Download Forces and the Laws of Motion

Chapter 4, Part 1
Newton’s Laws of Motion
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Force
Force: push or pull
Force is a vector – it has magnitude and
direction
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Newton’s First Law of Motion
If you stop pushing an object, does it stop
moving?
Only if there is friction! In the absence of any
net external force, a moving object will keep
moving at a constant speed in a straight line.
This is also known as the law of inertia:
objects in motion tend to stay in motion;
objects at rest tend to stay at rest.
In order to change the velocity of an object –
magnitude or direction – a net force is required.
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Newton’s Second Law of Motion
Acceleration is directly proportional to force:
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“Forces and Motion”
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Newton’s Second Law of Motion
An object may have several forces acting on it;
the acceleration is due to the net force:
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Forces and the Laws of
Motion
•  Problem
–  A 873-kg (1930-lb) dragster, starting from
rest, attains a speed of 26.3m/s (58.9 mph)
in 0.59 s. What is the magnitude of the
average net force on the dragster during
this time?
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Forces and the Laws of
Motion
•  Solution
m = 873kg
v f − vi 26.3m / s − 0m / s
vf = 26.3m/s a =
=
= 44.58m / s 2
t
0.59s
vi = 0m/s
t = 0.59s
2
4
F = ma = (873kg )(44.58m / s ) = 3.9 x10 N
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Newton’s Third Law of Motion
For every action, there is an equal and
opposite reaction.
Forces always come in pairs, acting on
different objects. These forces are called
action-reaction pairs.
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Newton’s Third Law of Motion
Some action-reaction pairs:
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Free-body diagrams:
A free-body diagram shows every force
acting on an object.
•  Isolate the object of interest
•  Sketch the forces as vectors
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Example of a free-body diagram:
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Weight
The weight of an object on the Earth’s surface
is the gravitational force exerted on it by the
Earth.
Fw
Fw
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(g is the absolute value of Earth’s
gravitational acceleration, –9.81 m/s2)
Normal Forces
The normal force is
the force exerted
by a surface on an
object.
Fw
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Normal Forces
The normal force may be equal to, greater than,
or less than the weight.
Fw
Fw
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Fw
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Fw
Normal Forces
The normal force is always perpendicular to the
surface.
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Forces and the Laws of
Motion
•  Example Problem
If two horizontal forces of 225N and 165N are
exerted in opposite directions on a crate,
what is the net force on the crate?
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Forces and the Laws of
Motion
•  Solution
225N
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165N
Forces and the Laws of
Motion
•  Solution
60N
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Forces and the Laws of
Motion
•  Problem
On Earth, a scale shows that Ben weighs
712N. What is Ben’s mass?
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Forces and the Laws of
Motion
•  Solution
Fw = 712N
g = a = 9.81m/s2
Fw
712N
m=
=
= 72.6kg(160lb)
2
g 9.81m / s
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Forces and the Laws of
Motion
•  Problem
The acceleration of gravity on the moon is
–1.60m/s2. What would the scale indicate
that Ben weighs if he were on the moon
and his mass is the same as in the
previous question? (72.6kg)
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Forces and the Laws of
Motion
•  Solution
m = 72.6kg
gmoon = 1.60m/s2
2
Fw = mgmoon = (72.6kg)(1.60m / s ) = 116N
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Forces and the Laws of
Motion
Male lions and human sprinters can both
accelerate at about 10.0 m/s2. If a typical
lion’s mass is 170 kg and a typical sprinter’s
mass is 75.0 kg, what is the difference in the
force exerted on the ground during a race
between these two species?
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Forces and the Laws of
Motion
•  Solution
ml = 170 kg
mh = 75.0kg
a = 10.0m/s2
Flion − Fhuman
mlion a − mhumana
(170kg)(10.0m / s 2 ) − (75.0kg)(10.0m / s 2 ) = 950N
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Homework
pp. 126-127 Multiple Choice 1-11 (odd)
pp. 128-132 9, 11, 17, 29, 39, 43
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