CP Physics Chapter 3,4,5 Notes

 

I.                   Demos

a.       Eggs

b.      Hammer

c.       Weights and string

d.      Tighten hammer head

II.            Newton’s First Law of Motion

a.       Old idea that constant motion needs a force

                                                              i.      if an object started moving one would reasonably look for cause for its motion

                                                            ii.      Aristotle looked at motion as either natural or violent

1.      ex. natural = rocks fall down, no one dos anything

2.      ex. violent = you throw a rock, you had to do something

                                                          iii.      Galileo looked at motion through the idea of INERTIA

1.      Once anobject is set in motion it continues todo so without any added effort

b.      INERTIA = the resistance to a change in motion

c.       Newton’s 1st Law = An object in motion stays in motion and an object at rest stays at rest unless acted on by an outside net force

III.             Mass, Weight, and Volume

 

mass

volume

weight

Unit

g or kg

m3 or L

Newtons

Definition

amt of inertia

space

Force of gravity

Changes?

no

only for liquids and gases

yes, depends on location

IV.             Check questions

a.       Why are we shorter at night?(Try finding an object just out of reach at night, then reach for it in the morning)

b.      What would be the motion of a ball tossed in empty space and away from gravity?

c.       Demo – Stand against wall and jump!

V.             Newton’s 2nd Law of Motion

a.       Review –

                                                              i.      If something is accelerating at 10 m/s2 that means it accelerates by increasing its velocity by 10 m/s every second

                                                            ii.      Acceleration means the velocity is changing. All objects travel at the same velocity unless they are acted on by an OUTSIDE NET FORCE!

b.      Two student demo – drop book and pen, which hits the ground first?

                                                              i.      That means that a is DIRECTLY proportional to F, F^, a^

1.      F ~ a

                                                            ii.      That also means that a is INVERSELY proportional to inertia or mass, m^, a v

1.      1/m ~ a

                                                          iii.      This leads us to the equation

1.      F/m ~a, but Newton actually proved that was it, nothing else so F/m =a…

2.      Normally written

 


a.     F = ma

 

b.      IMPORTANT --- F is really NET force

 

c.       units à mass = kg, a = m/s2, F = kg m/s2 = Newton = N (add to units table!!!)

 

c.       Net Force

                                                              i.      Demo – Push on the table. Now push harder. What happens?

                                                            ii.      What is the net force on the table? How do you know?

                                                          iii.      What if you pushed on the table on one side with 10 N and another student pushed on the other side with 6 N. What would be the net force? How do you know?

                                                          iv.      If you don’t push on the table, what forces act on the table?

                                                            v.      Is the table moving? How many forces minimum does it need not to move as long as there is gravity?

                                                          vi.      NORMAL force = the force perpendicular to supporting surface, doesn’t ALWAYS equal gravity, but many times it does (ex. box on incline will slide down because gravity is pulling it down)

1.      Normal means perpendicular (math term)

2.      Gravity is ALWAYS straight down but Normal is NOT always straight up (ex. incline)

3.      Scale reading = your NORMAL force

4.      Questions:

a.       If you are weighing yourself on a scale, how does the scale reading compare to your actual weight due to gravity when

                                                                                                                                      i.      you are on a flat surface

                                                                                                                                    ii.      you are on an incline

                                                                                                                                  iii.      you are on an elevator accelerating up

                                                                                                                                  iv.      you are on an elevator accelerating down

                                                                                                                                    v.      you are on an elevator moving up that then comes to a stop

                                                                                                                                  vi.      you are on an elevator moving down and then comes to a stop

                                                                                                                                vii.      your are on an elevator moving at a constant velocity

d.      Tension

                                                              i.      Demo – Spring balance and weight with one or two strings. What do they read?

                                                            ii.      When an object is suspended by rope or strings, the strings must make the net force on the object zero.

                                                          iii.      Check questions

1.      If a 400 N board is suspended by 1 rope, what is the tension in each rope?

2.      2 ropes?

3.      4 ropes?

e.       Accelerated vs nonaccelerated motion

                                                              i.      Demo – pull a block across the table at a constant velocity with a spring balance

1.      What force is being applied to the block?

2.      What is the acceleration of the block?

3.      What is the net force on the block?

4.      What other forces are acting on the block?

5.      What must be the frictional force on the block?

6.      Free body diagram of block:

 

 

 

 

 

 

                                                            ii.      Check questions:

1.      A plane is flying at a constant speed of 900 km/hr and the thrust of the engines is a constant 80,000 N. What is the acceleration of the airplane?

2.      What is the combined force of air resistance that acts all over the plane’s outside surface?

3.      picture:

f.       Frictional force

                                                              i.      Friction is represented with Ff or f

                                                            ii.      Friction ALWAYS opposes motion

                                                          iii.      Friction is ALWAYS parallel to the surface (so it is always perpendicular to the Normal force)

                                                          iv.      f = mN

1.      Ff = frictional force in N

2.      m = coefficient of friction (no units)

3.      N = normal force

                                                            v.      Looking at this equation, what would a graph of friction vs normal force look like? Why?

g.      Force of gravity

                                                              i.      Weight is known as force of gravity

                                                            ii.      We already know that F = ma

                                                          iii.      so…. Fg = mag or as your book likes to do Fg = mg, where g = 10 m/s2 or whatever acceleration is on any given planet.

                                                          iv.      Check questions:

1.      If I weigh 850 N on Earth, what is my mass?

2.      If my mass is 70 kg, what is my weight on Earth?

3.      If my mass is 60 kg on Earth, what is my mass on the moon where gravity is 1/6 of g? What is my weight on the moon?

h.      Terminal velocity

                                                              i.      What happens if you stick your hand out the window while your car moves down a neighborhood street? How is this different from moving down the freeway? Why?

                                                            ii.      Air resistance depends on:

1.      How big the object is

2.      How fast you are going

                                                          iii.      Imagine a person falling from a building (assume they forgot their parchute). What forces are acting on them?

1.      picture:

 


                                                                               

 

 

 


2.      What happens to their speed as time goes by?

3.      What happens to their air resistance?

4.      What happens when their air resistance = their weight?

                                                          iv.      Terminal velocity =

1.      no acceleration

2.      constant velocity

3.      air resistance = weight (force of gravity)

                                                            v.      Cat’s reach terminal velocity very quickly, that’s why they can jump down from high places without being hurt

                                                          vi.      What position will give you the highest terminal velocity? lowest?

i.        Sample Problems

                                                              i.      What is the acceleration for a 10-N book if it is pushed with a 5 N force, assuming no friction.

1.      G: Fg = 10 N, Fa = 5 N

2.      F: a =?

3.      R: Fnet = ma, Fg = mg

4.      S: a = Fnet/m, m = Fg/g

5.      C: m = 10N/10m/s2 = 1kg

     a = 5N/1kg =

         5 m/s2

 

                                                            ii.      What is the mass of a boy and his wagon if he is accelerated at 0.2 m/s2 by a force of 15 N if the force of friction is 5N?

1.      Text Box: 15 NText Box: 5 NG:

 

Fnet = 10 N, a = 0.2 m/s2

2.      F: m = ?

3.      R: Fnet = ma

4.      S: a = Fnet/m

5.      C: m = 10N/0.2m/s2 = 50kg

VI.        Newton’s 3rd Law

a.       Demos

                                                              i.      Bending fingers back in air and with table

                                                            ii.      Push against wall – Everyone knows I am pushing on the wall but only physics types realize that the wall is simultaneously pushing on me also

                                                          iii.      Walk on the floor – what is pushing me forward?

                                                          iv.      Release a filled balloon – why does it move? how does it move?

b.      Newton’s 3rd Law – For every action, there is an equal and opposite reaction

c.       Action/Reaction pairs

                                                              i.      Just because two forces are equal an opposite, doesn’t make them an action reaction pair  

                                                            ii.      Rules for action/reaction pairs

1.      must occur in pairs

2.      may never cancel

3.      only involve TWO objects

                                                          iii.      What is the reaction/action pair for a person firing a rifle?

1.      The rifle pushes the bullet (it flies off)

2.      The bullet pushes the rifle (kicks)

                                                          iv.      What is the reaction/action pair for kicking a football?

1.      The foot kicks the ball (it flies off)

2.      The ball kicks the foot (Why doesn’t the kicker fly backwards?)

3.      How do these two forces compare?

4.      If they are equal and opposite, why isn’t the net force on the ball = 0 (it can’t be since it does fly off!)

                                                            v.      A refrigerator is being pushed at a CONSTANT velocity with an applied force of 100 N

1.      What is the force of friction?

2.      What is the net force on the refrigerator?

3.      Are the applied force and the friction force an action/reaction pair?

                                                          vi.      What causes your weight? What is the reaction force to this action force?

                                                        vii.      What causes the Earth to stay in motion? What is the reaction force to this action force? What is the result of both forces?

d.      Demo – Tug of war

                                                              i.      Text Box: ?Drawing:

 

 


                                                            ii.      If each group is pulling with 1000N what does the spring balance read? Why?

                                                          iii.      What is the reaction force to a team member pulling on the rope?

                                                          iv.      What makes the rope move at all?

                                                            v.      How do you win?