6.2 Friction  (Page 9/12)

Show that the acceleration of any object down an incline where friction behaves simply (that is, where $f_{\text{k}}={\mu }_{\text{k}}N)$ is $a=g(\text{sin}\theta -{\mu }_{\text{k}}\text{cos}\theta ).$ Note that the acceleration is independent of mass and reduces to the expression found in the previous problem when friction becomes negligibly small $({\mu }_{\text{k}}=0).$

Calculate the deceleration of a snow boarder going up a $5.00\text{°}$ slope, assuming the coefficient of friction for waxed wood on wet snow. The result of the preceding problem may be useful, but be careful to consider the fact that the snow boarder is going uphill.

A machine at a post office sends packages out a chute and down a ramp to be loaded into delivery vehicles. (a) Calculate the acceleration of a box heading down a $10.0\text{°}$ slope, assuming the coefficient of friction for a parcel on waxed wood is 0.100. (b) Find the angle of the slope down which this box could move at a constant velocity. You can neglect air resistance in both parts.

If an object is to rest on an incline without slipping, then friction must equal the component of the weight of the object parallel to the incline. This requires greater and greater friction for steeper slopes. Show that the maximum angle of an incline above the horizontal for which an object will not slide down is $\theta ={\text{tan}}^{\mathrm{-1}}{\mu }_{\text{s}}.$ You may use the result of the previous problem. Assume that $a=0$ and that static friction has reached its maximum value.

Calculate the maximum acceleration of a car that is heading down a $6.00\text{°}$ slope (one that makes an angle of $6.00\text{°}$ with the horizontal) under the following road conditions. You may assume that the weight of the car is evenly distributed on all four tires and that the coefficient of static friction is involved—that is, the tires are not allowed to slip during the deceleration. (Ignore rolling.) Calculate for a car: (a) On dry concrete. (b) On wet concrete. (c) On ice, assuming that ${\mu }_{\text{s}}=0.100$ , the same as for shoes on ice.

Calculate the maximum acceleration of a car that is heading up a $4.00\text{°}$ slope (one that makes an angle of $4.00\text{°}$ with the horizontal) under the following road conditions. Assume that only half the weight of the car is supported by the two drive wheels and that the coefficient of static friction is involved—that is, the tires are not allowed to slip during the acceleration. (Ignore rolling.) (a) On dry concrete. (b) On wet concrete. (c) On ice, assuming that ${\mu }_{\text{s}}=0.100$ , the same as for shoes on ice.

Repeat the preceding problem for a car with four-wheel drive.

A freight train consists of two $8.00\times {10}^5\text{-kg}$ engines and 45 cars with average masses of $5.50\times {10}^5\text{kg}\text{.}$ (a) What force must each engine exert backward on the track to accelerate the train at a rate of $5.00\times {10}^{\mathrm{-2}}\text{m}\text{/}{\text{s}}^2$ if the force of friction is $7.50\times {10}^5\text{N}$ , assuming the engines exert identical forces? This is not a large frictional force for such a massive system. Rolling friction for trains is small, and consequently, trains are very energy-efficient transportation systems. (b) What is the force in the coupling between the 37th and 38th cars (this is the force each exerts on the other), assuming all cars have the same mass and that friction is evenly distributed among all of the cars and engines?

Consider the 52.0-kg mountain climber shown below. (a) Find the tension in the rope and the force that the mountain climber must exert with her feet on the vertical rock face to remain stationary. Assume that the force is exerted parallel to her legs. Also, assume negligible force exerted by her arms. (b) What is the minimum coefficient of friction between her shoes and the cliff?

A contestant in a winter sporting event pushes a 45.0-kg block of ice across a frozen lake as shown below. (a) Calculate the minimum force F he must exert to get the block moving. (b) What is its acceleration once it starts to move, if that force is maintained?

The contestant now pulls the block of ice with a rope over his shoulder at the same angle above the horizontal as shown below. Calculate the minimum force F he must exert to get the block moving. (b) What is its acceleration once it starts to move, if that force is maintained?

At a post office, a parcel that is a 20.0-kg box slides down a ramp inclined at $30.0\text{°}$ with the horizontal. The coefficient of kinetic friction between the box and plane is 0.0300. (a) Find the acceleration of the box. (b) Find the velocity of the box as it reaches the end of the plane, if the length of the plane is 2 m and the box starts at rest.