Units to read: 14, 15, 16, 17,18

1
Units to read 14, 15, 16, 17,18
2
Mass and Inertia

• Mass is described by the amount of matter an
  object contains.
• This is different from weight weight requires
  gravity or some other force to exist!
• Ex while swimming, your weight may feel less
  because the body floats a little. Your mass,
  however, stays the same!
• Inertia is simply the tendency of mass to stay in
  motion

3
The Law of Inertia

• Newtons First Law is sometimes called the Law of
  Inertia
• A body continues in a state of rest, or in
  uniform motion in a straight line at a constant
  speed, unless made to change that state by forces
  acting on it
• Or, more simply, a body maintains the same
  velocity unless forces act on it
• A ball rolling along a flat, frictionless surface
  will keep going in the same direction at the same
  speed, unless something pushes or pulls on it
• Gravity!

4
Another View of Newtons First Law

• If an objects velocity is changing, there must
  be forces present!
• Dropping a ball
• Applying the brakes in a car
• If an objects velocity is not changing, either
  there are no forces acting on it, or the forces
  are balanced and cancel each other out
• Hold a ball out in your hand, and note that it is
  not moving
• Force of gravity (downward) is balanced by the
  force your hand applies (upward)!

5
Circular Motion

• Tie a string to a ball and swing it around your
  head
• Law of inertia says that the ball should go in a
  straight line
• Ball goes in a circle there must be forces!

• Wheres the force?
• Its the tension in the string that is changing
  the balls velocity
• If the string breaks, the ball will move off in a
  straight line (while falling to the ground)

6
Acceleration

• The term acceleration is used to describe the
  change in a bodys velocity over time
• Stepping on the gas pedal of a car accelerates
  the car it increases the speed
• Stepping on the brakes decelerates a car it
  decreases the speed

• A change in an objects direction of motion is
  also acceleration
• Turning the steering wheel of a car makes the car
  go left or right this is an acceleration!
• Forces must be present if acceleration is
  occurring

7
Newtons Second Law

• The force (F) acting on an object equals the
  product of its acceleration (a) and its mass (m)
• F m ? a
• We can rearrange this to be
• a F/m
• For an object with a large mass, the acceleration
  will be small for a given force
• If the mass is small, the same force will result
  in a larger acceleration!

• Though simple, this expression can be used to
  calculate everything from how hard to hit the
  brakes to how much fuel is needed to go to the
  Moon!

8
Newtons Third Law

• When two bodies interact, they create equal and
  opposite forces on each other
• If two skateboarders have the same mass, and one
  pushes on the other, they both move away from the
  center at the same speed
• If one skateboarder has more mass than the other,
  the same push will send the smaller person off at
  a higher speed, and the larger one off in the
  opposite direction at a smaller speed
• Why?

This works for planets, too!
9
Orbital Motion and Gravity

• If the ball leaves the cannon with a slow
  velocity, it falls to the ground near the
  mountain
• If the cannonball has a higher velocity, if falls
  farther from the mountain.
• What if we gave the cannonball a very large
  velocity, so large that it misses the Earth?
• The cannonball would be in orbit around the
  Earth, and it would be falling!

• Astronauts in orbit around the Earth are said to
  be in free fall, a weightless state.
• Are they falling? Yes!
• Imagine a cannon on top of a mountain that fires
  a cannonball parallel to the ground
• The cannonball leaves the cannon and is pulled
  toward the ground by gravity

10
Surface Gravity

• Objects on the Moon weigh less than objects on
  Earth
• This is because surface gravity is less
• The Moon has less mass than the Earth, so the
  gravitational force is less
• We let the letter g represent surface gravity, or
  the acceleration of a body due to gravity
• F mg

• On Earth, g 9.8 m/s2
• g on the Moon is around 1/6 as much as on the
  Earth!

11
Consider a basketball player dribbling a ball.
Which of the following statements is not true

• a. The ball bounces because the court floor
  pushes up on it every time it hits
• b. The floor experiences no acceleration due to
  the dribbling ball because its mass is so large
  compared to that of the ball.
• c. The ball exerts a force on the player's hand
  each time the two connect
• d. The player's hand exerts an equal force each
  time the two connect.

12
Which of the following properties of an
astronaut changes when she is standing on the
Moon, relative to when the astronaut is standing
on Earth?

• a. Weight
• b. Mass
• c. Inertia
• d. All of the above

13
Centripetal Force

• If we tie a mass to a string and swing the mass
  around in a circle, some force is required to
  keep the mass from flying off in a straight line
• This is a centripetal force, a force directed
  towards the center of the system
• The tension in the string provides this force.
• Newton determined that this force can be
  described by the following equation

14
Orbits

• Orbital velocity

We can use this expression to determine the
orbital velocity (V) of a small mass orbiting a
distance d from the center of a much larger mass
(M)
15
Calculating Escape Velocity

• From Newtons laws of motion and gravity, we can
  calculate the velocity necessary for an object to
  have in order to escape from a planet, called the
  escape velocity

16
Newtons Universal Law of Gravitation

• Every mass exerts a force of attraction on every
  other mass. The strength of the force is
  proportional to the product of the masses divided
  by the square of the distance between them
• Simply put, everything pulls on everything else
• Larger masses have a greater pull
• Objects close together pull more on each other
  than objects farther apart

• This is true everywhere, and for all objects
• The Sun and the planets exert a gravitational
  force on each other
• You exert a gravitational force on other people
  in the room!

17
What Escape Velocity Means

• If an object, say a rocket, is launched with a
  velocity less than the escape velocity, it will
  eventually return to Earth
• If the rocket achieves a speed higher than the
  escape velocity, it will leave the Earth, and
  will not return!

18
The Origin of Tides

• The Moon exerts a gravitational force on the
  Earth, stretching it!
• Water responds to this pull by flowing towards
  the source of the force, creating tidal bulges
  both beneath the Moon and on the opposite side of
  the Earth

19
High and Low Tides
As the Earth rotates beneath the Moon, the
surface of the Earth experiences high and low
tides
20
The Sun creates tides, too!

• The Sun is much more massive than the Moon, so
  one might think it would create far larger tides!
• The Sun is much farther away, so its tidal forces
  are smaller, but still noticeable!

• When the Sun and the Moon line up, higher tides,
  call spring tides are formed
• When the Sun and the Moon are at right angles to
  each other, their tidal forces work against each
  other, and smaller neap tides result.