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Thursday, 26 October 2006

Now Playing: 5.5 to 5.7 Notes
Topic: Chap 5

CP Lecture Notes Chap 5 continued:

 

5.5 Applying Force—Pressure

 

Pressure is the amount for force per unit of area.

 Pressure =       force                               .

                   Area of application

 

In equation form:

 

P = F

       A

 

 

 

Which exerts more pressure on the shelf, the blue physics book or the pink one (see picture below left)?

 

If each book weighs 5 N and the books measure 20 cm wide by 20 cm long by 5 cm thick, what is the pressure that each book exerts on the shelf?

        

5.6    Free Fall Explained

 

Why is the acceleration due to gravity (g) the same for all objects when you ignore air resistance?

 

It has to do with the force-to-mass ratio:

  

(see middle picture below)

  

Question:  If you were on the moon and dropped a hammer and a feather from the same elevation at the same time, would they strike the surface of the moon at the same instant?

 

5.7    Falling and Air Resistance

 

Terminal speed or terminal velocity is reached when the downward gravitational force on the object is exactly balanced by the upward force of air resistance.

 

 

 

In the below right,

a golf ball and

a Styrofoam ball (the larger one)

are dropped at the same time.

 

The lighter Styrofoam ball

experiences a greater resistance force

and falls at a slower rate.

         

Question:  If a heavy person and a light person open their parachutes together at the same altitude and each wears the same size parachute, who will reach the ground first?

 

If the force of air resistance is the same for a falling baseball and a falling tennis ball, which will have the greater acceleration?  Hint:  the baseball is heavier.


Posted by physicscp at 5:49 PM EDT
Updated: Thursday, 26 October 2006 5:53 PM EDT

Now Playing: Forces Worksheet
Topic: Chap 5

This is from the pink workbook.  If you do the exercise in this order, then I think you will understand:

 

 



 


Posted by physicscp at 7:14 AM EDT
Sunday, 22 October 2006

Now Playing: 5.1 Force Causes Accleration Notes
Topic: Chap 5

Notes on Chapter 5 of Conceptual Physics

 

Recall,  the definition of acceleration from Chapter 2

 

                        acceleration = change in velocity

                                                   time interval

 

or,                                     a =  Δv

                                                Δt

 

 

5.1    Force Causes Acceleration

 

An acceleration is caused by a FORCE applied. 

 

More specifically, by a NET FORCE

 

So,              acceleration ~ net force

 

Or,                               a ~ Fnet

 

This means that the more force applied, the greater the acceleration.

 

There is a direct relationship between Force and acceleration.


Posted by physicscp at 9:25 AM EDT

Now Playing: 5.2 Mass Resists Acceleration
Topic: Chap 5

5.2…Mass Resists Acceleration

 

Given equal Force,

 

Acceleration will be less, the more mass piled on

 

Acceleration will be more, the less mass

 

So,                acceleration ~      1      .

                                             mass

 

 or,                                 a ~     1    .

                                                m

 

There is an inverse relationship between mass and acceleration.


Posted by physicscp at 9:24 AM EDT

Now Playing: 5.3 Notes on Newton's Second Law
Topic: Chap 5

5.3  Newton’s Second Law

 

Newton put these two relationships together in Newton’s Second Law of Motion:

 

The acceleration produced by a net force on an object

q       Is directly proportional to the magnitude of the net force

q       Is in the same direction as the net force

q       Is inversely proportional to the mass of the object

 

In equation form:

 

                                    acceleration = net force

                                                               mass

 

Or,                                                a = Fnet

                                                             m

 

UNITS!

 

We have said before that the SI unit for Force is the Newton.

 

From the Equation above, substituting in units, we see that

 

                                                 m/s2 =    N  .

                                                                kg

 

That means, that the Newton is a derived unit which is equal to a

 

                                                kg m/s2

 

 

From p 63:  Questions

 

  1. If a car can accelerate at 2 m/s2 , what acceleration can it attain if it is towing another car of equal mass?
  2. What kind of motion does a constant force produce on an object of fixed mass?

Posted by physicscp at 9:21 AM EDT

Now Playing: 5.4 Friction Notes
Topic: Chap 5

5.4     Friction

 

Friction is a force that acts on materials that are in contact with one another.

 

What does friction depend on?

 

q       The type of materials pressed together

q       The amount of force pressing the materials together

 

Fluid Friction

Fluids are liquids (like water) or gases (like air), because they flow.

 

Fluid friction occurs as an object pushes aside the fluid it is moving through.

 

If an object moves through a fluid at constant velocity,

q       then the friction force of the fluid is exactly balanced by the applied force

q       the Net Force is zero

q       there is no acceleration

 

 

 

 

 

 

Page 64 Questions:

 

  1. Two forces act on a book resting on a table:  its weight and the support force from the table.  Does a force of friction act as well?
  2. Suppose a high flying jet cruises with a constant velocity when the thrust from its engines is a constant 80,000 N. 

 

q       What is the acceleration of the jet? 

q       What is the force of air resistance acting on the jet?


 


Posted by physicscp at 9:18 AM EDT

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