Chapter 3: Force and Motion

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Chapter 3 Force and Motion
The First Law of Motion An object at rest will
remain at rest and an object in motion will
remain in motion (at constant velocity) unless
acted upon by an external force. Forces are the
causes of changes in motion. forces on an
object arise from interactions with other
objects. The inertia of an object is its
resistance to changes in its motion. Mass is a
measure of inertia.
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Mass as a measure of inertia two interacting
objects, initially at rest
vA
vB
B
B
A
A
relative changes in motion vA /vB mB /mA
Units kilogram kg (other units slugs,
sometimes pounds mass) 1kg 2.2 lb.
Force the influence which causes change in
motion change in velocity change in speed or
direction A net force (or unbalanced force)
creates changes in motion If the resultant of
all forces acting on an object is zero, there is
no change in motion the object is in equilibrium.
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The Second Law of Motion The net force acting on
an object equals the product of the mass and the
acceleration. F m a (net force)
(mass)(acceleration) Units 1 Newton 1N 1 kg
m/s2 1N 1/4 pound
Example 3.1 A 60 g tennis ball approaches a
racket at 30 m/s, is in contact with the racket
for 5 ms and then rebounds at 30 m/s. What is
the average force the racket exerts on the ball?
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Example 3.2 A human cannon ball with mass 70 kg
is fired from a compressed-air cannon whose
barrel is 20m long. He emerges from the cannon
at 40 m/s. Find his average acceleration and the
force exerted on him during the firing of the
cannon.
Example 3.3 A horizontal force of 10 N is
applied to a 4.0 kg block which is initially at
rest on a smooth level surface. Find the speed
of the block and its displacement after it has
gone for 6.0 s.
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Weight the force exerted by earth (via gravity)
on an object In free fall, gravity is the only
force acting on the object F ma mg w Weight
(mass)(acceleration of gravity)
Example 3.4 A loaded elevator whose total mass
is 800 kg is suspended by a cable whose maximum
permissible tension is 20,000 N. What is the
greatest upward acceleration possible? What is
the greatest downward acceleration? (draw
free-body diagrams)
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The third law of motion action/reaction When an
object exerts a force on a second object, the
second object exerts a force on the first of
equal magnitude, but opposite direction. For
every action there is an equal but opposite
reaction. Forces arise from interactions!
Forces on an Object
Action/Reaction
reaction force of table on computer
reaction force of table on computer
force of computer on table
weight of computer
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TFB
mB 30kg
()
B
()
T
A
mA 15kg
wA -TFA mA g-T
w
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T
()
wB-TFB
T-wAFA
()
w
A
B
mA 15kg
mB 30kg
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Friction opposes motion due to surfaces sticking
together Kinetic Friction surfaces are moving
relative to each other a.k.a. Sliding Friction
Static Friction surfaces are not moving
relative to each other. Static Friction prevents
stationary objects from moving until sufficient
force has been applied.
Friction
Applied Force
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Coefficient of Friction Frictional forces depend
upon how hard the surfaces are being pressed
together -gt force perpendicular to the surface
normal force the types of surfaces that are in
contact -gt coefficient of friction
Material static ms kinetic mk wood on
wood 0.5 0.3 wood on stone 0.5
0.4 steel on steel (smooth) 0.15 0.09
rubber tire on dry concrete 1.0 0.7
rubber tire on wet concrete 0.7 0.5
steel on Teflon 0.04 0.04
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Example 3.11 A 100 kg wooden crate is at rest on
a level stone floor. (a) What is the minimum
horizontal force needed to start the crate
moving? (b) What is the minimum horizontal
force needed to keep the crate in motion at
constant horizontal speed? (c) What will happen
if a horizontal force of 500 N is applied to the
crate?
Example 3.14A wooden chute is being built along
which wooden crates are to be slid down at
constant speed. (a) What angle should the chute
make with respect to the horizontal? (b) What
force needs to be applied to a 100 kg crate to
start it sliding down this chute?
FN
Ff
mg sinq
q
mg cosq
q
mg