Chapters One and Two

1

An Introduction to Basic Physics Force and
Motion
2
Sections

1. Describing and Measuring Motion
2. Acceleration
3. The Nature of Force
4. Force, Mass, and Acceleration
5. Friction and Gravity
6. Action and Reaction

3
What is Motion?

• An object is in motion when its distance from
  another object is changing.
• Motion depends on your reference point (an object
  used for comparison in determining if something
  is in motion).

4
Describing Distance and Speed

• The SI unit for distance is the meter.
• Speed is calculated as
• Distance/time

5
Average Speed

• Since most objects in motion do not travel at a
  constant speed, we calculate an average speed
  which is calculated as
• Total distance/Total time

6
Average Speed (cont.)

• If Lance Armstrong cycles a total of 889 Km in
  the Tour De France in 49 hours, What is his
  average speed?

7
Velocity

• When you know the speed and the direction of an
  object, you know its velocity.

8
Graphing Motion

• Slope refers to the steepness of a line on a
  graph. When graphing motion (distance vs. time)
  the steeper the slope, the faster the motion.
• Slope rise/run

9
Graphing Motion
10
Graphing Motion
Time Boy 1 Distance Boy 2 Distance
1 min .2 Km .2 Km
2 min .4 Km .4 Km
3 min .6 Km .6 Km
4 min .8 Km .8 Km
5 min .8 Km 1.0 Km
6 min .8 Km 1.2 Km
7 min .9 Km 1.4 Km
8 min 1.1 Km 1.6 Km
9 min 1.3 Km 1.8 Km
10 min 1.6 Km 2.0 Km
11
Acceleration

• Acceleration refers to the rate at which velocity
  changesa change in velocity over time.

12
Acceleration (cont.)

• Speeding up Slowing down
• Changing direction

13
Acceleration (cont.)
14
Acceleration (cont.)

• If a roller coaster is traveling at 7 m/s at its
  highest point, and 3 seconds later its speed has
  increased to 28 m/s, what is its average
  acceleration?

15
Graphing Acceleration
16
The Nature of Force

• A force is a push or a pull. Forces are described
  by magnitude and direction.

17
Forces

• Forces can add together (when acting in the same
  direction), or cancel each other out (when acting
  in opposite directions).

0

18
Forces

• The overall force acting on an object after all
  of the forces have been added together is called
  the net force. Unbalanced forces always result in
  motion.

19
Forces

• Equal forces acting on an object are called
  balanced forces. Balanced forced do not result in
  motion

20
Newtons 1st Law of Motion

• Inertia is the tendency of an object to resist
  change in its motion.
• An object at rest will stay at rest, and an
  object in motion will stay in motion, unless
  acted upon by an unbalanced force.

21
Inertia An everyday experience
22
Inertia and Mass

• Mass refers to the amount of matter in an object.
  The more mass an object has, the more inertia it
  has

23
Newtons Second Law

• The net force on an object is equal to the
  product of its acceleration and its mass.
• Force (N) Mass (Kg) X Acceleration (m/s2)
• OR
• Acceleration(m/s2) Force (N)/mass (Kg)

24
Newtons Second Law

• If the wagon has a mass of 62 Kg, and the girl is
  pulling it with a force of 170 N, what is the
  rate at which the wagon is accelerating?

25
Newtons Second Law

• What force is required for this 275 Kg motorcycle
  to accelerate at a rate of 18 (m/s2)?

26
Friction

• The force that one surface exerts on another is
  called Friction.
• Even objects that appear smooth have many
  irregularitieswhich get caught on the
  irregularities of the opposing surface, and
  resist motion..

27
Friction

• Friction always acts in the opposite direction of
  the force/motion.

28
Friction An Everyday Experience
29
Friction Three types
Sliding friction
Rolling Friction
Fluid Friction
30
(No Transcript)
31
Gravity

• Gravity is the force that pulls objects toward
  each other. The more massive the object, the more
  gravitational pull it has.

32
Freefall

• When the only force acting on an object is
  gravity (ignoring air resistance) it is said to
  be in freefall.

33
Acceleration Due to Gravity

• Objects in freefall will accelerate at 9.8m/s2.
  All objects accelerate at the same rate
  regardless of mass (again, ignoring air
  resistance).

34
Acceleration Due to Gravity

• If the person is in freefall for 3.8 seconds,
  what is his approximate speed just before the
  bungee cord slows his fall?

35
Projectile motion

• An object that is thrown horizontally is called a
  projectile.

36
Projectile Motion

• When an object is simply dropped from the same
  height as another object that is thrown, they
  will hit the ground at the same time!

37
Air Resistance

• Objects falling through the air experience a
  type of fluid friction called air resistance.

Air Resistance
38
Terminal Velocity

• When air resistance is so great that an object no
  longer accelerates in freefall, the object has
  reached terminal velocity.

39
Weight or Mass?..

• Mass refers to the amount of matter an object
  contains, while weight refers to the
  gravitational pull on an object.
• Gravitational pull on an object can be measured
  in newtons. Since FM x A, one calculates weight
  (N) by multiplying mass (Kg) times the
  acceleration due to gravity (9.8 m/s2).

40

• What is the approximate gravitational force
  acting on this motorcycle if it has a mass of 225
  Kg?

Think..
41
How much would I weigh on the moon? Because the
moon has one sixth the gravity of Earth, you
would weigh six times less than what you weigh on
Earth. This is why the astronauts were able to
move easily in their heavy space suits. In the
photo to the right, John Young, commander of the
Apollo 16 lunar landing mission, jumps up from
the lunar surface as he salutes.
42
Universal Gravitation

• The law of universal gravitation states that the
  force of gravity acts between all objects in the
  universe.

43
Newtons 3rd Law of Motion

• If one object exerts force on another object,
  then the second object exerts a force of equal
  magnitude in the opposite direction on the first
  object.

44
Newtons 3rd Law affects everything!
45
Momentum

• In assessing what Newton referred to as quantity
  of motion we now call momentum.
• Momentum equals mass X velocity

46
Momentum

• What has more momentum, a 12,000 Kg jet traveling
  at 700 m/s, or a 295,000 Kg cruise ship traveling
  at 6 m/s?

47
Conservation of Momentum

• The Law of Conservation of Momentum states the
  total momentum of any group of interacting
  objects remains the same. (in the absence of
  outside forces)

48
Conservation of Momentum