1. Oregon State University PH 211, Class #18
Some comments/hints on Lab 6-III…
• Don’t assume FSmax unless you’re told so!
• Except for FG, all forces are contact forces; objects exerting
those forces much be touching the mass you are diagramming.
• Make sure all forces are accounted for…
Third-law pairs (one on each diagram)!
A surface exerts at least FN; often FS or FK as well.
• The direction of a is often given, but not the direction of v!
Your choice of direction for v will determine the direction of FK.
11/2/18
Oregon State University PH 211, Class #18

2. Oregon State University PH 211, Class #18
The mass m is hanging at
rest from this system of ideal
strings and ideal pulleys. Evaluate
the following statement (T/F/N).…
m = FT/(4g)
A. True
B. False
C. Not enough information
(See solution for Prep 6-7, #3e.) FT m
11/2/18
Oregon State University PH 211, Class #18

3. (See next slide for answer.)
A single unknown applied force is pushing horizontally on Block A (mA = 10.0 kg). Block A, in turn, is pushing horizontally on block B (mB = 15.0 kg). Both blocks are sliding together across a frictionless horizontal surface in the positive x-direction with an acceleration of (+)3.00 m/s2. Find the normal force FN.BA. N
2. –45.0 N N
4. –30.0 N
5. None of the above.
(See next slide for answer.)
11/2/18
Oregon State University PH 211, Class #18

4. Oregon State University PH 211, Class #18
A single unknown applied force is pushing horizontally on Block A (mA = 10.0 kg). Block A, in turn, is pushing horizontally on block B (mB = 15.0 kg). Both blocks are sliding together across a frictionless horizontal surface in the positive x-direction with an acceleration of (+)3.00 m/s2. Find the normal force FN.BA. N
2. –45.0 N N
4. –30.0 N
5. None of the above.
A FBD and x-direction equation set for block B will show that FN.AB must be N, so by Newton’s Third Law, FN.BA = –45.0 N
11/2/18
Oregon State University PH 211, Class #18

5. The top of block 2 is attached to the
side of block 1 by an ideal string.
The blocks are accelerating together
as block 1 slides across a horizontal shelf
(which offers friction against block 1).
Both angles remain constant as block 1 slides.
Evaluate (T/F/N) the following statement.
“The blocks are moving to the right.”
A. True
B. False
C. Not enough information.
(See next slide for answer.) F q1 m1
mS , mK q2 m2
11/2/18
Oregon State University PH 211, Class #18

6. Oregon State University PH 211, Class #18
The top of block 2 is attached to the
side of block 1 by an ideal string.
The blocks are accelerating together
as block 1 slides across a horizontal shelf
(which offers friction against block 1).
Both angles remain constant as block 1 slides.
Evaluate (T/F/N) the following statement.
“The blocks are moving to the right.”
A. True
B. False
C. Not enough information.
A FBD and x-direction equation set for m2 will show that a2.x (and thus the acceleration of both blocks) is to the right. But that could mean either: the blocks are moving to the right and increasing in speed; or moving to the left and decreasing in speed. F q1 m1
mS , mK q2 m2
11/2/18
Oregon State University PH 211, Class #18

7. “The block’s speed is increasing.”
Two equal-magnitude forces (F = 40.0 N)
are applied as shown on a block (m = 3.00 kg).
The coefficients of friction between the wall
and the block are µS = 0.65 and µK = 0.35,
and local g = 9.80 m/s2.
Evaluate the following claim (T/F/N):
“The block’s speed is increasing.”
A. True
B. False
C. Not enough information.
(See After Class 18 for solutions.) m F F
11/2/18
Oregon State University PH 211, Class #18