5.1 Friction

When a force of 20 N is applied to a stationary box weighing 40 N, the box does not move. This means the coefficient of static friction

1. is equal to 0.5.
2. is greater than 0.5.
3. is less than 0.5.
4. cannot be determined.

A block is given a short push and then slides with constant friction across a horizontal floor. Which statement best explains the direction of the force that friction applies on the moving block?

1. Friction will be in the same direction as the block's motion because molecular interactions between the block and the floor will deform the block in the direction of motion.
2. Friction will be in the same di…

A physics major is cooking breakfast when he notices that the frictional force between his steel spatula and his Teflon frying pan is only 0.200 N. Knowing the coefficient of kinetic friction between the two materials, he quickly calculates the normal force. What is it?

Suppose you have a 120-kg wooden crate resting on a wood floor. (a) What maximum force can you exert horizontally on the crate without moving it? (b) If you continue to exert this force once the crate starts to slip, what will the magnitude of its acceleration then be?

A team of eight dogs pulls a sled with waxed wood runners on wet snow (mush!). The dogs have average masses of 19.0 kg, and the loaded sled with its rider has a mass of 210 kg. (a) Calculate the magnitude of the acceleration starting from rest if each dog exerts an average force of 185 N backward on the snow. (b) What is the magnitude of the acceleration once the sled starts to move? (c) For both…

Calculate the deceleration of a snow boarder going up a $5.0º$, slope assuming the coefficient of friction for waxed wood on wet snow. The result of the exercise may be useful, but be careful to consider the fact that the snow boarder is going uphill. Explicitly show how you follow the steps in Problem-Solving Strategies.

Calculate the maximum acceleration of a car that is heading up a $\text{4}º$ slope (one that makes an angle of $\text{4}º$ 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 concr…

A freight train consists of two $8 . \text{00} \times \text{10}^{5} -\text{kg}$ engines and 45 cars with average masses of $5 . \text{50} \times \text{10}^{5} \text{ kg}$. (a) What force must each engine exert backward on the track to accelerate the train at a rate of $5 . \text{00} \times \text{10}^{- 2} m / s^{2}$ if the force of friction is $7 . \text{50} \times \text{10}^{5} N$, 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-eff…

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