PEGGY’S PHYSICS HOMEWORK.
Ch. 4 Questions
10) The heavier cart's acceleration is two times slower than the lighter cart. This
is because it is two times heavier than the lighter cart, and according to Newton's
second law, acceleration is inversely proportional to mass.
11) The amount of friction the floor exerts on the crate depends on the kinds of
material both are made of and how they are pressed together. The friction may
or may not be equal to the 200N exerted onto the box, but it is oppositely
directed to the 200N push. The reaction to my push is the crate's push on me, not
the friction.
12) The magnitude of both forces is the same, however, the small sports car
suffers more damage. It also has a much greater acceleration because it has less
mass than the truck.
16) a. Since I can't draw because I'm answering on a wiki, I'll just say that there's
supposed to be another arrow pointing up, representing the support force.
b. Support force, or Normal Force.
17)
a. There's the upward tension in the string and the downward pull of gravity on
its weight.
b. Yes.
c. Because the magnitude of the upward tension in the string should equal the
weight of the bag.
18)
a. There's the downward pull of gravity and the upward pull of a force. The
arrow pointing up is greater than the arrow pointing down.
b. The force the accelerates the stone upward is greater than the force of gravity
- it has to be in order to allow the upward acceleration of the stone.
1) D
2) B
3) B
4) C
5) C
6) A
7) A
8) C
9) A
10) D
How My Toy Car Is Related to Concepts In Chapter Three
Chapter 3 talks a lot about acceleration, net force and mass, and what relates all
of these things. When I think about how it relates to my toy car, I think about the
factors that influence its acceleration and therefore, speed. First of all, the mass
of my toy car - the heavier my car is, the harder it'll be for it to accelerate.
Secondly, there's the matter of net force. There's the force of gravity and then the
support force of the floor. Then there's the force of the motor that's acting on the
car. All of these forces add up to the net force, and the net force helps determine
the rate of acceleration. All of these things are factors that determine my car's
acceleration. Thus, these are all things I need to keep in mind to get the most
effective results out of my toy car.
Determining The Acceleration Of My Toy Car
To determine the acceleration of my toy car, I took the information from
LoggerPro and copied it onto Excel. From there, I created a graph that
represented the speed of my car, and looked at the form of the scattered points.
The graph is as follows:
Following that, in order to estimate the maximum acceleration, I took a segment
of the points I found to be the most steep [to represent the highest acceleration
rate], which was the segment from 1 to 2.5 seconds. I also estimated the speeds
[as I wasn't sure of the unit of speed, I guessed that it was meters per second] of
the point at that time. For the 1 second mark, I estimated the speed to be 0.5mps,
for the 2.5 seconds mark, I estimated the speed to be 1.1mps. From there, I
divided the differences of the times by the differences of the speeds. The
equation goes as follows:
(1.1 - 0.5) / (2.5 - 1)
= 0.6 / 1.5
= 0.4
Thus, my estimate of the maximum acceleration is 0.4m/s/s, or in other words,
0.4
m/s squared.
p36 Questions
14) A different scaffold that weighs 300 N supports two painters, one 250 N and
the other 300 N. The reading on the left scale is 400 N. What is the reading on the
right scale?
The reading on the right scale is 450 N. This is because the total gravitational
force adds up to be 850 N, so in order to achieve mechanical equilibrium, the
upward force must be 850 N as well. In order to achieve this, the right scale must
be 450 N to make the upward force a total of 850 N, thus canceling out each
other to achieve equilibrium.
15) Nellie Newton hangs at rest from the ends of the rope as shown. How does
the reading on the scale compare to her weight?
The reading on the scale is the same as her weight. This is because the upward
tension in the ropes is the same magnitude as her weight.
16) Harry the painter swings year after year from his bosun's chair. His weight is
500 N and the rope, unknown to him, has a breaking point of 300 N. Why doesn't
the rope break when he is supported as shown at the left? One day Harry is
painting near a flagpole, and, for a change, he ties the free end of the rope to the
flagpole instead of to his chair as shown at the right. Why did Harry end up
taking his vacation early?
In the diagram on the right, there are two ropes supporting Harry instead of one,
increasing the breaking point to 600 N, sufficient enough to support his weight of
500 N. In the diagram to the right, however, there is only one rope supporting
him, because he tied the other end to the flagpole. This lowers the breaking point
back down to 300 N, not enough to support Harry, and as a result, the rope
breaks and he falls.
p35 Questions
4) If a huge bear were chasing you, its enormous mass would be very
threatening. But if you ran in a zigzag pattern, the bear's mass would be to your
advantage. Why?
Its larger mass makes the bear less agile than me, which makes it easier for me to
get away. The bear's speed would also decrease if I ran in a zigzag pattern, since
it's more difficult to travel fast with a large mass.
6) Consider a ball at rest in the middle of a toy wagon. When the wagon is pulled
forward, the ball rolls against the back of the wagon. Interpret this observation
in terms of Newton's first law.
Newton's first law states that every object continues in a state of rest, or in a
state of motion in a straight line at constant speed, unless acted on by a nonzero
net force. In this situation, the ball and the wagon are in a state of rest. However,
when the wagon is pulled, it is acted on by a nonzero net force, thus moving both
it and the ball contained inside.
7) Why do you lurch forward in a bus that suddenly slows? Why do you lurch
backward when it picks up speed? What law applies here?
Newton's first law applies in this situation. When I'm sitting in the bus, I am in
constant motion; the movement of the bus was the force that set me in motion.
Thus, when the bus slows suddenly, I lurch forward because I am still in constant
motion while the bus is not. Vice-versa, when the bus picks up speed, I lurch
backward because I continue to be in what is similar to a state of rest when the
bus suddenly changes.