Chapter 7: Q1CQ (page 177)
Can a normal force do work? If not, why not? If so, give an example.
The normal force works when the displacement is in the direction of the normal force.
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A shopper in a supermarket pushes a cart with a force of directed at an angle of below the horizontal. The force is just sufficient to balance various friction forces, so the cart moves at constant speed.
(a) Find the work done by the shopper on the cart as she moves down a -long aisle.
(b) The shopper goes down the next aisle, pushing horizontally and maintaining the same speed as before. If the friction force doesn’t change, would the shopper’s applied force be larger, smaller, or the same?
(c) What about the work done on the cart by the shopper?
Preparing to clean them, you pop all the removable keys off a computer keyboard. Each key has the shape of a tiny box with one side open. By accident, you spill the keys onto the floor. Explain why many more keys land letter-side down than land open-side down.
Let represent the direction horizontally north, represent northeast (halfway between north and east), and so on. Each direction specification can be thought of as a unit vector. Rank from the largest to the smallest the following dot products. Note that zero is larger than a negative number. If two quantities are equal, display that fact in your ranking. (a) (b) (c) (d) (e) .
(a) For what values of the angle between two vectors is their scalar product positive? (b) For what values of is their scalar product negative?
In a control system, an accelerometer consists of a 4.7 g object sliding on a calibrated horizontal rail. A low-mass spring attaches the object to a flange at one end of the rail. Grease on the rail makes static friction negligible, but rapidly damps out vibrations of the sliding object. When subject to a steady acceleration of
, the object should be at a location away from its equilibrium position. Find the force constant of the spring required for the calibration to be correct.
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