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

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

(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. 1. 45.0 N 2. –45.0 N 3. 30.0 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

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. 1. 45.0 N 2. –45.0 N 3. 30.0 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 +45.0 N, so by Newton’s Third Law, FN.BA = –45.0 N 11/2/18 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. (See next slide for answer.) F q1 m1 mS , mK q2 m2 11/2/18 Oregon State University PH 211, Class #18

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

“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