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5.1.3 Dynamics
You should be able to:
(a) demonstrate an understanding that mass is the property
of a body which resists change in motion.
(b) define and use the equation density = mass/volume.
(c) recall and use the equation F = ma, in situations where
mass is constant, appreciating that force and acceleration
are always in the same direction.
(d) define the newton.
(e) describe and use the concept of weight as the effect of
a gravitational field on a mass.
(f) recall and use the relationship weight = mass x
gravitational field strength.
(g) describe qualitatively the motion of bodies falling in a
uniform gravitational field with fluid
resistance.…read more

Slide 2

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How is Mass different from Weight?
Mass Weight
·The mass of an object refers ·The weight of an object is the
to the amount of matter that is force of gravity acting upon that
contained by the object. object.
·Mass is related to "how much ·Weight is related to the pull of
stuff is there". the Earth (or any other planet).
·The mass of an object ·Weight changes according to
(measured in kg) will be the same the gravitational field
no matter where in the universe
that object is located.…read more

Slide 3

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Mass and Inertia
Newton's First Law of Motion states that
"An object at rest tends to stay at rest and an object in
motion tends to stay in the same state of motion unless
acted upon by an unbalanced force.
" Objects "tend to keep on doing what they're doing."
This tendency to resist changes in their state of motion is
described as inertia.…read more

Slide 4

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Newton and Galileo
The dominant thought prior to Newton's day was that it
was the natural tendency of objects to come to a rest
position; a force was necessary to keep an object
Galileo, before Newton, developed the concept of
inertia. Galileo reasoned that moving objects eventually
stop because of a force called friction.
Homework activity:
Research how Galileo developed the concept of inertia.
(Use keywords Galileo, inertia and friction in a web
search.)…read more

Slide 5

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Imagine a place in the cosmos far from all
gravitational and frictional influences.
Suppose that an astronaut in that place throws a
rock. The rock will:
According to Newton's
first law, the rock will
continue in motion in
the same direction at
constant speed.
a) gradually stop.
b) continue in motion in the same direction at
constant speed.…read more

Slide 6

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A 0.5 kg object is moving horizontally with a speed
of 4 m/s. How much net force is required to keep
the object moving at this speed and in this
Answer: 0 N
An object in motion will maintain its state of
motion. The presence of an unbalanced force
changes the velocity of the object.…read more

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