00_lab #14-final exam review solutions

PHS 110 Spring 2013 1 U3 Lab #13 v. kgw

vel (m/s)

5

10

10

5

0

time (s)

Name Date Lab Group: _______

Lab Session #14: Recitation Final Exam Review

Please complete these on a separate sheet of paper.

Unit 1—Scientific Thinking 1. Suppose that one unsharpened pencil is 5.2 paperclips long.

a. How long would 2 unsharpened pencils be in paperclips? Show any calculation(s).

b. A door opening is 7.3 unsharpened pencils wide. How big is the opening in paperclips?

2. The graph below shows the relationship between the velocity and

time of a ball rolling down a ramp.

a. Determine the vertical intercept and the (approximate) slope. Show

your work on the graph.

b. Write the mathematical equation that states the relationship

described by the graph. Use meaningful variables and units!

c. Write a clear, English sentence that describes the meaning of the

slope and the vertical intercept.

d. What would be the velocity of the ball after it

rolls for 20 seconds? Explain or show your

work.

e. Calculate the approximate time at which the ball

would reach a velocity of 50 m/s (assuming the trend

here continues).

Unit 2—Motion 3. Robin, rollerskating down a marked sidewalk, was observed at the

following positions at the times listed below:

Time (sec) 0 1 2 5 9 10

Position(m) 10 12 14 20 28 30

a. Plot a position vs. time graph for the skater on the axes provided.

b. Write a mathematical model (equation) that describes the skater's motion.

c. Was his speed constant over the entire interval? How do you know?

d. Sketch a velocity vs. time graph for the skater. Put NUMBERS and labels

on your axes.

e. Calculate the position of the skater at t = 20.0 sec if he kept traveling at the same rate.

f. Calculate the skater’s total displacement at t = 20.0 seconds. Is this the same as part e? Why

4. At t = 0 a car has a velocity of 30 m/s. At t = 6 s, its velocity is 14 m/s.

a) Draw a quantitative velocity-time graph for the car.

b) What is its average acceleration during this time interval? c)

How far did it travel during this interval?

t (s)

v (

m/s

)

+

-

t

a) 10. 4 paper clips

b) ≈38 paper clips

Intercept: 2 m/s

Slope: 0.8 m/s/s

v = (0.8 m/s/s)·t + 2 m/s

Velocity at t = 0 is 2 m/s; object speeds up 0.8 m/s every second.

v = 0.8·20 + 2 = 18 m/s 50 = 0.8·t + 2 subtract 2 both sides

48 = 0.8·t divide by 0.8 both sides

60 seconds = t

10

20

30

4 8 12

x = (2 m/s)·t + 10 m

According to the data it was constant, 2 meters every second.

v (m/s)

t (s)

2

x = 50 m

Displacement = final position – initial position = 40 m

30

14

6

a = -2.6 m/s/s

displacement =

area of Δ + area of rectangle = 132 m


5. Conflicting Contentions Task

Consider the position vs. time graph to the right which shows the

motion of objects A and B. Three students make the following

observations about the motions of objects A and B:

Student A says: Object B moves at a steady pace, while object A

increases speed.

Student B says: No, object A and B move the same average speed,

because they are at the same place at 5.0 s.

Student C says: Yes, A and B are at the same place at 5.0 s, but A over takes B because he is slowing down.

a. With which, if any, of these students do you agree?

b. CORRECT the statements of any of the students with which you disagree.

c. Carefully explain your reasoning.

Unit 3—Forces (& Vectors) 6. Fred is on a train that travels east as shown

in the picture. Fred gets bored and decides

to try running across the train. He faces North and runs

at a steady speed of 4 m/s for five seconds. Answer the

following:

a. Draw a sketch showing the overhead view of the

train with vectors (arrows) sketched for Fred’s

velocities as seen by Sam and Joe.

b. Find Fred’s speed with respect to the train.

c. Find the Fred’s distance with respect to the

ground.

d. Find Fred’s velocity with respect to the train.

e. Find Fred’s velocity with respect to the ground.

7. Consider the "tension force" acting in each of the three springs if the spring scale held by the person reads

60 N in the figure below.

a.) Determine the tension in each of the springs.

Spring A __________; Spring B__________; Spring C __________

b.) Find the size of the force exerted by the person on the scale. __________

Position vs. Time x

t 5

A

B

Correction: object A decreases speed.

This statement is true—average speed is the same.

However, this isn’t the whole story of their motions.

Correction: B over takes A because A is slowing down.

None are entirely perfect, but student B is probably the best because his statement is true.

Vtrain= 3 m/s E

VFred= 4 m/s N Vresult= 5 m/s NE

4 m/s

25 meters (5 seconds of travel)

VFred= 4 m/s N

VFred= 5 m/s NE

60 N 30 N 30 N

60 N


8 . An air puck on a table with NO FRICTION experiences the following force situations. In each case,

draw a MOTION MAP and explain what type of motion occurs and why in terms of forces.

a. While resting on the air table, the puck experiences a constant push to the right.

b. The puck is moving to the right on the air table, but not one is pushing or pulling it.

c. The puck is moving to the right on the air table and experiencing a CONSTANT FORCE to the left.

d. The puck is moving to the east and experiences a brief kick (force) to

the south. Describe the motion AFTER the kick.

e. The puck is moving to the right on and leaves the edge of the table. In this case draw a FORCE

DIAGRAM for the puck after it leaves the table, rather than a motion map.

9. A body falls freely from rest on Earth. Suppose that air resistance is NOT a factor.

a. Find its speed in m/s at t = 3s.

b. Find the time required for it to reach a speed of 85 m/s.

c. Sketch the velocity vs. time graph for the object if it falls for 15 seconds. How fast will it be going at the 15

second mark? Label the axes with numbers.

d. Recall that the AREA under a velocity vs. time graph gives how far an object goes (displacement). Use this

fact and your velocity graph to find a falling object’s displacement after falling for four seconds.

e. Now calculate a falling object’s displacement after falling for eight seconds. Is it twice the value you

calculated in part d? Why or why not?

10. A 12, 000 kg bus slams on its brakes and slows from 30 m/s to 10 m/s in 10 s.

a. What is the acceleration of the bus, in meters per second each second?

b. What is the value of the net force acting on the bus to cause this acceleration?

Top View:

velocity

acceleration Puck will speed up to the right

Puck will travel at a constant speed

acceleration

velocity

Puck will slow down while traveling to the right

Puck will travel at a constant speed to the southeast velocity

Fearth on puck

velocity

30 m/s

8.5 seconds

150 m/s

velocity

time 15

150

80 meters

320 meters; no, not twice as much because object

speeds up, resulting in four times as much displacement

when time is doubled

-2 m/s/s

-24000 N (opposite the direction of motion)


11. Answer true or false to the following questions. If it is false, correct it so that it becomes true. On the last

one, pick the best choice.

(a) _______ A car is driving along a straight road at a constant speed . The force of the road up equals the force

of the earth down.

(b)_______ A car is driving along a straight road at a constant speed. The force of the road forward equals the

force of the air and road backward.

(c)________ If the student pushes on the wall with 10 N of force, the wall does not push back.

(d) ________ If a scale is placed between two cars when a big car pushes a small car, the scales shows that the

force of the small car on the big car must be smaller than the force of the big car on the small car.

(e)________ A box sits on a scale on a table. The scale reads 40 N. If a person lifts up on the box with 10 N of

force, then the scale will read 30 N.

(f)______ An object is going faster and faster to the right. You would expect:

a. more force to the left than the right.

b. the same force left and right.

c. more force to the right than the left.

d. more force up than down.

12. Two chunks of dry ice rest on a smooth table top. One has a mass of 1 kg, the other is 2 kg. A 4.0 N force is

applied to each for 4 seconds. (assume there is no friction)

a. What is the acceleration of each chunk?

b. Compare the velocities of the two chunks at the end of 4 seconds.

Unit 4—Energy Energy Transfer or Work = F•distance Eel = ½ k x

2 Eg = mgh Ek = ½ mv

2

Power = ∆∆∆∆Energy/∆∆∆∆t For Multiple choice use g = 10 N/kg

13. A spring whose spring constant is 850 N/m is

compressed 0.40 m.

a. Fill in the energy bar graphs for this situation.

b. Use energy considerations to calculate the

maximum speed this spring can give to a 0.50 kg

ball.

c. If the spring were compressed twice as much, how many times greater would the velocity of the ball be?

Show your calculations.

True

True

True

False

False

Balanced forces for constant speed.

Wall pushes back with 10 N of

the same size as

c

1 kg chunk: a = 4 m/s/s 2 kg chunk: a = 2 m/s/s

1 kg chunk: v = 16 m/s 2 kg chunk: v = 8 m/s

v = 16.5 m/s

v = 33 m/s The velocity will be twice as much, also.


14. An object is lifted 20 meters. By what fraction does its gravitational energy change if its height is tripled?

15. Kevin (mass 82 kg) climbs Mt. Humphrey near Flagstaff. Kevin’s vertical elevation increases by 540 meters.

Kara (mass 60 kg) climbs the same trail. Kevin takes 90 minutes to make the climb, while Kara takes 70

minutes to complete it.

a. Who has more Eg at the top? Calculate and compare their energies.

b. Who exerted more power getting there? Calculate and compare their power.

Unit 5—Electricity 16. Determine which of the bulbs will light in each of the following and trace the path followed by the charge.

17. Suppose the circuit below has just been completed. The capacitor was charged prior to connection. The

bulbs are bright. Assume that the charges have just started to move and that they have moved a very short

distance through the corresponding wire.

Fill in each blank (corresponding to a point on the circuit) with where the charges originated in the circuit from

the description choices at right.

Circuit

Location

Origin of Charge

Choices:

A

• a capacitor plate

• the battery

• a connecting wire

• a bulb

• none of these

B

C

D

E

F

G

2

2

1 1

A

E

D

R

CB

R

G F

+++ +++

- - - - - -

The gravitational energy will also be tripled!

EgKevin = 442,800 J EgKara = 324,000 J

PKevin = 82 Watts PKara = 77 Watts

Bulb 1 is lit; bulb 2 is unlit Both bulbs are lit

Connecting wire

Connecting wire

bulb

Connecting wire

Capacitor Plate

Connecting wire

bulb


18. List the circuits below in order of LEAST overall resistance to MOST overall resistance. Don’t worry about

the number of batteries.

19. A capacitor is charged through two bulbs (Figure 2a) and then discharged through a single bulb (Figure 2b).

Show appropriate starbursts and arrowtails for each circuit, and mark each of the following statements as

True or False (T or F). If a statement is False, re-write it as a correct statement.

a. _____ The same amount of charge flows during charging and discharging.

b. _____ More charge flows through the single bulb than through the two bulbs.

c. _____ Charge flows at a greater rate through the single bulb than through the two bulbs.

d. _____ The single bulb shines brighter than either bulb in the two-bulb circuit.

e. _____ The total resistance of the two bulbs is greater than that of the single bulb.

f. _____ A compass would show a larger deflection for Figure 2a than for Figure 2b.

g. _____ The brightest bulb(s) indicate the fastest charge flow.

A B C

D E

Least E, A, C, D, B most

True

True

False

True

True

True

False

2b 2a

Same amount of charge..

00_lab #14-final exam review solutions

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