Download Forces and Newton`s Laws of Motion

Teacher Reference Page
This powerpoint is a presentation of Forces and
Newton’s Laws. It probably goes through more
than is necessary for students to know on the
proficiency test, but is not as detailed as what
would be discussed in a physics class.
I made no effort to make this super-fancy
because I just don’t know how. Sorry!!
There are links to the RPDP site at the end of this
presentation. There are also a few links to sites
that show animations of various principles
described in this unit.
Please use your physics peers for help in
understanding, presentation, or resources.
That’s what we’re here for.
Forces and Newton’s
Laws of Motion
What is a Force?
A
force is defined as a push or a pull.
 A force is something that CHANGES
the motion of an object. This could
mean that the object changes its
speed by slowing down or speeding
up. It might also mean that the
object changes its direction of travel
after a force has been applied.
Can Objects Move Without a
Force?
 Yes,
a force only CHANGES the way
an object moves. Therefore, if an
object is already moving, it will
continue to move at the same speed
in the same direction.
 Changes in speed or direction are
known as ACCELERATIONS
 Therefore, FORCES cause objects to
ACCELERATE.
Newton’s First Law
 An
object at rest will stay at rest,
and an object in motion will stay in
motion in a straight line, UNLESS
acted upon by an outside force.
 Again, it takes a force to get an
object moving (change in motion) or
a force to change the speed or
direction of motion (also a change in
motion)
Why don’t objects keep moving
when I throw them on Earth?
 Because
of Newton’s 1st Law.
Gravity is a force (a push or pull
acting between all objects with
mass). If you throw an object
through the air, gravity is a force that
can change of the motion of the
object.
Why do rolling objects slow down?
 Because
of Newton’s 1st Law.
Friction is a force that impedes the
motion of objects when two surfaces
are in contact with one another.
 If we lived in a world without friction,
then objects would actually keep
moving at the same speed. Because
friction is a force, it changes the
motion of the object.
More on Friction
 Friction
always acts in a direction
opposite to motion.
 Objects that weigh more have a
greater frictional force acting on
them.
 Friction even acts when objects are
at rest. To move an object, you
must provide enough force to
overcome friction.
Mass and Inertia
 Mass
is the amount of matter in an
object.
 Inertia is the mass of an object.
 Inertia is also a measure of an
object’s resistance to CHANGING its
motion.
More Mass and Inertia
 If
something has a lot of mass, it has
a lot of resistance to changing its
motion – it has a lot of inertia.
 Therefore, heavy objects are hard to
get moving if they are at rest, OR
they are hard to stop moving if they
are already in motion.
Newton’s 2nd Law
 The
net force (all of the forces acting
on an object) cause the object to
have an acceleration.

F = ma

F = total force

m = mass of the object

a = acceleration
Newton’s 2nd Law


It may be easier to explain Newton’s
2nd Law in terms of acceleration (the
CHANGE in the object’s motion)
a = F/m
a = F/m
 Because
acceleration and Force are
directly related in the equation, a big
force results in a big acceleration.
Or a small force results in a small
acceleration.
 This means a big push causes a big
CHANGE in motion like a drastic
speed increase or decrease, or a
sudden change in direction.
a = F/m
 Because
acceleration and mass are
indirectly related, applying a force to
a SMALL mass will result in a LARGE
acceleration. Applying a force to a
LARGE mass will result in a SMALL
acceleration.
 The units for ANY type of Force is the
Newton.
Example
 If
I apply a 1 Newton force to a
1 kilogram object, what is its
acceleration?
a = F/m = 1/1 = 1 m/s2
This is not a very large acceleration and
would result in only a minor change of
motion.
Another Example

If a 500 N force is applied to a
1 kilogram object, what is the acceleration
of the object?
a = F/m = 500/1 = 500 m/s2
This is a very large acceleration which
means that the object will have a drastic
increase or decrease in motion.
USE COMMON SENSE
 Does
it take big forces to move big
objects?
OF COURSE!!!
 Does
a bigger force (push or pull)
cause a greater change in motion?
YES!!
Use Diagrams
A
free body diagram shows all of the
forces acting on an object.
 An
object always accelerates in the
direction of the net force.
Example
What the picture says…
 The
object clearly moves to the right
because that is the direction of the
bigger force. In fact the net
sideways force would be 13 N to the
right because we would be
subtracting the forces in opposite
directions.
The picture also says…
 The
downward force of the weight is
equal to the upward force from the
table. By subtracting two equal
forces, we get a net force of zero.
 REMEMBER, a zero net force means
no acceleration – no CHANGE of
motion. This is why the object
moves sideways, but not up and
down.
But Just as Important…
 An
object may have two equal forces
acting on it and still be moving. It
just isn’t CHANGING the way it is
moving if these equal forces act from
opposite directions.
For Example
 In
cruise control, your engine puts
out a certain force to make your car
go forward. But if your speed is
constant, friction and air resistance
MUST BE putting an equal force back
on your car to prevent any CHANGE
of motion.
REAL LIFE
 Why
do you keep pedaling a bike
when an object in motion wants to
stay in motion.
 Because….friction
is a force trying to
slow you down, so you need a force
to keep you motion going.
Newton’s Third Law
 “If
object A exerts a force on object
B, then object B exerts an equal
force on object A in the opposite
direction.”
 This is more commonly known as,
“for every action there is an equal
and opposite reaction”.
What does the 3rd Law Mean?
 Does
that mean that if I punch a wall
with 300 Newtons of force, the wall
punches me back with 300 N of
force?
YES – that’s why your hand
hurts. The wall is
exerting a force to
change the motion of
your hand.
Classic Question
 If
a bee flies straight into a moving
car, who experiences the greater
force? The bee or the car?
ANSWER – they both experience the
same force. That’s what
Newton’s Third Law states.
How does the 3rd Law Make Sense
 How
can both the bee and the car
experience the same force? It
doesn’t make sense.
Answer – Remember that forces cause
CHANGES in the motion of an object.
Also remember that the more mass
an object has – the less the change
will be.
(Continue on next slide)
Let’s say the bee and the car each
experience a force of 10 N during the
collision. How would a 10 N force
affect a bee whose mass is
0.002 kg?
a = F/m = 10 N/.002kg
a = 5000 m/s2
This is a HUGE change in motion for
the bee. But think about it. The bee
DOES undergo a huge change in
motion after striking the car, so this
makes sense.
Now apply the same 10 Newton force to the
car whose mass is 2000 kilograms.
a = F/m = 10 N/2000 kg
a = .005 m/s2
This is a very SMALL acceleration. In other
words, the car’s motion CHANGES very
little. This makes sense too. Why would a
bee striking a car really impact the way
the car is moving?
AVOID CONFUSION
 It
is a common misconception to
think that the bigger force comes
from the bigger object..- NOT TRUE!
 The FORCES must be equal when
two objects interact. What is not
equal are the accelerations, or the
effect of the forces on motion,
because of the difference in mass
between the two objects.
Common Misconceptions
Students believe that objects in
motion have an “impetus” force.
They think that if an object is
moving, it is retaining some of the
force that initially set the object in
motion, and that’s why it continues
to move. They don’t recognize that
once the object is in motion, it while
stay that way UNTIL an outside force
makes its motion change.
Common Misconceptions
Big objects always exert the bigger
force when they strike a small
object.
Truth – the forces are the same. It is
the “effect” of those forces that are
different. The larger object has a
smaller acceleration, and therefore, a
small CHANGE in its motion after the
force is applied.
Common Misconceptions
Forces deal with velocity.
Truth – forces deal with accelerations, which
are changes in speed or direction.
Therefore, an object can have a constant
velocity without a net force acting on it.
This doesn’t mean no forces act on the
object, it means that the forces act
equally from opposite directions to cancel
each other. So an object with no
acceleration can either be moving at a
constant speed or not moving at all.
The following links go directly to the
RPDP Tips Site and other Web
Sites with Physics Animations and
Examples.
http://rpdp.net/sciencetips_v2/P12B1.htm
http://nzip.rsnz.org/es/applets/index.htm
http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/
http://phet.colorado.edu/new/simulations/index.php?cat=Motion
Please don’t hesitate to see Scott or Kris for other resources if it helps.