Download 4.1 Force

Physics 114 – Lecture 7
• Chapter 4 Dynamics: Newton’s Laws of Motion
• We now investigate why bodies undergo motion.
• Why do bodies accelerate? They do so under the
action of applied unbalanced forces.
• What is a force?
• Examples of a force – friction, gravity, a force exerted
by a spring, etc.
• Forces can be exerted by an inanimate object, e.g., by
a hammer striking a nail
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Physics 114 – Lecture 7
• §4.1 Force
• Pushing or pulling an object, lawn mower, wheelbarrow,
etc., requires an applied force, magnitude and direction.
• Magnitude – one way is to use a calibrated spring scale
• Force also has an associated direction
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Physics 114 – Lecture 7
• §4.2 Newton’s First Law of Motion
• Relationship between force and motion?
• Aristotle (384-322 B.C.) believed that a force was
necessary to maintain a body in constant motion on a
horizontal surface. Aristotle maintained that the greater
the speed the greater the required force
• Galileo, in about 1630, about 2000 years later,
disputed this suggestion, arguing that it was just as
natural for a body to move with constant velocity –
magnitude
 and direction – as it was to be at rest. Note
that v= 0 is a reasonable value for a constant velocity
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Physics 114 – Lecture 7
• Consider a hockey puck moving across a rough
surface with some initial velocity. How far does it
travel before coming to rest – note that it travels in a
straight line?
• Now consider the puck moving over the surface to
which some lubricant has been applied. → It will
travel further before coming to rest.
• Now consider a puck moving across smooth ice. → It
will travel still further.
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Physics 114 – Lecture 7
• What is the factor that has been influencing these
changes?
• We now recognize this factor to be a progressive
reduction in the force of friction between the body
and the surface.
• Galileo considered that, in principle, friction between
the surface and the puck might be eliminated
altogether. In such a situation no unbalanced force
acted on the body, which would then maintain its
motion with constant velocity, until it was disturbed,
i.e., an unbalanced force was then applied to the body
to change its state of motion.
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Physics 114 – Lecture 7
• Newton’s First Law
• Any object continues in its state of rest, or in a state of
uniform velocity – in a straight line – provided that no
unbalanced force acts on it
• Inertial Reference Frames
• Reference frames, in which Newton’s First Law holds,
are known as inertial reference frames
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Physics 114 – Lecture 7
• §4.3 Mass
• Mass?
• Newton used the term to describe the quantity of
matter in a body. It is better used as a measure of the
inertia of a body. The larger the mass, the larger is the
force required to give it with a particular acceleration.
• Units of m? in the SI system the unit of mass is the
kilogram – kg.
• Note that mass and weight are distinct quantities
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Physics 114 – Lecture 7
• §4.4 Newton’s Second Law of Motion


F  ma

• F is the net or total force acting on the body
• Notice that this is a vector equation where a vector, the
acceleration, is multiplied by a scalar, the mass. Thus
the force and the acceleration act in the same direction
• We see that if the mass is held constant then the
acceleration is proportional to the force and in the
same direction as the force
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Physics 114 – Lecture 7
• If the force is held constant then the magnitude of the
acceleration is inversely proportional to the mass, but
the force and the acceleration still act in the same
direction
• Units of force? In the SI system the units of force are
the newton ≡ kg m/s2
• Notice that Newton’s First Law is a special case of
Newton’s Second Law, because velocity = 0 or v =
constant both require acceleration = 0 → F = ma = 0
• Lets look at some examples

• Note – F  m a → Fx = max, Fy = may
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