Download KEYPhysics SP09 Inv-7 ExpanIV (WP)

Physics
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Inv-7 Expan. IV False Statements Regarding Newton’s Laws
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Each of the following statements has 1, 2 or 3 conceptual flaws. In complete sentences, explain what is wrong with
each statement.
1.) A rocket moves through space because the exhaust from the rocket’s engine pushes against the atmosphere.
A rocket moves through space because the rocket pushes against the gases it emits and Newton’s 3rd law is in effect.
The escaping (and usually burning) gases become the needed second body. It can also be considered a recoil situation in
which the overall momentum of the system is conserved. Therefore, the rocket does not need the atmosphere to push
off of.
2.) If an unbalanced force in opposition to the motion of an object were applied for a sufficiently long time, the inertia
of the object would reach zero.
As long a a body has mass it will have inertia. I have a hard time thinking of anything without this quality of
having a resistance to a change in it’s motion. If an unbalanced force in opposition to the motion of a object were
applied for a sufficiently long time, the MOMENTUM of the object would reach zero since momentum is MOVING
inertia (p = mv)
3.) The number of oscillations made by an inertial balance in 8 seconds is inversely proportional to the horizontal
component of the weight placed on the tray of the balance.
There is no horizontal component of weight. The number of oscillations made by an inertial balance in 8 seconds is
inversely proportional to the mass placed on the tray of the balance. Mass has no horizontal component since it is a
scalar and not a vector. In other words, the mass is a property of a substance and properties have no particular
direction. For instance, I am extremely good looking and very muscular. These are PROPERTIES of my body and my
handsomeness and have no direction. (Just checking to see if you were still reading this.)
4.) According to Newton’s 3rd Law of Motion, if a small steel ball were thrown horizontally and bounced off the center
of a large steel ball, which had been sitting motionless on a frictionless surface, both balls would accelerate at the
same rate, but in opposite directions, after the impact.
It is true that the forces exerted by the balls are equal and opposite (Newton's third law); however, according to
Newton’s second law, F=ma, the different masses of the balls would cause them to have different accelerations
inversely proportional to their masses.
5.) The force due to gravity is the same on all objects near the surface of the earth.
The FORCE due to gravity is mass times the acceleration due to gravity (g), so objects with different masses have
different forces due to gravity. If the statement said that the ACCELERATION due to gravity was about the same on
all objects near the surface of the earth, then it would have been correct.
6.) The apparent snapping back of my head, when I throw the trunk of my body forward is a physical reality of
Newton’s 3rd law of motion.
The apparent snapping back of my head when I throw the trunk of my body forward is a physical reality of Newton’s
1st law. This can not be Newton’s third law because there is only one body.
7.) An object will accelerate when any force acts on it, and that acceleration will be directly proportional to the mass
of the object, according to Newton’s first law of motion.
An object will accelerate when AN UNBALANCED, EXTERNAL force acts on it, and that acceleration will be
INVERSELY proportional to the mass of the object according to Newton’s 2ND LAW of motion.