Motion and Forces

1
Motion and Forces

• Motion - a change in position of an object,
  caused by an object
• Force a push or a pull, caused by another
  object
• Force Can Start Motion
• Stop Motion
• Change its direction

2
Kinds of Force
TAKE NOTES!

• There are many different kinds of force. There
  are pushes
• and pulls
• electromagnetic forces
• weak forces
• strong forces
• and gravitational forces,
• Some forces are referred to as contact forces and
  some are non-contact forces

3
Forces related to basic processes
TAKE NOTES!

• Blood flow in the body
• Plate movement (tectonics)
• Volcanic eruptions
• Earthquakes
• Mountain building
• Emergence of seedlings
• Erosion

4
A net force results in a change of motion

• The net force is the left over amount when all
  the forces are added
• (no net force)
• If there is a net force, the object will change
  its direction or velocity. This is called
  acceleration

acceleration
5
Newtons First Law of Motion Law of Inertia
(Was really discovered by Galileo in
1600) An object will remain at rest or
move in a straight line unless it is acted on
by a force. Force - a
push or a pull Newton (N) 1 Newton 1 kg x
m/s2 Gives ENERGY to an object Contact force -
involves objects touching each other Long
range forces - include gravity, magnetism,
electricity Inertia - measures an objects
tendency to remain at rest or keep moving - it
is related to mass
6

• Balanced forces - when the forces are equal
• but in opposite directions, they will cancel
• each other out. Does not cause motion.
• Unbalanced forces - uneven forces acting
  on an object that did not cancel each
  other out. The object will move in the
  direction of the larger one. Cause motion.

TAKE NOTES!
7
Friction - is a force that resists motion between
surfaces that are touching. Static
Friction - hinders a stationary object from
moving on a surface when a force is applied
to the object. Sliding Friction-
occurs when two surfaces slides past
each other Rolling Friction - is
the force that enables a wheel to roll past
on a surface-same direction Air
Resistance - molecules in air colliding with
the forward-moving surface of an object,
slowing its motion.
8
Newtons Second Law of Motion
An object acted on by an unbalanced force will
accelerate in the direction of the force,
according to the formula. Acceleration
Force (Newton) (10N) 5N/kg or 5m/s2
Mass (Kilogram) (2kg) Force of Gravity
Force m x (9.8 m/s2) 9.8m/s2
acceleration Mass vs. Weight - Mass is the
measure of the amount of matter in an object
it remains the same. Weight is a force
mass x (9.8 m/s2) gravity (Mass is a measure of
inertia.)
9
If a 50 kg person in the middle of an ice skating
rink pushes a 20 kg box with a force of 10 N,
what will the acceleration be? Acceleration
of the box Acceleration force
mass a (10 N) (20
Kg) a .5m/s2 Acceleration of the
person Acceleration force
mass a (10 N) (50
Kg) a .2m/s2
10 N Force
50 Kg person
20 Kg box
10
Weight is a type of force
TAKE NOTES!
Mass is a measure of how much matter an object
contains.
Weight is the result of the force of gravity
pulling on a mass.
The greater the force of gravity, the greater the
weight.
mass
weight
matter
gravity
11
Newtons Third Law of Motion
TAKE NOTES!
For every action, there is an equal, but
opposite reaction. Force always acts in equal
and opposite pairs. (Not the same as
balanced forces - different objects.)
When you walk, you push backward on the
ground, and the ground pushes forward on
you.
If you put two bar magnets with opposite poles
facing one another, they will move toward each
other.
12
TAKE NOTES!
Gravity is a force of attraction between objects.
The amount of the force depends on the mass of
each object and the distance between them.
13
Gravity is a non-contact force

• The earth pulls on the moon while the moon also
  pulls on the earth.

The sun pulls on the earth and moon (and other
planets) and the earth and moon pull on the sun
The earth pulls on other planets and they also
pull on the earth
14
Gravity and the third law - Gravity is pulling
you to earth and Earth is also pulled toward
you. The same force that keep planets in orbit,
is the same force that pulls objects down to
Earth. You apply a force to earth, but because
earth is more massive than you are, your force
has little or no effect. (While F is greater, so
is m the mass.)
Newtons laws apply to all objects, including
distant galaxies. Just as the Sun exerts a
gravitational force on Earth, Earth exerts an
Equal and opposite force on the Sun. This causes
a variation in the Suns rotation.
15
Combining the Laws
2. You leave the ground, you accelerate in the
direction of this unbalanced force 2nd law -
accelerate in direction of the unbalanced force.
3. When your feet hit the ground, the ground
exerts an upward force on you. When you stop
accelerating, all of the forces are
balanced. 1st law - you remain at rest.
1. You push on the ground, it pushes up on
you. 3rd law - equal/opposite reaction
16
Combining the Laws
1st Law - The canoe will stay until a force
moves it. This force comes from the push of the
puddles.
3rd Law - The paddlers push water backward - an
action. This backward force produces an
equal and opposite force- a reaction in the
water - that move the canoe forward.
2nd Law - The canoe changes speed or direction or
both. The people add mass and the paddlers
change the canoes acceleration by adding a lot
of force