Dr. TenenbaumPhysics2013-2014

Exam 3:

Forces(+90pts)

vf=v0+at
x=v0t+a/2*t2
x = ½(v0+vf)t
vf2=v02+2ax
Fnet=ma
Ff=µN
1 mile = 1609m
1 mile = 5280ft
1 ft = 0.3048m
1 inch = 2.54cm

Name______

Period______(+1pt)

Dr. TenenbaumPhysics2013-2014

Remember, for full and partial credit:

(1)Remember units

(2)Show all work

Dr. TenenbaumPhysics2013-2014

Dr. TenenbaumPhysics2013-2014

  1. (+14pts)

Mr. Wood (70kg) stands on an elevator that is accelerating upward at a rate of 3m/s2.

  1. (+2pts) Draw the free body diagram for Mr. Wood
  1. (+2pts) The scale will read

HIGHER THANLOWER THANTHE SAME AS

what it reads at rest.

  1. (+4pts) Calculate what the scale should read (Hint: Fnet=ma).
  1. (+1pt) Is Mr. Wood in equilibrium?
  2. (+2pts) Now, assume the elevator is moving at constant velocity. Now, the scale will read

HIGHER THANLOWER THANTHE SAME AS

what it reads at rest .

  1. (+2pts) Calculate what the scale should read now.
  1. (+1pt) Is Mr. Wood in equilibrium now?

2. (+10pts)

At least once a week Dr. Tenenbaum almost slips and dies in the boys bathroom from a wet spot on the floor. What causes this?

  1. µ of his shoes against wet floor is too large
  2. µ of his shoes against wet floor is too small
  3. normal force of the floor on him is too large
  4. normal force of the floor on him is too small
  5. the water applies a sideways force that pushes the shoe
  6. this is a result that cannot be explained by theory.

3. (+10pts)

The following two forces act on an object. Draw a third force that will produce equilibrium if added to the two forces here.Calculate how large this force will be.

4. (+6pts)

In equilibrium:

  1. There are no forces acting.

Truefalse

  1. The object is at rest.

Truefalse

  1. The net force is 0.

Truefalse

  1. Velocity is 0.

Truefalse

  1. Acceleration is 0.

Truefalse

  1. It is possible to have equilibrium with only one force acting.

Truefalse

4.(+30pts)

In the scene in The Matrix, Trinity’s helicopter crashes. Neo is on the roof of a building. Neo grabs the cable that is connected to the crashing helicopter. mhelicopter=10,000kg, mNeo=75kg, mTrinity=59kg. At first, the entire helicopter is attached to the cable. The helicopter is almost completely in free fall with the cable pulling straight up on the helicopter. The cable pulls sideways (horizontally) on Neo. The tension in the cable between Neo and the helicopter is 1000N.

(a) (+4pts) Draw the free body diagram for Neo:

(b)(+4pts) Draw the free body diagram for the helicopter:

(c)(+4pts) Using Newton’s 2nd law, calculate the acceleration of the helicopter.

(d)(+4pts) Calculate the acceleration of Neo, assuming that the coefficient of friction, µ, between his shoe and the rooftop is 0.7.

(e)(+4pts) Trinity cuts the cable so that the helicopter is not attached. Trinity grabs onto the cable herself, which keeps her from falling. Draw the free body diagram for Trinity:

(f)(+2pts) Draw the free body diagram for Neo now:

(g)(+4pts) Using Newton’s 2nd law, calculate the tension in the cable now (Hint: Use your free body diagram for Trinity).

(h)(+4pts) Is it possible for Neo to experience this tension and stay in equilibrium or does this mean that Neo has broken the laws of physics inside the Matrix, proving that he is “The One”? Support your answer with a calculation.

5. (+20pts)

A door has a spring that pulls the door shut. Dr. Tenenbaum decides to put a textbook next to the door to hold it open. This does not work. The door gets pulled shut by the spring and the textbook just slides. Now, Dr. Tenenbaum puts a heavy filing cabinet next to the door to hold it open. This works. The door stays open and the file cabinet does not move. Explain why the filing cabinet is able to hold the door open but the textbook does not, using what you know of physics.