

Physics Demystified, 2nd edition 
Stan Gibilisco 

Explanations for Quiz Answers in Chapter 2 

1. A vector quantity always possesses two independent
characteristics: magnitude and direction. Of the four choices given here,
only velocity (the composite of speed and direction) qualifies.
Temperature, visiblelight intensity, and mass all need only the
expression of magnitude to be fully defined.
The correct choice is C. 

2. Our ship isn't accelerating, and the mass that
we see isn't accelerating relative to us. From our point of view, therefore,
no force whatsoever acts on the external object. It's coasting through space freely, just as we are. The
correct choice is D. 

3. In effect, the earth is in a
perpetual state of free fall, constantly "pulled" on by the sun's
gravitation. As the earth orbits the sun, its acceleration vector points
straight toward the sun at all times, and maintains a magnitude equal to
the gravitational acceleration of the sun at the earth's distance. If the
earth suddenly stopped revolving around the sun, the sun's gravitation
would continue to act, "pulling" the earth straight toward the sun at an
everincreasing speed equal to the sun's gravitational acceleration. The correct choice is C. 

4. Newton's first law tells us that an object
moving at a specific velocity will keep moving at that velocity unless an
outside force acts on it. When you powerdown the boat's motor, the only
force acting on the boat is its own friction with the water, which will
eventually bring the boat to a stop. Unfortunately for you, this friction
has a negligible effect over the short distance between you and the dock;
your boat might as well be sliding along a frictionless surface!
You've just seen a vivid demonstration of Newton's first law in action.
The correct choice is
A. 

5. At any given instant in time, a velocity
vector comprises instantaneous speed and instantaneous direction.
Those are the only two parameters that we must specify; nothing else
has any relevance. The correct choice is C. 

6. In threedimensional (3D) space, we can define
the magnitude of a vector in terms of the length of the line segment
representing it. The type of coordinate system makes no difference. The
correct choice is D. 

7. A kilogrammeter per second squared (1 kg
· m · s^{2}) equals one newton (1 N) by definition. A gram is 1/1000 of
a kilogram. Therefore, a grammeter per second squared (1 g · m ·
s^{2}) equals 1/1000 of a newton (0.001 N). The correct choice is
C. 

8. On the planet we're
visiting, the gravitational acceleration is 5.67 m · s^{2} (which we can also denote as 5.67 m/s^{2}). If we drop
an object and let it fall freely, and assuming that the atmosphere has no effect on the object's
acceleration, then the object will
accelerate downward at a rate of 5.67 m/s^{2} regardless of its mass. After one second
(1.00 s) of time, the object will have attained an instantaneous speed of
5.67 m/s. After 2.00 s, it will have gained another 5.67 m/s of speed, so
it will be falling at 5.67 + 5.67 = 11.34 m/s. We should round this value
off to three significant figures, getting 11.3 11.3 m/s. The correct
choice is D.


9. Assuming that the atmosphere has no effect on
the second brick, it will accelerate downward at exactly the same rate as
the first brick did, even though the second brick has twice the mass of
the first one. If we had dropped both bricks side by side, they'd have
fallen right along with each other at identical instantaneous speeds all
the way down until impact (conducting an extraterrestrial rerun of
Galileo's famous 17thcentury experiment in Pisa). The correct
choice is A. 

10. If you hit the brakes while driving a vehicle
along a straight, level highway, the vehicle will (if the brakes work
properly) lose speed but keep going in the same direction. The velocity
vector will therefore maintain the same direction but grow smaller in
magnitude. The correct choice is B. (Don't get fooled by the wording of
this question. It asks what happens to the velocity vector, not the
acceleration vector!) 
