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.

• As the unit of force, the newton is equal to a kilogram-meter per second squared.

Don’t hate.

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

• Isaac Newton:– Arguably the most important

scientist in human history.– Wealthy Nobleman.

• 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.

- Inventor of the spork.- Criminal Mastermind.- Brew-master of fine ales.-

Philanthropist.- International ladies man.

Newton’s Laws of Motion

Quick Quiz:•True or False:

– If an object is at rest, it will stay at rest until something causes it to move.

– 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

TRUE

•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.

Inertia and Newton’s First Law

This is an object.

It is at rest.

The blue arrow represent forces, does the motion of the object change?

•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.

Inertia and Newton’s First Law

This is an object.

It is at rest.

The blue arrows represent forces, does the motion of the object change?

•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.

Inertia and Newton’s First Law

This is an object.

It is at rest.

The blue arrows represent forces, does the motion of the object change?

•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.

Inertia and Newton’s First Law

This is an object.

It is at rest.

The blue arrows represent forces, does the motion of the object change?

•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.

Inertia and Newton’s First Law

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?

•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.

Inertia and Newton’s First Law

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?

•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.

Inertia and Newton’s First Law

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?

•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.

Inertia and Newton’s First Law

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?

•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.

Inertia and Newton’s First Law

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?

•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.

Newton’s Second Law

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?

•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

Newton’s Second Law

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?

There is one, 10.0N force and one, 15.0N force acting on it, what is the acceleration of the object?

•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

Newton’s Second Law

This is an object.

It has 10.0kg of mass.

Newton’s Second Law

Find the net Force (Fnet).

15.0N

Fnet is 5.0N

- 10.0N

= 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?

There is one, 10.0N force and one, 25.0N force acting on it, what is the acceleration of the object?

•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

Newton’s Second Law

This is an object.

It has 10.0kg of mass.

Newton’s Second Law

Draw the Force vectors tip to tail

Insert values.

Draw the resultant vector.Use the Pythagorean

Theorem.

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

Insert values.

Draw the resultant vector.Use inverse tangent

21.8° left from vertical

Bell-Ringer 2/2

1.Is the giant, anniversary, sock-monkey 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?

Answer the questions regarding the giant, anniversary, sock-monkey on the front bench.

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.

• 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 free-body 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/ind

ex.jsp?uREC_ID=174394&type=u&termREC_ID=&pREC_ID=links

Free-Body Diagrams

•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.

Newton’s Third Law

This is an object.

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.

•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.

Newton’s Third Law

This is an object.

The white object will exert an equal and opposite force on the orange object.

The orange object exerts a 9.0N force on the white one.

This object is on top.

•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.

Newton’s Third Law

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.