Transcript ppt - MrMaloney.com

FORCES
episode III
The return of Newton
Physics
Mr. Maloney
Objectives
You will be able to …
 describe what happens when all forces are
balanced out.
 apply concepts of equilibrium to problems.
 observe and describe the consequences of
Newton’s 1st Law.
© 2002 Mike Maloney
Inertia questions



I put a book on a table and what happens?
I slide a puck across the ice what happens?
An astronaut gets pushed away from the shuttle
out in deep space what happens?
 A magician pulls a table cloth out from under
some plates and glasses, what happens?

In all these cases the motion, whether the object
is moving or at rest, stays constant.
 How can this be .. aren’t there forces?
© 2002 Mike Maloney
Net force is zero?
 What
if the net force acting on an object
is zero?
 This
could mean there are no forces acting
on the object
 Or the forces cancel each other out.
© 2002 Mike Maloney
Newton’s 1st Law (Inertia)
 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 NET force.
 It describes what happens when there
is NO NET force acting on something.
© 2002 Mike Maloney
Newton’s 1st Law
MATHEMATICALLY
 AT
REST TENDS TO STAY AT REST
Velocity
=0
No net outside force [F=0]
Acceleration (change in v) = 0
Object stays stationary
© 2002 Mike Maloney
Newton’s 1st Law MATHEMATICALLY (part2)
 IN
MOTION STAYS IN MOTION …
Velocity
≠0
No net outside force [F=0]
Acceleration (change in v) = 0
Object is in constant velocity motion
(speed and direction)
© 2002 Mike Maloney
Newton’s 1st Law
(Consequences)
 An
object at rest will stay there if you do
not disturb it
 A moving object’s motion will not change
(no a, no v and no change in direction) if
there is no outside net force acting on it.
 When the net force acting on something is
zero, the object is said to be in equilibrium.
 Where have you heard that before?
© 2002 Mike Maloney
Newton’s 1st Law
(Consequence Examples)
 Pulling
table cloth off table.
 Card flick
 Hammering head with books
 Car crash into a wall
 How is Homer’s crash messed up?
 Dancing Ball-Guy
© 2002 Mike Maloney
Extra Note on newton’s 1st law


Another consequence of Newton’s first law is that there is
no difference between constant velocity motion and no
motion … they both are cases of 0 acceleration.
This seems weird, but think about driving in a car.




When you are at rest sitting in the seat you don’t really feel
anything.
When you speed up you can “feel” the force speeding you up as
the seat pushes on you.
But when you get to 55 and set the car on cruise control, you do
not feel that force anymore. You are moving at a constant
velocity and no force is need to keep you moving.
In a force sense, 0 velocity is just another type of
constant velocity, nothing special about it.
© 2002 Mike Maloney
Extra Note on newton’s 1st law


Another consequence of Newton’s first law is that there is
no difference between constant velocity motion and no
Newton’s
1st0law
is really just a
motion…
they are both
acceleration.
This seems
about driving
a car.
specialweird,
casebutofthink
Newton’s
2nd inLaw,




When you are at rest sitting in the seat you don’t really feel
anything.
When you speed up you can “feel” the force speeding you up as
the seat pushes on you.
But when you get to 55 and set the car on cruise control, you do
not feel that force anymore. You are moving at a constant
velocity and no force is need to keep you moving.
when the acceleration is equal to 0
and the net force is also 0.
In a force sense, 0 velocity is just another type of
constant velocity, nothing special about it.
© 2002 Mike Maloney
Objectives
Can you …
 describe what happens when all forces are
balanced out.
 apply concepts of equilibrium to problems.
 observe and describe the consequences of
Newton’s 1st Law.
© 2002 Mike Maloney
APPENDIX
© 2002 Mike Maloney
Inertia
 The
tendency of an object to resist a
change in motion is called its inertia.
 Objects with greater masses generally
have greater inertias.
 For our purposes, an object’s inertia is
basically measured by it mass.
 BACK
© 2002 Mike Maloney
Net Force

NET FORCE refers to the vector sum total
of all forces acting on an object. It is often
expressed as F
 For example, if there were two leftward forces
of 10 lb each, the NET FORCE would be 20
lb leftward.
 If there were one 10 lb rightward force and
one 8 lb leftward force, the NET FORCE
would be 2 lb rightward.
 What about if the forces were in X and Y?
 BACK
© 2002 Mike Maloney
Applied Force
 APPLIED
FORCE refers to a generic
force that is acting on an object.
 An APPLIED FORCE is basically any
force that is not a WEIGHT, NORMAL
FORCE, FRICTIONAL FORCE,
SPRING FORCE, or other named force.
 BACK
© 2002 Mike Maloney