Phy 213 Quiz 3       Forces I   5    points each      Sept. 24, 2004

 

A man stands on a bathroom scale inside an elevator.  When the elevator is at rest, the scale reads 750 N (170 lb).  Compare the scales reading to 750 N when:

 

1.  The elevator is moving upward at an increasing speed.

 

 

2.  The elevator is moving upward at a decreasing speed.

 

 

3.  The elevator is moving upward at constant speed.

 

 

4.  The elevator is moving downward at decreasing speed.

 

 

 

5.  The elevator is moving downward at constant speed.

 

6.  On a freebody diagram, the name we give the scales reading is the ________________ force because the direction of this force is always perpendicular to the plane of contact.

 

7. 


8.   A 20 kg mass is pulled along on a friction free horizontal plane by a horizontal force of 50 N.  

 

a.  Draw a FBD of the mass that includes normal and gravitational forces when they exist.

b. Is the mass in equilibrium?

c.  Sum the forces acting on the mass in both x and y directions if you have forces in both directions.

d.  What are these sums equal to?

e.  What is the acceleration of the mass?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

9.   An block slides down a  friction free plane with an acceleration of 4 m/s2.  What angle does the plane make with the horizontal ?

 

a.  Draw a FBD of the block that includes normal and gravitational forces when they exist.

b. Is the block in equilibrium?

c.  Sum the forces acting on the block in both x and y directions if you have forces in both directions.

d.  What are these sums equal to?

e.  What is the angle the plane makes with the horizontal ?

(The following problems were taken from the infamous Saga of Hank & Sally .  For those of you that weren't here for these exciting episodes, the bag of goodies has a mass  of 50 kg.)

 

10.   But alas, the extra weight proves too much for the branch, which breaks, plunging bear, bag, orangutan and limb to the ground.  The bear leaps up and bounds away, dragging the bag behind him, applying a force of 60N to the rope, causing it to follow him reluctantly at a constant speed of 7 m/s.  (The impact causes the orangutan to lose her grip.)

 

a.  Draw a FBD of the bag.

b.  Is the bag in equilibrium?

c.  Sum the forces acting on the bag.

d.  What are these sums equal to?

e.  Find the net friction force on the bag .

 

 

 

 

 

 

 

 

 

 

 

 

11.  The dazed, but unharmed, orangutan finally recovers her composure, and leaps to her feet.   Unlike the bear, she is unhampered by the bag, she quickly catches up and dives onto the bag.  The added 30 kg mass of the orangutan increases the friction force which slows the bear, (still applying a 60N force to the rope) down at a rate of 2 m/s2.

 

Draw a FBD of the bag.

Is the bag-orangutan mass in equilibrium?

Sum the forces acting on the bag-orangutan mass.

What is this sum equal to?

Find the net friction force acting on the bag-orangutan mass.