# HTW Fall 2007 MidTerm A Answers _08-34_

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```					Phys 3550 How Things Work
Fall 2007
1st Midterm (A) – Tuesday October 23rd 2007

Concepts and Definitions                              8. A car is traveling east. What would happen
if
1. What is Newton’s 3rd Law of Motion? - For          1. there is a (net) force to the east?
every action there is an equal an opposite         2. there is a (net) force to the west?
reaction                                           3. there is a (net) force to the north?

2. What is the Principle of Conservation of           In each case, does the change in kinetic
Energy? - Energy cannot be created or              energy of the car increase, decrease or stay
destroyed                                          the same?
1. Since the force is in the same direction
3. What is the term given to the energy               as the motion the car speeds up. KE
associated with vertical position? -               increases
gravitational potential energy                     2. Since the force is in the opposite
direction to the motion the car slows
4. When is it possible to say that the velocity       down. KE decreases
of a car is 30 mph? - Never. 30 mph is the         3. This the force is perpendicular to the
speed. To get velocity you would also need         motion. There is no change in speed, but
direction                                          the car will steer to the north (change of
direction. The KE is unchanged.
5. Can an object have a constant speed and
also have an acceleration? Under what           9. If you were told that a car has traveled 16
conditions might this occur? - Yes, when           miles in ¾ hour, which quantity would you
changing direction                                 be able to deduce?
1. Its average acceleration
6. Imagine that you have stone tied to a              2. Its average speed
string, and you are swinging the stone             3. Its average velocity
around your head in a horizontal circle.           4. Its displacement (change in position)
1. Is the stone accelerating? Why? - Yes,
because the velocity (direction) is            This is really a definition question. You are
constantly changing                            told a speed (a scalar) and time (a scalar).
2. Is there a force on the stone. If so            You have no directional information at all.
where does the force come from? -              From that you can only calculate another
From Newton's 2nd Law, there must be a         scalar, and so the only option is 2) average
force F = ma                                   speed. The other three are vector
3. What would be the path of the stone if          quantities, and need direction as well.
you let go of the string? - Sideways,
tangential to the circle                    10. List all the circumstances when an object
might experience an acceleration.
7. According to Newton’s Laws, what happens            1. speeding up
to a moving object if all forces acting on it       2. slowing down
are removed? - Continues to move in a               3. changing direction
straight line at constant speed
Phys 3550 How Things Work
Fall 2007
1st Midterm (A) – Tuesday October 23rd 2007

11. Fill in the blanks                                 also smaller.
1. In stopping a car kinetic energy is
converted to heat by the brakes.        Numerical Pr oblems
2. When water falls over a waterfall GPE
energy is converted to kinetic energy 14. If a 2000 kg car has a KE of 25 kJ what is its
and then to heat.                             speed? Can you say what its velocity is?
3. In a coal or oil fired power station            1. KE = ½ mv²
Chemical      energy     is   ultimately      2. 25,000 = ½ 2000 v²
converted to electrical energy.               3. v² = 2 * 25,000 / 2000 = 25
4. When you lift a heavy suitcase                  4. v = 5 m/s
chemical energy is converted to GPE.          5. Cannot tell velocity without direction.
5. If you drive a car around a bend at
constant speed, Kinetic energy is not 15. A man of mass 60 kg runs up a hill. If the
converted to anything, there is no work       top of the hill is 15 m above the ground,
done.                                         how much work does he do? What is the
power which he needs if it takes him 40
12. If a car goes around a corner at constant          seconds to run up the hill?
speed, does either the momentum or the             1. W = GPE gain = mgh
kinetic energy change?                             2. W = 60 * 9.8 * 15 = 8820 J
1. Neither change                                  3. P = W/t = 220.5 W
2. The momentum is a constant but the
kinetic energy changes                    16. A 4 kg weight hangs from a string. You pull
3. The kinetic energy is a constant but the        on the string so that the mass moves
momentum changes                              upwards with a constant deceleration of 6
4. Both change                                     m/s². What is the tension in the string?
1. Net force = m a = 4 * 6 = 24 N
KE depends only on mass and speed. It does         2. Since the weight is moving up and
not change. On the other hand momentum                 decelerating, the force must be
depends on velocity and speed, and since               downwards
direction is change then so is momentum            3. W = mg = 4 * 9.8 = 39.2 N
4. Tnet = F – W
13. The most serious accidents for car                 5. F = 39.2 – 24 = 15.2 N
passengers who are not wearing a seat belt
occurs when the passenger strikes the          17. A car has a mass of 1500 kg. The engine
front windshield. Why does this occur, and         generates a force of 600 N. Calculate
what physical principle explains why an            1. The acceleration of the car. - a = F/m =
air bag helps avoid injuries in these                  600 N / 1500 kg = 0.4 m/s²
collisions?                                        2. The change in velocity in a time of 6 s. -
Δv = a t = 0.4 m/s² * 6 s = 2.4 m/s
The air bag slows you down gently, taking a        3. If it starts this time interval moving at
substantial time to do so. That means to for           6 m/s, what is its final speed? - vf = vi +
a given change in velocity the acceleration            Δv = 6 m/s + 2.4 m/s = 8.4 m/s
is smaller, and so the forces involved are         4. Sketch the speed as a function of time.
Phys 3550 How Things Work
Fall 2007
1st Midterm (A) – Tuesday October 23rd 2007

5. How far does it move in this 6 s
interval?
1. The sketch is a rectangle and a
triangle
2. area of rectangle = 6 * 6 = 36 m
3. area of triangle = ½ * 6 * 2.4 = 7.2 m
4. total distance = 36 + 7.2 = 43.2 m

18. A 60,000 kg rocket is propelled forward
with an acceleration of 4 m/s².
1. What is the force on the rocket? - F =
ma = 60,000 * 4 = 240,000 N
2. if the rocket moves a distance of 60 km,
how much work is done? - W = Fd =
240,000 N * 60,000 m = 1.44 x 1010 J
3. if the rocket starts from rest, what is
the final speed of the rocket?
1. KE = ½ mv²
2. 1.44 x 1010 = ½ 60,000 v²
3. v² = 2 * 1.44 x 1010 / 60,000 = 480,000
4. v = 693 m/s

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