Transcript Forces and Motion - Canyon ISD / Overview

Forces and Motion
Newton’s First Law
Causes of Motion
Until now, we have been learning how to
describe motion
• Displacement
• Velocity
• Acceleration
Now we will learn what causes an object to
move
• Force
Forces
When we think of the word force, we usually
imagine a push or pull exerted on an object.
For example, you exert a force on a ball when you
throw or kick it.
You exert a force on a chair when you sit on it.
(The chair also exerts a force on you!)
Force represents the interaction of an object with
its environment
Forces Cause Changes in
Velocity
A force often changes an objects state of motion.
For example, when you kick a football on a tee,
the force of your foot causes the stationary ball to
soar quickly into the air.
Forces can also slow or stop an object or change
its direction.
In all of these cases, forces cause a change in
velocity, or acceleration, of the object.
The Newton
The metric unit of
force is the Newton,
named after Sir Isaac
Newton.
Newton’s work
contributed much to
our modern
understanding of force
and motion.
The Newton
The Newton is defined as
the amount of force that,
when acting on a 1kg mass,
produces an acceleration of
1 m/s2.
Therefore, 1N = 1kg*m/s2.
The weight of an object is
expressed in Newtons with
one pound being equal to
about 4.448 N.
Types of Forces
There are two types of forces that can cause
changes in the motion of an object.
Contact forces involve pushing or pulling an
object. This force requires direct contact with the
object moving. A wagon being pulled by a child
would involve a contact force.
Field forces do not require physical contact
between two objects. Gravity, and the attraction
or repulsion of electrically charged objects would
involve field forces.
Force Diagrams
If you give a toy car a light push, it does not move
as fast as it does when you give it a harder push.
The effect of a force depends on the magnitude
(how large or small) of the force.
The effect of the force also depends on the
direction of the force. The car moves differently if
pushed from the front instead of the rear.
Force is a Vector
Because the effect of force depends on its
magnitude and direction, force is a vector
quantity.
Force diagrams can be a useful tool to determine
how forces on an object can be resolved and
motion of the object can be determined.
Because force is a vector, we can add and subtract
them exactly as we did when we studied vectors
last six weeks.
Free-Body Diagrams
Free-body diagrams
are a special type of
force diagram.
Free-body diagrams
include only the forces
that are causing
motion in the object.
Forces that are not
effecting the motion of
the object are left out.
Newton’s First Law
An object at rest tends
to stay at rest and an
object in motion tends
to stay in motion with
the same speed and in
the same direction
unless acted upon by
an unbalanced force.
Balanced Forces
What is an unbalanced
force?
In pursuit of an answer, we
will first consider a physics
book at rest on a table top.
There are two forces acting
upon the book. One force the Earth's gravitational pull
- exerts a downward force.
The other force - the push of
the table on the book (the
normal force) - pushes
upward on the book.
Balanced Forces
These forces are equal but
opposite of each other so
they cancel each other out.
The net external force
acting on the book is zero.
The net external force is
the vector sum of all the
forces acting on an object
Objects that have a net
external force of zero are
said to be in equilibrium.
Balanced Forces
An object in
equilibrium
can still be
moving,
But it will be
moving at a
constant
velocity ( no
acceleration).
Unbalanced Forces
An object with an
unbalanced force acting
on it will usually be
undergoing some kind of
acceleration (speeding up,
slowing down, or
changing direction).
The forces acting on the
object do not balance out
to zero.
There will be a net
external force greater than
zero.
Inertia
Newton’s first law is
sometimes called the law
of inertia.
Inertia is the tendency of
an object to resist changes
in its motion.
The more mass an object
has the more inertia it has.
If it isn’t moving, it wants
to stay motionless.
If it is moving, it wants to
continue moving at the
same velocity and
direction.
Galileo Developed Inertia
Galileo, the premier
scientist of the
seventeenth century,
developed the concept of
inertia. Galileo reasoned
that moving objects
eventually stop because of
a force called friction.
Galileo’s Experiments
In experiments using a pair of
inclined planes facing each other,
Galileo observed that a ball will
roll down one plane and up the
opposite plane to approximately the
same height.
If smoother planes were used, the
ball would roll up the opposite
plane even closer to the original
height. Galileo reasoned that any
difference between initial and final
heights was due to the presence of
friction.
Galileo postulated that if friction
could be entirely eliminated, then
the ball would reach exactly the
same height.
Galileo and Motion
Galileo's reasoning
continued - if the
opposite incline was
set at a 0-degree angle,
then the ball would
roll forever in an effort
to reach the original
height.
Force Is Not Needed
Newton's first law of motion
declares that a force is not
needed to keep an object in
motion.
Slide a book across a table
and watch it slide to a rest
position.
The book in motion on the
table top does not come to a
rest position because of the
absence of a force; rather it
is the presence of a force that force being the force of
friction - which brings the
book to a rest position.
Force Is Not Needed
In the absence of a force of
friction, the book would
continue in motion with the
same speed and direction forever!
A force is not required to keep
a moving book in motion; in
actuality, it is a force which
brings the book to rest.
Inertia
Have you ever accelerated
suddenly while holding a
drink without a lid?
If you have, you have
experienced inertia first
hand ( you also got wet!)
The drink wants to stay
motionless while the car
accelerated forward.
“Objects want to keep on
doing what they are
doing”
Inertia
A car hitting a wall is
another example of
inertia.
If you don’t have a
seat belt, you want to
continue moving at the
same speed the car
was moving before it
hit the wall.
Ouch!!
Demonstrations
First Demonstration
Hand-Cinderblock-hammer
Second Demonstration
Hover-puck
Assignment
Using your new-found knowledge of forces
explain:
Why was my hand not hurt when I smashed the
cinderblock?
Explain the motion of the hover-puck. How far did it
go? Was a force needed to start the puck in motion?
Was a force needed to keep the puck moving? What
stopped the motion of the puck? How far would the
puck go if no forces were acting on it after it was
pushed?