Homework-10-soln

Homework-10-soln - Homework: Week 10 Chapter 11 SOLUTIONS...

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Homework: Week 10 – Chapter 11 – SOLUTIONS 1. (11-5) A person holds a 1.42-N baseball in his hand, a distance of 34.0 cm from the elbow joint, as shown in the figure below. The biceps, attached at a distance of 2.75 cm from the elbow, exerts an upward force of 12.6 N on the forearm. Consider the forearm and hand to be a uniform rod with a mass of 1.20 kg. (a) Calculate the net torque acting on the forearm and hand. Use the elbow joint as the axis of rotation. (b) If the net torque obtained in part (a) is nonzero, in which direction will the forearm and hand rotate? (c) Would the net torque exerted on the forearm and hand increase or decrease if the biceps were attached farther from the elbow joint? SOLUTIONS (a) biceps forearm ball 2 m0 . 3 4 0 (12.6 N)(0.0275 m) (1.20 kg) 9.81 m (1.42 N)(0.340 m) 2 s 2.14 N m ττ τ =− − ⎛⎞ =− ⎜⎟ ⎝⎠ (b) Negative net torque means clockwise motion: the forearm and hand will rotate downward. (c) Attaching the biceps farther from the elbow would increase the moment arm and increase the net torque. 2. (11-12) The L-shaped object in the figure below consists of three masses connected by light rods. What torque must be applied to this object to give it an angular acceleration of 12 if it is rotated about (a) the x axis, (b) the y axis, or (c) the z axis (which is through the origin and perpendicular to the page)? SOLUTIONS (a) 0 2 . r a d / s 22 11 2 2 33 , IIm r m r τα ==++ 2 2 2 (9.0 kg)(1.0 m) (2.5 kg)(0) (1.2 kg)(1.2 kg)(0) 9.0 kg m x I =+ + = 2 2 rad 1.20 (9.0 kg m ) 11 N m s =⋅ = 2 (b) 2 (9.0 kg)(0) (2.5 kg)(2 m) (1.2 kg)(0) 10 kg m y I += 2 2 rad 1.20 (10 kg m ) 12 N m s =
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(c) 22 2 (9.0 kg)(1.0 m) (2.5 kg)(2.0 m) (1.2 kg)(0) 19 kg m z I =++ = 2 2 2 rad 1.20 (19 kg m ) 23 N m s τ ⎛⎞
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This note was uploaded on 07/02/2011 for the course PHYS 201 taught by Professor Woodahl during the Fall '09 term at Indiana.

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Homework-10-soln - Homework: Week 10 Chapter 11 SOLUTIONS...

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