Evans SPH 3U1

Physics Grade 11

Unit 2: Dynamics

Note 8: Newton's 1st & 2nd Laws, FBDs

 

Reference: Chapter 3.1, 3.2, 3.3

 

Demonstrations:

Egg Demo

$20 bill water bottle demo

Card on thumb with coin on top. Flick card out.

Brick and Hammer. Place hand under large brick. Hit the brick with the hammer.

 

Discuss:

Skateboard hits a stone while you are moving

Bus stops suddenly

Throw a ball straight up in the air when moving in a car (demo)

Throw a ball straight up in the air while accelerating/decelerating (demo)

 

Force:
Push or pull of a definite magnitude and direction.  Force is a vector quantity.  Derived SI unit is Newton, N (kgm/s2)
Four Fundamental Forces of Nature:: Gravitational, Electromagnetic, Strong Nuclear (keeps atoms together), Weak Nuclear 
Contact Forces:
        Normal force (opposed force of gravity Fg = mg, perpendicular to the surfaces of objects in contact) 
        Friction (opposes motion or 'intended' motion)
        Tension (pulling force by rope/string, cannot push)

Balanced force: forces that are equal in magnitude but opposite in direction, therefore the forces cancel out.
Net or Unbalanced force: The resultant force acting on an object is the unbalanced or net force.  It is the vector sum of all the forces acting on the object.
Newton’s First Law (Inertia): When no external unbalanced net force acts on an object, its velocity (or state of rest) remains constant.
                                                  When the net force is balanced the object will be remain at its current state (rest or constant velocity).
                                                  When the net force in unbalanced there will be changing velocity (i.e. acceleration)

                                                  Explain Newton's 1st Law as it applies to car headrests

Free Body Diagrams (FBDs): These diagrams are used to solve ALL force problems.
                                                   FBD Examples:      1) Book on table     2) Person pulling wagon     3) Hockey puck coming to rest                                                                                                      4) Duck, book, chair (old text)      5) Inclined Plane     6) Pulley

Newton’s Second Law: When an external unbalanced force acts on an object, it accelerates. Show metre stick demo.

                        we define the Newton so that k is 1

 

Steps to solve problems: Draw diagram and free body diagram. Label all forces.  Resolve into components (one component should be in direction of acceleration). Determine net forces. 

Example :
A 300N[E] force is exerted on a 50.kg object. A frictional force of 200N is opposing the motion. If the object starts from rest and moves for 5.0 seconds, find the: a) object's acceleration    b) velocity in 5.0 seconds    c) distance travelled 

More Problems on Newton's 2nd Law:
Example 1:
A car with a mass of 1000 kg is moving in a straight line at a constant speed of 30 m/s.  It is brought to rest in 25 s.  What constant force is acting to stop the car?
Example 2:
Inga is pulling a monument with a force of 100N {E30N}.  Anna is pulling the same monument with a force of 100N [E30S].  William is pulling the same monument with a force of 10N[W].  What is the net force on the monument?
Example 3:
A car can accelerate by 3 m/s2. What is its acceleration if it is towing another car like itself?
Example 4:
You have to push with a force of 200N to slide a refrigerator across the floor at a constant velocity. What is the force of friction acting on the refrigerator?
Example 5:
Forces of 100N[N] and 80N[W] act simultaneously on an object of mass 10 kg.  What is the acceleration of the object?

Forces, 1st Law, FBDs Questions:
Pg 120 #1bc,2,3
Pg 122 #7,10,13,16
Pg 129 #7,9,12,13,14

2nd Law Questions:
Pg 133 #2,5
Pg 135 #1,2
Pg 136 #5,8,9,11,12

Old Text Pg 147 #44-48 (2D Questions)