4 outline

1

4 outline• Newton’s 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

Newton’s Second Law: the acceleration of an object is proportional to the Net External Force acting on it, and inversely proportional to the object’s mass.

massforcenet

acceleration

massforcenet

time

direction / speedin change

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.

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.• /

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 object’s 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• /

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• /

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• /

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

• Newton’s 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)