cerutti (cpc566) – HW03 – TSOI – (58160)
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001
(part 1 of 3) 10.0 points
Consider the following statement made re-
garding a book at rest on a level table:
The two forces exerted on the
book are the normal force directed
up and the weight of the book di-
rected down.
These are equal and
opposite to one another.
By New-
ton’s third law they are an action-
reaction pair, so the normal force is
always equal to the weight of the
book.
Do you agree with the statement?
1.
Agree; a normal force only exerts enough
force to keep the object from falling through.
The normal force and weight are always equal,
even when other forces are present.
2.
Agree; for every action there is an equal
and opposite reaction (Newton’s third law).
Since gravity pulls down on the book, to be
“equal and opposite” the force of gravity must
equal the normal force.
3.
Disagree; if an additional force acts down
on the book,
the normal force must also
counter this extra force.
correct
4.
Disagree;
the weight must be slightly
greater than the normal force to keep the book
in contact with the table. However, the forces
are an action-reaction force pair because they
are acting on one object.
Explanation:
If we were to put a weight on the book, the
normal force would not be simply the weight
of the book as the statement implies (see part
2).
Note:
This force pair is
not
a third law
action-reaction pair. The force pairs in New-
ton’s third law always act on two different
objects!
The action-reaction pair is ”Earth
exerts a force on the book” and ”the book
exerts a force on the Earth.”
Caution:
The explanation introduces some-
thing not mentioned in the problem.
002
(part 2 of 3) 0.0 points
Consider a book on top of a level table while
the book is being pressed
straight down
with
a force
F
by your hand.
Book
F
The following figures show several attempts
at drawing free-body diagrams for the book.
Which figure has the correct directions for
each force?
Note:
The magnitude of the forces are not
necessarily drawn to scale.
1.
normal
gravitational
hand
2.
gravitational
hand
normal
3.
gravitational
hand
normal
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cerutti (cpc566) – HW03 – TSOI – (58160)
2
4.
hand
normal
gravitational
5.
hand
normal
gravitational
6.
normal
gravitational
hand
correct
Explanation:
The normal force opposes both the gravita-
tional force down and the
F
hand
down.
003
(part 3 of 3) 0.0 points
What forces change when comparing the free
body diagram before the hand was placed on
the book to after?
1.
normal
2.
F
hand
3.
gravitational, normal, and
F
hand
4.
gravitational and normal
5.
gravitational and
F
hand
6.
normal and
F
hand
correct
7.
None of these
8.
gravitational
Explanation:
There will be a change in both the nor-
mal force and the
F
hand
.
When the hand
is removed, the normal force is equal to the
weight of the book.
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- Spring '08
- Turner
- Physics, Force, ty, Ty Mb Ma Tx
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