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Copyright © 2010 Pearson Education, Inc.
Chapter 4, Part 1 Newton’s Laws of Motion
Copyright © 2010 Pearson Education, Inc.
Force
Force: push or pull
Force is a vector – it has magnitude and direction
Copyright © 2010 Pearson Education, Inc.
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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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?
Copyright © 2010 Pearson Education, Inc.
Forces and the Laws of Motion
• Solution m = 873kg vf = 26.3m/s vi = 0m/s t = 0.59s
a =vf − vit
=26.3m / s− 0m / s
0.59s= 44.58m / s2
NxsmkgmaF 42 109.3)/58.44)(873( ===
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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.
(g is the absolute value of Earth’s gravitational acceleration, –9.81 m/s2)
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Normal Forces
The normal force is the force exerted by a surface on an object.
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Normal Forces The normal force may be equal to, greater than, or less than the weight.
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Copyright © 2010 Pearson Education, Inc.
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 165N
Copyright © 2010 Pearson Education, Inc.
Forces and the Laws of Motion
• Solution
60N
Copyright © 2010 Pearson Education, Inc.
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 F = 712N g = a = 9.81m/s2
m =Fg=
712N9.81m / s2
= 72.6kg(160lb)
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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
F =mgmoon = (72.6kg)(1.60m / s2 ) =116N
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Forces and the Laws of Motion
• Problem – Male lions and human sprinters can both
accelerate at about 10.0 m/s2. If a typical lion weighs 170 kg and a typical sprinter weighs 75.0 kg, what is the difference in the force exerted on the ground during a race between these two species?
Copyright © 2010 Pearson Education, Inc.
Forces and the Laws of Motion
• Solution ml = 170 kg mh = 75.0kg a = 10.0m/s2
(170kg)(10.0m / s2 )− (75.0kg)(10.0m / s2 ) = 950N
humanlion FF −
amam humanlion −
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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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