How do forces affect motion

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					How do
Forces always
   in pairs.
 One force pushes
 in one direction,
   and the other
pushes with equal
  strength in the
opposite direction.
demonstrates action
and reaction because
 one foot pushes on
  the ground as the
 ground pushes the
    body forward.
  Earth pushes
forward against
your foot with an
 equal reaction
A push or pull is called a
 force and forces make
  things move faster or
  slower, stop, change
direction, or change size
       and shape.
A force that acts at a distance and
  cannot be seen. The closer the
 objects, the greater the magnetic
The needle of a compass moves
  because of Earth’s magnetism.
Materials that create this force are
   said to be magnetic and are
          called magnets.

The closer the objects, the greater
        the magnetic force.
The force that
opposes motion
between two
surfaces that are
The rougher the
surfaces are, the
harder they press
  together, the
  more friction
  there will be.
A smooth surface
allows you to go
  faster, while a
  rough surface
    would be
Occurs between liquids
 and gases as well as
 between solids.

Without friction you
 would not be able to slow
 or stop the motion of
Friction can be reduced by
     using lubricants, for
  example motor oil, wax,
grease, by making surfaces
   smoother, or by using
Lubrication, for example, oil or
 grease, reduces the effects of
 friction. Without lubrication,
 moving parts of machines
 would slow down or stop very
How does gravity
  affect motion?
 Gravity is a force that
   makes any object
attract, or pull toward,
        another object.

All objects, large and
  small, pull on all
    other objects.
 Gravity can
make things
slow down or
• The attraction is not noticeable unless one
  of the objects is very large.

               Earth is so big it
               keeps everything
                    on the
• Introduction to forces and gravity 3:20

• Some types of forces (for review)
• Types of Force
The changing position of
an object over time when
     compared to its
  beginning reference
 Measure of how far
something travels in a
certain period of time
The course or path that an
 object is moving and can
     be determined by
   reading a compass in
 terms of “north”, “south”,
     “east”, or “west”.
Direction can be
 described using these
 terms: right, left, up,
 down, forward,
 backward, and toward
   Is the object “above”,
 “below”, “beside”, “behind”,
 “ahead of” another object?
What is its length away from
       the other object?
Sir Isaac Newton’s
 Laws of Motions

 Unbalanced forces change the rate and
    direction of the motion of objects

Several forces can act on an object at the
                same time.

  Forces are balanced when they don’t
     change the motion of the object.
Unbalanced forces cause changes in

 How can unbalanced forces cause
    motion in the picture below?
         OBJECT at REST
If an unbalanced force acts on an object at
  rest the object will move in the direction of
                   the force.

  Like a golf ball being hit by Ms. Woods

           A stronger force (push or pull) will make
           it move faster.
           (Ms. Woods always hits the sweet spot!)
If an object is moving, an unbalanced force
     will change the motion of the object in
  different ways depending on how the force
                   is applied.
The unbalanced force may speed the object
      up, slow it down, or make it change
  • If the force is applied in the same
  direction as the object is moving, the
           object will speed it up.
• If the force is applied in the opposite
  direction as the object is moving, the
     object will slow it down or stop it.
 • If the force is applied to the side of
 the moving object, the object will turn.
Newton’s First Law
   of Motion

 The tendency of an
   object to stay in
  motion or to stay
  still unless acted
      upon by an
 unbalanced force
  Every object
  whether it is
  moving or at
rest has inertia.
  In order to start
something moving
   or stop it from
 moving, you need
   to use force to
    overcome its
Earth’s gravity is so
    strong it can
 overcome inertia
 and make things
 fall toward Earth.
On the earth, invisible
forces, such as gravity
and the drag of air or a
    rough surface,
  overcome inertia to
 slow objects down or
 pull them toward the
  Two objects
 with about the
same amount of
  mass attract
   each other

Amount of
When two objects have
different masses, even
though the force of the
 gravity between them
  is equal, the object
  with the lesser mass
 moves more than the
  object with greater
An object’s mass
 stays the same
 everywhere in
  the universe.
  An object’s mass
   stays the same;
 however, its weight
 changes according
to the pull of gravity.
    Weight is the
measurement of the
amount of force that
gravity exerts on you.
Falling Objects
 And the role
 How do we
 interpret a
motion graph?
• A distance-time graph tells us how far an
  object has moved with time.
• The steeper the graph, the faster the
• A horizontal line means the object is not
  changing its position - it is stationary.
• A downward sloping line means the object
  is returning to the start.
      Q: Who won the race?
A: Albert won – he reached 100 m
Q: Which runner stopped for a rest?
A: Charlie stopped for a rest at 50 m.
Q: How long did he stop for?
A: He stopped for 5 seconds.
Q: How long did Bob take to complete the
A: 14 seconds.
Q: Calculate Albert’s average speed
A: speed (m/s) = distance (m)/time (s)
   speed (m/s) = 100m/12s
   • Air Resistance
• Liquid Resistance
  • Surface Texture
    • Surface Area
     • Lubrication
 Air Resistance:
  Molecules of gas
   in the air push
against the surface of
 moving objects and
      slow their
     Texture of the Surface
• The rougher the surface, the
  greater the friction. Smooth
  surfaces reduce friction. Carpets
  have more friction than tile floors.
  Soles of shoes have rough
  textures to increase friction
  between the shoes and the floor
  so that it is possible to walk
  without slipping.
    Amount of Surface Area
• The greater the surface area, the
  greater the friction. If tires of a car
  or truck are larger, more surface
  area of the tire will touch the road
  making friction greater. Trucks
  have larger tires to make it easier
  for them to stop or slow down.
How does surface texture affect the
  amount of friction on a moving
• The Magic School Bus Plays Ball (24 min)
Experiments with friction on rubber tires and
  different conditions of asphalt of the road

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