Dynamics Concepts Practice WS Answers
1.
Gravity is the only force on the stone and it acts in the downward
direction. Thus, there is a downward net force on the stone, and it will
accelerate in that direction, according to Newton’s 2nd Law (Fnet = ma).
2.
When the stone strikes the ground, it will be brought to a sudden stop;
in other words, it will experience an upward acceleration. The forces on
the stone are the downward force of gravity and an upward force of the
ground on the stone. In order for the stone to have an upward acceleration, there must be a net force on it in the upward direction (Newton’s
2nd Law—Fnet = ma). The force of gravity plus the force of the ground
equals the net force in the upward direction, since the force of the
ground is greater than the force of gravity.
3.
The vertical forces of gravity and normal force on the puck are equal
and opposite, so they produce zero net force in the vertical direction,
thus the puck does not accelerate in that direction. The only horizontal
force is the force of friction in the direction opposing its velocity. This
force is unbalanced and produces an acceleration in that direction (2nd
Law—Fnet = ma), causing the puck to slow down and stop.
4.
The vertical forces of gravity and normal force on the box are equal and
opposite, so they produce zero net force in the vertical direction, thus the
box does not accelerate in that direction. The only horizontal force is
the force of static friction in the direction opposing its velocity (and the
direction of attempted sliding). This force is unbalanced and produces
an acceleration in that direction (2nd Law—Fnet = ma), causing the box to
slow down and stop.
5.
The vertical forces of gravity and normal force on the box are equal and
opposite, so they produce zero net force in the vertical direction, thus the
box does not accelerate in that direction. The only horizontal force is
the force of kinetic friction in the direction opposing its velocity. This
force is unbalanced and produces an acceleration in that direction (2nd
Law—Fnet = ma), causing the box to slow down as it is sliding.
6.
The winner of this race will depend on the “driver’s” ability to
accelerate their wheelbarrow, that is, to speed it up, slow it down at the
corners, and change direction at the corners. Newton’s 2nd Law states
that acceleration is inversely proportional to an object’s mass. So The
Biebs should win this race because his cart is empty (low mass) while
Taylor’s cart is full (large mass). Justin will be able to accelerate his
cart more quickly with a given amount of force than Taylor can.
7.
You will be more likely to evade the moose by following a zigzag path
than if you ran in a straight line, because the zigzag path involves many
accelerations—slowing down, changing direction, and speeding up
repeatedly. Since the moose has a very large mass, it will have difficulty
accelerating as quickly as you, as Newton’s 2nd Law states: there is an
inverse relationship between an object’s mass and its acceleration.
8.
WE DID THESE IN CLASS—LOOK AT YOUR NOTES!
9.
I would tell Ben the following: “Calm down, big fella! You’re right
about Newton’s 3rd Law saying that every force has an equal and
opposite reaction force, but you’re dead wrong about those forces
canceling each other out. The force of your hand on the ball is a force
ON THE BALL, and the reaction force to that—the force of the ball on
your hand—is a force ON YOUR HAND. These forces cannot cancel
each other because they act on 2 different objects! You should have
taken a physics class in college instead of “basket weaving for athletes”!
Now go and practice throwing that deep ball to Antonio Brown!”
10. An object’s mass has no dependence on its location, since mass is just a
measure of the amount of matter in an object. An object’s weight does
depend on its location. Since weight is just the gravitational force on an
object, weight can change with location if the strength of the gravitational field changes, such as when an object is taken to a different planet.
11. No, the velocity of an object is not necessarily in the same direction as
that of the net force on the object. For example, when an object is
thrown straight up into the air, its velocity is initially upward. The only
force on the object at that time is the downward force of gravity (as long
as air resistance is neglected), so the direction of the net force on the
object is downward. The object’s acceleration will always be in the same
direction as the net force on it, as stated in Newton’s 2nd Law of Motion.
12. No, if an object is not accelerating that just means that there is no net
force on the object. There could be forces on the object that are
balanced, such as when a book sits on a desk: the downward force of
gravity pulls on the book and the desk pushes up on the book, but these
forces are balanced, so the book will not accelerate.
13. No, a car moving at 55 mph has the same inertia as when the car is at
rest. Inertia depends solely on the mass of an object, not its speed.
14. The rope is under tension. The amount of tension force in the object is
equal to the force being applied at one of the ends of the object. Since a
scale placed in the center of the rope would indicate the amount of
tension in it, the scale would read 1500 N.
15. When a bug collides with a car, the bug exerts a force on the car (FB-C).
Newton’s 3rd Law of Motion states that there is an equal and opposite
reaction force—the force of the car on the bug (FC-B). So the forces are
equal in magnitude. Their accelerations, however, will differ. Since the
car has a much larger mass than the bug, it will have a small
acceleration compared to the bug’s acceleration, as can be shown by
Newton’s 2nd Law.
16. Tension exists in an object when it is pulled outward at both ends. The
object responds by pulling inward at each end. The tension in the object
is equal to the force being applied at one of the ends. Compression exists
in an object when it is pushed inward at both ends. The object responds
by pushing outward at each end. The compression force in the object is
equal to the force being applied at one of the ends.
17. (a) When the skydiver leaves the basket, the force of gravity is
unbalanced and pulls her in the downward direction, causing her to
accelerate in that direction at a rate of 9.80 m/s². (b) When she reaches
her terminal velocity, the upward force of air resistance is equal to the
downward force of gravity on her, so the forces are balanced and she is
no longer accelerating. She will continue to fall with a constant velocity.
(c) Before the skydiver reaches terminal velocity, the downward force of
gravity is greater than the upward force of air resistance, so there is a
downward net force on her, causing her to accelerate in the downward
direction. This net force is less than it was when she initially left the
basket, because she is moving faster now and so the force of air
resistance has increased. As she continues to fall faster, the upward
force of air resistance will keep increasing until she reaches terminal
velocity.