the net force F and inversely
proportional to the mass m, (i.e.: F = ma).
– It is very important to conceptualize Net force as
the sum of all forces applied on an object.
This is an object.
It has 1.00kg of mass.
Since the sum of no forces is zero, there is
no acceleration.
Newton’s Second Law
Divide both sides by mass.
Insert values.
In which direction is the acceleration?
Newton’s Second Law
•Second law:
– The acceleration a of a body is parallel and directly
proportional to the net force F and inversely
proportional to the mass m, (i.e.: F = ma).
– It is very important to conceptualize Net force as
the sum of all forces acting on an object
This is an object.
It has 2.00kg of mass.
There is one, 10.0N force acting on it, what
is the acceleration of the object?
Newton’s Second Law
Divide both sides by mass.
Insert values.
In which direction is the acceleration?
Newton’s Second Law
•Second law:
– The acceleration a of a body is parallel and directly
proportional to the net force F and inversely
proportional to the mass m, (i.e.: F = ma).
– It is very important to conceptualize Net force as
the sum of all forces acting on an object
This is an object.
It has 10.0kg of mass.
There is one, 10.0N force and one, 15.0N force
acting on it, what is the acceleration of the object?
Newton’s Second Law
Find the net Force (Fnet).
15.0N - 10.0N = 5.0N
Fnet is 5.0N
Use the net force to calculate the acceleration
experienced by the mass.
Newton’s Second Law
Divide both sides by mass.
Insert values.
In which direction is the acceleration?
Newton’s Second Law
•Second law:
– The acceleration a of a body is parallel and directly
proportional to the net force F and inversely
proportional to the mass m, (i.e.: F = ma).
– It is very important to conceptualize Net force as
the sum of all forces acting on an object
This is an object.
It has 10.0kg of mass.
There is one, 10.0N force and one, 25.0N force
acting on it, what is the acceleration of the object?
Newton’s Second Law
Draw the Force vectors tip to tail
Draw the resultant vector.
Use the Pythagorean Theorem.
Insert values.
Fnet = 26.9N
Use the net force to calculate the acceleration
experienced by the mass.
Newton’s Second Law
Divide both sides by mass.
Insert values.
In which direction is the acceleration?
Newton’s Second Law
Draw the Force vectors tip to tail
Draw the resultant vector.
Use inverse tangent
Insert values.
21.8° left from vertical
Bell-Ringer
2/
2
Answer the questions regarding the giant,
anniversary, sock-monkey on the front bench.
1. Is the giant, anniversary, sockmonkey at rest?
2. Is the net force zero or non-zero?
3. What are the external forces on the
giant, anniversary, sock-monkey ,
if any at all?
Normal Force
• The bunny is at rest because the force of gravity
is balanced by a force coming up from the front
bench.
• Any force applied to an object from a surface is
called a normal force.
• All normal forces are exerted on an object from
a surface with which the object is in contact.
• All normal forces are directed perpendicularly
from the contact surface.
Free-Body Diagrams
• When doing Physics problems involving forces, it is
advantageous, and sometimes absolutely necessary to
draw a free-body diagram.
• I was going to make a power-point on making a freebody diagram, but I found something better online.
• Everybody, get into groups and go back to the
computer tables.
Free-Body Diagrams
Go to the LPHS website:
– www.lincolnparkhs.org
– Navigate through these selections:
Academics>Teachers/Staff>Mr. Miklusak>Links
– Make sure to change the section to the “Free-Body
Diagrams” category.
– You can try this thing as well.
– http://www.lincolnparkhs.org/apps/pages/index.jsp
?uREC_ID=174394&type=u&termREC_ID=&pREC
_ID=links
Newton’s Third Law
•Third law:
– Third law: The mutual forces of action and
reaction between two bodies are equal, opposite and
collinear.
– This has been paraphrased into: Every action has an
equal and opposite reaction.
The orange object
exerts a 10.0N force on
the white object.
The white object will exert an equal and
opposite force on the orange object.
This is an object.
Newton’s Third Law
•Third law:
– Third law: The mutual forces of action and reaction
between two bodies are equal, opposite and collinear.
– This has been paraphrased into: Every action has an equal
and opposite reaction.
This object is on top.
This is an object.
The orange object exerts a 9.0N force on the white one.
The white object will exert an equal and opposite force
on the orange object.
Newton’s Third Law
•Third law:
– Third law: The mutual forces of action and reaction
between two bodies are equal, opposite and collinear.
– This has been paraphrased into: Every action has an equal
and opposite reaction.
These objects are in motion.
The orange object exerts a 100.0N force on the white
object.
The white object will exert an equal and opposite force
on the orange object.