Transcript FORCES - Mr. Maloney

Apparent weight/
normal force
Physics
Mr. Maloney
Objectives
You will be able to …
 describe how an inanimate object can
apply a force
 apply Newton’s 2nd Law to determine an
object’s apparent weight (Normal Force)
 solve problems involving apparent
weight.
© 2002 Mike Maloney
The “Normal” Force
 How
does the table know how hard to
push to hold something up?
© 2002 Mike Maloney
The “Normal” Force

Surfaces are able to
apply a surface normal
force because they
deform, if only slightly.
 This deformation
stretches or
compresses molecular
bonds, which in turn
apply a force trying to
get the molecular
arrangement back into
place.
© 2002 Mike Maloney
Apparent weight

We have already said that Weight (Fg) = mg,
but when we talk of weight in everyday life what
do we really mean?
 What happens to your weight in a elevator?
 Or when you are on a hill?
 Or when someone pushes on your shoulders?
 Or when you are carrying a lot of books?
 Or after you have jumped out of an airplane?
 The weight we feel, or our APPARENT WEIGHT is
another name for the NORMAL FORCE on us.
 This is also the force that a bathroom scale reads.
© 2002 Mike Maloney
Normal Force [FN]

For example, the
normal force of a book
sitting on a table is the
force exerted on the
book by the table.
 According to Newton’s
2nd law, this is equal to
its weight at rest in this
situation.
The table pushes
up on the book.
The weight of the book
pushes down on the table
© 2002 Mike Maloney
FN
Fg
Normal Force [FN]

What would happen if
we added a mass on
top of the book?
 The normal force
would increase to
balance out both the
book and the mass’
weight.
 Lets test with a few
students
The table pushes
up harder now.
FN
Fg, book
© 2002 Mike Maloney
Fg, mass
http://mrmaloney.com/mr_maloney/simulations/jar-files/ejs_mem2_elevator4.jar
Normal Force on Elevator
Think about this situation … work in groups
Picture yourself as a penguin riding on an
elevator. What would your apparent weight be
under the following six (6) conditions?
1. The Elevator is at Rest.
2. The Elevator is rising at a constant velocity
3. The Elevator is accelerating up
4. The Elevator is accelerating down
5. The Elevator is in free fall
6. The Elevator is accelerating sideways
© 2002 Mike Maloney
Normal Force [FN ] (CONTD.)
 We
said the normal force on a book on a
table is equal to its weight, and if you
pushed on it or laid a mass on top of the
book the normal force would increase.
 What would happen to FN if the book were
on a hill or a ramp?
© 2002 Mike Maloney
Normal Force [FN] (CONTD.)
 Lets
go back to our block on an incline.
 What forces are acting on the block if
we don’t include friction?
© 2002 Mike Maloney
Normal Force [FN] (CONTD.)
 Right
if friction is
negligible, only
gravity and FN are
acting on it.
 But how big is the
normal force? How
can we find out?
FN
Fg
© 2002 Mike Maloney
Normal Force [FN] (CONTD.)
 Part of the Weight is acting
down the incline {Fgx}
 Part is acting into the
incline {Fgy}
 Which one acts in the same
direction as the normal
force?
 Right, Fgy
 And how do we find Fgy
with respect to the weight?
© 2002 Mike Maloney
FN
30°
Fgy
Fg
Fgx
Normal Force [FN] (CONTD.)
 From our vector lessons,
Fgy = Fg ∙ cos(θ)
 In this case
 Fgy = Fg ∙ cos(30)
= Fg ∙(0.87)
= 0.87 ∙ Fg
 Which corresponds with
our theory that it is only
part of the weight.
 Sim of Block on Hill
© 2002 Mike Maloney
FN
30°
Fgy
Fg
Fgx
Objectives
Can you …
 describe how an inanimate object can
apply a force
 apply Newton’s 2nd Law to determine an
object’s apparent weight (Normal Force)
 solve problems involving apparent
weight.
© 2002 Mike Maloney