Chapter 2 Question Set
5. What happens to the speed of an object as it falls freely? To its acceleration?
Answer:
The object’s speed increases until it reaches a terminal velocity. The acceleration remains
constant. Even though the change in velocity approaches zero, the acceleration due to
gravity is constant. It is the force due to air resistance that causes terminal velocity.
10. (a) Imagine that Charlotte drops a ball from a window on the twentieth floor of a building
while at the same time Fred drops another ball from a window on the nineteenth floor of that
building. As the balls fall, what happens to the distance between them (assuming no air
resistance)? (b) Next imagine that Charlotte and Fred are at the same window on the
twentieth floor and that Fred drops his ball a few seconds after Charlotte drops hers. As the
balls fall, what happens to the distance between them now (again, assuming no air
resistance)?
Answer:
(a) The distance between them remains the same since the acceleration due to gravity
(9.8 m/sec2) is the same for both and the time they fall is the same for both. (distance
= ½ (acceleration X time2).
(b) The distance between the balls increases with Charlotte’s ball going faster than Fred’s
at any given point in time. For example, if Fred drops his ball two seconds later than
Charlotte, her ball will already be going at a velocity of (v = at: (9.8m/sec2) X 2 sec =
19.6 m/sec). After the third second, Fred’s ball will be going 9.8 m/sec while
Charlotte’s ball will now be traveling at velocity (9.8 m/sec2) X 3 sec = 29.4 m/sec.
The earlier the ball is dropped, the longer time there is for acceleration due to gravity
to affect it’s speed.
15. Distinguish between mass & weight.
Answer: (p 59)
Mass may be thought of as a quantity of matter. The weight of an object is the gravitational
force with which the earth attracts its mass.
20. Two children wish to break a string, Are the more likely to succeed if each takes one end
of the string and they pull against each other, or if they tie one end of the string to a tree and
both pull on the free end? Why?
Answer:
Neither. The apply the same force on the string whether they each exert a force against the
string in opposite directions or they both exert a force against the string in the same direction.
In the second scenario, the tree is exerting the force in the opposite direction for them.
25. A track team on the moon could set new records for the high jump or pole vault (if they
did not need space suits, of course) because of the smaller gravitational force. Could
sprinters also improve their times for the 100-m dash?
Answer:
No. Their masses remain the same on the moon as on the earth. The force their legs could
exert would be unchanged so their acceleration due to that force would be unchanged. They
would not be able to run faster.
30. According to the theory of gravitation, the earth must be continually “falling” toward the
sun. If this is true, why does the average distance between the earth and sun not grow
smaller?
Answer:
It does not grow smaller because the orbital velocity of the earth around the sun keeps trying
to move the planet away from (tangential to) the sun.
35. An airplane makes a vertical circle in which it is upside down at the top of the loop. Will
the passengers fall out of their seats if there is no belt to hold them in place?
Answer:
No. Because they and the airplane are “falling” at the same rate.