# Homework Help: Did I do these right?

1. Nov 2, 2006

### wakejosh

I dont expect anyone to really go through all this and double check everything, but If anyone notices anything that is wrong maybe brushing over it please tell me. Thanks

1. On planet Z, the standard unit of length is the foose. Ann the
Astronaut is 5.90 feet tall on earth. She lands on planet Z and is measure to be 88 foosi (plural of foose) tall. Her crew member Rachael is 94 foosi tall. How tall is Rachael in Earth measurements?

e. 6.3

2. Note the expression: y = A/x2. Which statement is most consistent
with this expression?

a. y is less than x

3. The speed of a boat is often given in knots. If a speed of 5 knots were expressed in the SI system of units, the units would be:

c. m/s

4. A ball is pushed with an initial velocity of 4.0 m/s. The ball rolls down a hill with a constant acceleration of 1.6 m/s2 . The ball reaches the bottom of the hill in 6.0 s. What is the ball's velocity at the bottom of the hill?

c. 14 m/s

5. A European sports car dealer claims that his car will accelerate at a constant rate from rest to a speed of 100 km/hr (28 m/s) in 8.00 s. What is the speed after the first 5.00 s of acceleration?

d. 17.4 m/s

6. An object is dropped from a height and falls for several seconds. Which of the following statements can be true at some time during the fall.

Its velocity is more than its acceleration.
Its velocity is less than its acceleration.
Its velocity is the same as its acceleration.

d. All of the above can be true. (But not at the same exact time)

7. Which of the following is an example of a vector quantity?

c. displacement

8. Arvin the Ant is on the picnic table. He travels 30 cm eastward,
then 25 cm northward, and finally 15 cm westward. What is the magnitude of Arvin's net displacement?

d. 29 cm

9. A river flows due east at 3.0 m/s. A boat crosses the river which is 300 m wide, by maintaining a constant velocity of 10 m/s due north relative to the water. If no correction is made for the current, how far downstream does the boat move by the time it reaches the far shore?

d. 90 m

10. A 2.0 kg bowling ball experiences a net force of 4.0 N. What will
be its acceleration?

b. 2.0 m/s2

11. If we know that an object is moving at a constant velocity, we may
assume:

a. The net force acting on the object is zero.

12. As a car skids with its wheels locked trying to stop on a road
covered with ice and snow, the force of friction between the icy road and the tires will usually be: (Please note that "normal" and "perpendicular" have the same meaning)

c. less than the normal force of the road times the coefficient of
kinetic friction

13. As an object is lowered into a deep hole in the surface of the
earth. How does the potential energy of the object at the bottom of the hole compare with its potential energy at the top of the hole.

b. The potential energy at the bottom is less.

14. A 60 kg woman runs up a flight of stairs having a rise of 4.0 m in
a time of 6.0 s. What average power did she supply?

a. 400 W

15. A horizontal force of 100 N is applied to move a 45 kg cart across
a 9.0 m level surface. What amount of work is done by the 100 N force?

d. 900 J

16. A 50 kg swimmer dives horizontally off a 75 kg rowboat. If the
speed of the rowboat immediately after the swimmer dives off is 1.0 m/s, what speed did the diver leave the rowboat?

b. 1.5 m/s

17. The impulse experienced by an object is equivalent to its change
in:

d. momentum

18. In a system with two moving objects, when a collision occurs
between the two objects, what is conserved?

b. the total momentum is always conserved

19. A grindstone spinning at the rate of 8.3 rev/s has what
approximate angular speed?

20. A ventilation fan has blades 0.25 m long rotating at 2 rad/s. What is the centripetal acceleration of a point on the outer tip of a blade?

a. 1.0 m/s2

21. An airplane is a wide sweeping "outside" loop can create zero "g"
(weightlessness) inside the aircraft cabin. What must be the radius of curvature of the flight path for an aircraft moving at 70 m/s to create a condition of "weightlessness" inside the aircraft?

c. 500 m

22. A ventilation fan with a moment of inertia of 0.034 kg-m2 has a
net torque of 0.11 N-m applied to it. What angular acceleration does it experience?

23 A bowling ball has a mass of 7.0 kg and a radius of 0.1 m. If it
rolls down the lane without slipping at a linear speed of 4.0 m/s, what is its angular velocity?

24. An ice skater spins at 2.5 rev/s when his arms are extended. He
draws his arms in and spins at 6 rev/s. By what factor does his moment of inertia change in the process?

c. 0.42

2. Nov 2, 2006

### BerryBoy

2 - I don't see why that statement has to be true (whats the other options)?

5 - I Realise that it is multiple choice, I take it there's no $17.5ms^{-1}$

6 - Don't like this question, but I think you answered it right; saying velocity is the same as acceleration, is like saying an apple is the same as a banana!

12 - Why is it less?

21 - Agree unless there was a 490m option.

24 - Disagree! The mans rotational energy stays constant i.e:
$$\frac{1}{2} I_1 \omega_1^2 = \frac{1}{2} I_2 \omega_2^2$$

So the factor it changes by is:
$$\frac{\omega_1^2}{\omega_2^2}$$

not $$\frac{\omega_1}{\omega_2}$$

Apart from that, looks good!

Regards,
Sam

3. Nov 2, 2006

### Andrew Mason

I would have another look at 2, 4, 5, 6, 12, and 14

In 2, if x2 means x2 what about the case when x<1?

In 4, the answer would be 14 with an accuracy of two signficant figures. The problem is that if the initial speed was 0, the speed after 6 seconds would be 9.6 m/sec. If you add 4.0 m/sec, you get 13.6 m/sec not 14.

5 should be 17.5

6, you cannot compare velocity and acceleration like that.

12 - by definition the friction force is equal to $\mu_kN$

14. the answer is right if you are using g = 10 m/sec^2 but it should be 390 if 9.8 m/sec^2 is used.

the others look right.

AM

4. Nov 2, 2006

### wakejosh

here is the full question:

2. Note the expression: y = A/x2. Which statement is most consistent
with this expression?

a. y is less than x b. if x is halved, y is multiplied by 4
c. y is greater than x d. if x is doubled, y is multiplied by a factor of 2
e. y is equal to x

5. Nov 2, 2006

### wakejosh

here is the full number 6:

6. An object is dropped from a height and falls for several seconds.
Which of the following
statements can be true at some time during the fall.

a. Its velocity is more than its acceleration.
b. Its velocity is less than its acceleration.
c. Its velocity is the same as its acceleration.
d. All of the above can be true. (But not at the same exact time)
e. None of the above can be true for a falling object

6. Nov 2, 2006

### geoffjb

As Andrew pointed out, (a) is not right. It's (b) or (d); look at both, and calculate which it must be.

7. Nov 2, 2006

### wakejosh

here is 12:

12. As a car skids with its wheels locked trying to stop on a road
covered with ice and snow, the force of friction between the icy road and the tires will usually be: (Please note that "normal" and "perpendicular" have the same meaning)

a. greater than the normal force of the road times the coefficient
of static friction
b. equal to the normal force of the road times the coefficient of
static friction
c. less than the normal force of the road times the coefficient of
kinetic friction
d. greater than the normal force of the road times the coefficient
of kinetic friction
e. equal to the normal force of the road times the coefficient of
kinetic friction.

I know its Kinetic friction, but i wasnt sure completely on the comparison to the normal force of the road. I thought since the road was icy it should be less than normal, Is this wrong?

8. Nov 2, 2006

### wakejosh

well i know it cant be B so it must be D, and that makes sense when i run a few numbers through it.

9. Nov 2, 2006

### geoffjb

The thing is, velocity and acceleration have different units. $m/s \neq m/s^{2}$, even if the value of each is the same. What the question should measure (in order for [d] to be correct) are the values of velocity and acceleration.

Last edited: Nov 2, 2006
10. Nov 2, 2006

### geoffjb

Umm... why can't it be (b)? It is $x^{2}$, right?

11. Nov 2, 2006

### wakejosh

isnt the skater doing work on the system by pulling in his/her arms? and none of my choices are 0, my choices are:

a. 2.4 b. 1.0 c. 0.42 d. 0.25 e. 0.12

12. Nov 2, 2006

### BerryBoy

Technically speaking (as Andrew pointed out), you cannot compare velocity & acceleration, they are completely different. But you have to make a judgement call and decide if the question is asking about the numbers when velocity & acceleration are in S.I. units - Personally I'd put 'e' and appeal if I were marked wrong.

Regards,
Sam

13. Nov 2, 2006

### wakejosh

hmm.. you know I dont know for sure. this was emailed to me so I dont know if its supposed to be X*2 or X^2 ... any guesses on what he is looking for here?

14. Nov 2, 2006

### BerryBoy

:uhh: Of course he is! I'm talking absolute rubbish this morning; sorry.

But I don't know if I'd choose 2.4 or 0.42, since $2.4 \times 2.5 = 6$

15. Nov 2, 2006

### geoffjb

Based on mathematical custom, I would guess he meant $x^{2}$. Typically, one would write $2x$ (constant first) when simply multiplying. Fortunately, there is no answer that would be correct if it was $2x$. That leaves only one choice.

16. Nov 2, 2006

### BerryBoy

By the look of the answers, my money would be on $x^2$, making (b) a nice candidate.

Anyone else?