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Chapter 6: Q80P (page 147)
Calculate the magnitude of the drag force on a missile 53 cmin diameter cruising at 250 m/sat low altitude, where the density of air is. Assume.
The magnitude of the drag force is .
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In the early afternoon, a car is parked on a street that runs down a steep hill, at an angle ofrelative to the horizontal. Just then the coefficient of static friction between the tires and the street surface is 0.725. Later, after nightfall, a sleet storm hits the area, and the coefficient decreases due to both the ice and a chemical change in the road surface because of the temperature decrease. By what percentage must the coefficient decrease if the car is to be in danger of sliding down the street?
In about 1915, Henry Sincosky of Philadelphia suspended himself from a rafter by gripping the rafter with the thumb of each hand on one side and the fingers on the opposite side (Fig. 6-21). Sincosky’s mass was . If the coefficient of static friction between hand and rafter was , what was the least magnitude of the normal force on the rafter from each thumb or opposite fingers? (After suspending himself, Sincosky chinned himself on the rafter and then moved hand-over-hand along the rafter. If you do not think Sincosky’s grip was remarkable, try to repeat his stunt)
Continuation of Problem 8. Now assume that Eq. 6-14 gives the magnitude of the air drag force on the typical stone, which presents to the wind a vertical cross-sectional area ofand has a drag coefficient C of. (a) In kilometers per hour, what wind speedalong the ground is needed to maintain the stone’s motion once it has started moving? Because winds along the ground are retarded by the ground, the wind speeds reported for storms are often measured at a height of. Assume wind speeds are times those along the ground. (b) For your answer to (a), what wind speed would be reported for the storm? (c) Is that value reasonable for a high-speed wind in a storm?
A block is pushed along a horizontal floor by a force of magnitude at an angle with the horizontal (Fig. 6-19). The coefficient of kinetic friction between the block and the floor is . Calculate the magnitudes of (a) the frictional force on the block from the floor and (b) the block’s acceleration.
You must push a crate across a floor to a docking bay. The crate weighs 165 N. The coefficient of static friction between crate and floor is 0.510, and the coefficient of kinetic friction is 0.32. Your force on the crate is directed horizontally. (a) What magnitude of your push puts the crate on the verge of sliding? (b) With what magnitude must you then push to keep the crate moving at a constant velocity? (c) If, instead, you then push with the same magnitude as the answer to (a), what is the magnitude of the crate’s acceleration?
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