4 outline

1
4 outline

• Newtons 2nd Law
• friction
• dynamics of falling objects
• RQ 1, 2, 3, 4, 5, 6, 8, 11, 13, 14, 16, 18, 20,
  21, 24, 26, 27, 30, 31, 33.
• Ex 2, 3, 20, 31, 32, 33, 42, 43, 44.
• Problems 2, 3, 5.

2
example uses

• design crumple zones (cars barriers)
• power requirements for cars
• parachute design
• elevator design
• /

3
force acceleration

• Net force ? acceleration
• acceleration net force
• e.g. The net force on a car is doubled. The
  acceleration of the car will then also double in
  size.
• ( means directly proportional to)

4
0
Newtons Second Law the acceleration of an
object is proportional to the Net External Force
acting on it, and inversely proportional to the
objects mass.
acceleration
5
Effect of a Net Force

• object at rest begins to move
• moving object changes its
• speed
• or direction
• or both.
• change in velocity is in the same direction as
  the net force on the object.

6
(No Transcript)
7
Finding Net Force Using Diagrams

• Example of adding two perpendicular Forces A 3,
  B 4
• A) Walk 3 steps forward.
• B) Turn left or right, walk 4 steps in this
  direction.
• C) Walk directly between the starting and ending
  locations counting your steps.
• Distance in steps in (C) is the Net of the two
  forces.

8
Ex. Net Force Rightward

• Car moving to right
• net force is to right
• Speed increases

9
Ex. Net Force Leftward

• Car moving to right
• net force is directed left
• Car slows down

10
Ex. Net Force Downward

• Ball is tossed to right
• net force is down
• object turns downward

11
Mass and Weight

• Mass is the quantity of matter.
• Mass measures inertia.
• Mass is measured in kilograms (kg)
• Weight is the force on mass due to gravity.
• Weight is measured in newtons (N) or pounds (lb).
• Weight Mass
• 1 kg of mass has a weight of 2.2 lbs

12
Comparing Accelerations of Objects in Free Fall.

• Downward force is weight.
• a weight/mass
• but an object with twice the mass will have twice
  the weight
• so the accelerations are the same
• We call this acceleration g.
• g is about 10m/s/s downward.

13
Friction

• Objects in contact like each other, i.e. they
  form a bond.
• They resist being moved when in contact.
• Ex. A Chest sitting on a wood floor seems glued
  down. It is harder to get it moving than it is to
  keep it moving.
• These resistance forces are called frictional
  forces.

14
Direction of Frictional Forces

• Frictional forces oppose the direction of motion
  of object if it is moving.

15
Direction of Frictional Forces (cont.)

• If the object is at rest there still may be a
  frictional force. If there is a frictional force
  then it is in a direction opposite to the
  direction of the net applied force due to other
  causes.

16
Categories of Friction

• Sliding Friction exists when one object slides
  against a second object, e.g. box along floor.
• Static Friction exists when a force is applied
  to an object, but that force is not large enough
  to break the frictional bond.
• Air friction force (speed of object)2.
• /

16
17
Weight Force

• Weight (N) mass (kg) x (10N/kg)
• Weight mg
• g 10 m/s/s 10 N/kg
• Acts at all times, e.g. object is falling,
  sitting, etc.
• Ex A 2kg object weighs 20N.
• Weight (2kg)(10N/kg) 20N
• /

18
Free-Fall

• only force is objects weight
• air drag is negligible in size
• Ex. A solid steel ball falling a short distance
  is in free-fall.
• Ex. A falling feather is not in free-fall since
  air drag is equal to its weight

19
Non Free Fall

• Whenever air drag is significant compared to
  weight the object will fall with acceleration
  less than 10m/s/s.
• Example A 5kg object weighs 50N but an air drag
  force of 10N acts on it.
• Acceleration NetForce/mass
• (50-10)N/5kg
• 40/5 m/s/s
• 8 m/s/s
• /

20
Ex. Falling Motion 1

• M 1kg, speed is small, air friction 0.
• Weight is 10N.
• Net force 10N 0 10 N
• Acceleration Net Force/Mass
• 10N/1kg
• 10 m/s/s
• N/kg m/s/s
• /

21
Ex. Falling Motion 2

• M 1kg, speed is large and air friction is 1/3
  the weight 10N/3 3.33N
• Net force 10N 3.33N 6.67N
• Acceleration Net Force/Mass
• 6.67N/1kg
• 6.67m/s/s
• /

22
Ex. Falling Motion 3

• M 1kg, speed is larger and air friction is
  equal to the object weight 10 N
• Net force 10N 10N 0N
• Acceleration Net Force/Mass
• 0N/1kg
• 0 m/s/s
• /

23
Terminal Speed/Velocity

• Air drag on falling objects increases
• until equal to the objects weight
• resulting in balanced forces
• acceleration 0, speed no longer changes
• This top speed is called the terminal velocity
  of the object
• and varies from object to object.

24
4 summary

• Newtons 2nd Law relates net force, mass, and
  acceleration. It also covers the 1st Law.
• frictional forces are proportional to the forces
  holding objects together
• falling objects accelerate until air drag equals
  their weight

25
p.11 practicing physics

• Mass is fundamental, does not depend on location
• Mass is not a vector and not a force, it is
  simply a number
• 1kg mass weighs 10N on earth (2.2lbs), less on
  the moon
• Weight mg (earth g 10m/s/s)