The Nature of Forces

1
The Nature of Forces

• Chapter 3 section 4

2
What is a force

• Force is a push or pull.
• Forces are either balanced or unbalanced.

3
Unbalanced Forces

• Unbalanced forces acting on an object will change
  the objects motion.
• Can cause an object to start moving, stop moving,
  or change direction.
• When 2 forces act in the same direction they add
  together.
• When 2 forces act in opposite directions they
  subtract
• (Actually, they add together, but one number is
  positive and one is negative. Confusing?)

4
What is the net force?

• The net force is the overall force after all the
  forces are added together.
• When there is a net force acting on an object the
  forces are unbalanced.

5
Balanced Forces

• Equal forces acting on one object in opposite
  directions are called balanced forces.
• Balanced forces acting on an object will not
  change the objects motion.

6
Newtons First Law of Motion

• An object at rest will remain at rest and an
  object moving at a constant velocity will
  continue moving at a constant velocity unless
  acted upon by an unbalanced force.

7
Inertia

• INERTIA is the tendency of an object to resist
  change in motion.
• Example when the car stops suddenly and you
  continue to move forward.

8
Mass

• Mass is the amount of matter in an object.
• SI unit is the kilogram.
• The amount of inertia an object has depends on
  its mass. The greater the mass the greater the
  inertia.

9
Friction and Gravity

• 3.5

10
Friction

• Friction is the force that one surface exerts on
  another when the two rub against each other.
• Friction opposes motion.
• Friction acts in a direction opposite to the
  objects direction of motion.

11
Without friction an object would continue to move
at a constant speed forever.

• The strength of the force of friction depends
  upon two factors the types of surfaces involved
  and how hard the surfaces push together.

12
Is friction useful?

• Depends on the situation helps us walk, ie.
  Tread on shoes and tires.

13
Different types of friction

• Sliding friction solid surface sliding over one
  another.
• Rolling friction object rolls over a surface.
• Fluid friction objects move through a fluid.

14
Gravity force that pulls the object toward the
earth.

• Free fall
• Projectile motion
• Air resistance
• weight.

15
Weight vs. Mass

• Weight is the measure of the force of gravity on
  an object.
• Mass is the amount of matter in an object.
• Weight mass x acceleration due to gravity.

16
Weight

• Mass is measured in grams
• Weight is a force measured in Newtons.
• A kilogram of mass on Earth weighs 9.8N

17
Universal Gravitation

• The law of universal gravitation states that the
  force of gravity acts between all objects in the
  universe.
• Gravitational force depends on mass and distance
  between the objects.

18
Force, Mass, and Acceleration

• 4.1

19
Newtons 2nd Law of Motion

• The net force on an object is equal to the
  product of its acceleration and its mass.

20
Force Mass x Acceleration

• Sometimes people refer to the equation as
  Newtons 2nd Law.

21
Units of measurement

• One Newton 1 kg x m/s/s
• Force ma
• Acceleration f/m

22
Air Resistance

• Objects falling through the air experience some
  type of fluid friction called air resistance.
• Air resistance is not the same for all objects.
  Depends on surface area the greater the surface
  area the greater the air resistance.

• Air resistance increases with velocity. So as a
  falling object speeds up, the air resistance
  against it increases. Eventually the air
  resistance equals the force of gravity. When
  forces are balanced there is no acceleration.

23
Terminal Velocity

• The greatest velocity an object reaches is called
  terminal velocity.

24
Free Fall

• When the only force acting upon the falling
  object is gravity the object is said to free
  fall.
• As an object falls it accelerates at a rate of
  9.8 m/s/s

25
Changes in force and mass

• How can you increase the acceleration of the
  wheel barrel?
• Increase Force
• Decrease Mass

26
Acceleration due to the force of gravity.

• 9.8 meters per second squared.
• After 1 sec. 9.8 m/s/s
• 2 sec. 19.6 m/s/s
• 3 sec 29.4 m/s/s
• Objects in free fall accelerate at the same rate
  regardless of mass.

27
42 Projectile and Circular Motions
28
Projectile Motion

• Projectile an object thrown horizontally.
• Will a projectile land on the ground at the same
  time as an object dropped straight down? Yes

29
Horizontal Motion

• Motion that is parallel to the Earths surface.
• Once the ball is thrown forward its horizontal
  velocity is constant if you ignore air resistance.

30
Vertical Motion

• When you let go of the ball the ball has
  horizontal motion but since gravity starts to
  pull it downward it has vertical motion.
• Not the ball has constant horizontal velocity
  with increasing downward vertical velocity.
• Demo Ball dropping straight down vs. ball being
  thrown as a projectile.

31
Motion along curves

• A change in acceleration is a change of speed or
  direction.

32
Centripetal Acceleration

• Acceleration towards the center of a curve.
• When riding a bike the rider experiences
  centripetal acceleration and they lean toward the
  inside of the curve.

33
Free Fall

• When an object is influenced only by gravity it
  is said to be in free fall.
• Space shuttles, satellites are in free fall
  they fall around the Earth

34
Centrifugal Force

• This the force acting away from the center of a
  curved or circular path.

35
Action and Reaction

• 10-4

36
Newtons third Law of Motion

• States that if one object exerts a force on
  another object, then the second object exerts a
  force of equal strength in the opposite direction
  on the first object.

37
Equal but Opposite

• Action and reaction forces will be equal and
  opposite, the same force acting on a greater mass
  results in a smaller acceleration.

38
Action Reaction In Action

• Walking you push on ground and the ground
  pushes on you.
• Squid Water jet

39
Do Action-Reaction Forces Cancel?

• Newtons third law refers to forces on two
  different objects.
• Example Soccerball
• If one player hits the ball force is upward.
  The ball exerts an equal but opposite downward
  force on the player. The action and reaction
  forces are acting on different objects and
  therefore cannot be added together.

40
Momentum mass x velocity

• A small mass can produce a large momentum if it
  is moving at a high velocity ie. Bullet.

41
Conservation of momentum

• The total momentum of any group of objects
  remains the same unless outside forces act on the
  objects.
• Conservation means the conditions before and
  after some event.

42
Two Moving Objects

• Two snowboarders traveling the same direction
  bump into each other.
• Momentum is conserved - The back person slows
  down while the front person speeds up.

43
Orbiting Satellites

• 10 - 5

44
Rocket Launch

• Rockets can rise into the air because the gases
  it expels with a downward force exert an equal
  but opposite force on the rocket.

45
What is a satellite?

• Any object that travels around another object in
  space.
• Centripetal force is any force that causes an
  object to move in a circle.

• Centripetal force is the gravitational force that
  pulls the satellite toward the center of the
  Earth.

46
Satellite Motion

• Satellites in orbit around Earth continually fall
  toward the Earth, but because Earth is curved
  they travel around it. A satellite is a
  projectile that falls around the Earth rather
  than into it.