Friction

Problems

practice

1. A wooden pallet carrying a load of 600 kg rests on a wooden floor.
   1. A forklift driver decides to push it without lifting it. What force must be applied to just get the pallet moving?
   2. After a bit of time, the pallet begins to slide. How fast is the pallet moving after 0.5 seconds of sliding under the same force you calculated in part a?
   3. If the forklift stops pushing, how far does the pallet slide before coming to a stop?
2. Determine the following quantities for a car driving on a level surface with a coefficient of static friction of 0.75 (¾) and a coefficient of kinetic friction of 0.67 (⅔).
   1. Determine the car's maximum starting acceleration with and without "burning rubber". How do these two methods of starting a car compare?
   2. Determine the car's minimum braking distance with normal brakes and antilock brakes as a function of initial speed. How do these two methods of stopping a car compare?
3. Write something different.
4. road-test-summary.txt
   This tab-delimited text file contains the stopping distance data for 123 cars tested by Road & Track magazine in 1998. Two initial speeds were used: 26.82 m/s (60 mph) and 35.76 m/s (80 mph). Use the data in this file and your favorite data analysis software to determine the coefficient of static friction of car tires on pavement.

conceptual

1. In the classical model of friction, surface area does not affect the force of friction between two surfaces in contact. Given this statement (which has been verified by experiment) answer the following questions.
   1. Ordinary car tires have tread. (Racing cars don't, but that's another problem.) Tread reduces surface area in contact with the road, but surface area is not a factor affecting friction. Adding a complex shape to the exterior of a tire increases the manufacturing cost. Nobody likes spending money unnecessarily. What's the point behind this design choice? Why do ordinary car tires have tread?
   2. Skis are long and skinny. Snow boards are wide and short. The point behind both sports is to slip across the surface of the snow with relatively little friction. Why wear a piece of apparatus with a large surface area if surface area doesn't affect friction?

numerical

1. A 100 kg physics teacher pushes a 1.0 kg physics textbook across a 222 kg physics demo table.
   1. If the teacher needs to apply a force of 0.50 N to start the book sliding, what is the coefficient of static friction between the book and the table?
   2. If the teacher needs to apply a force of 0.40 N to keep the book sliding at a constant 0.30 m/s across the table, what is the coefficient of kinetic friction between the book and the table?
   3. If the teacher then stops pushing the book, how far does the book slide before coming to rest?
2. A car driving on asphalt comes to a complete stop with locked brakes in 10 m. What was the initial speed of the car?
3. The table below shows some of the best and worst performing cars in an emergency braking test.

   Emergency braking test

   	weight/mass		initial speed		braking distance
   make and model	(lb)	(kg)	(mph)	(m/s)	(ft)	(m)
   Mercedes-Benz E550 Coupe	3810	1728	80	35.8	252	76.8
   Porsche 911 GT3	3075	1395	80	35.8	186	56.7
   Volkswagen Golf GTI 3-dr	3370	1529	60	26.8	143	43.6
   Alfa Romeo 8C Competizione	3495	1585	60	26.8	105	32.0

   Source:

   Road & Track (September 2010)

   1. Draw a free body diagram of a car in an emergency braking test.
   2. For all four cars, determine the …
      1. braking acceleration
      2. coefficient of friction between the tires and test track

statistical

1. wood-wood.txt
   A group of physics students measured the static and kinetic frictional forces as they dragged a wooden block across a wooden plank resting on a table. They placed weights on top of the block, which varied the normal force. Construct a graph with the frictional forces on the vertical axis and the normal force on the horizontal axis. Add a line of best fit and determine the coefficients of friction for wood on wood.
2. wood-stone.txt
   A group of physics students connected a force sensor to a 2.0 kg wooden block and recorded the force they used to drag the block across a smooth, level stone table. They began the experiment by pulling lightly on the block with a gradually increasing force until the block started to move. Once the block started moving, they tried to pull it at a constant velocity (and were reasonably successful). The results of one trial were saved in this tab delimited text file. Use this data to determine the …
   1. coefficient of static friction
   2. coefficient of kinetic friction
   … for wood on stone.

investigative

1. The horizontal force component of the heel as it strikes the ground when a person is walking has been measured and found to be approximately 15% of a person's weight.
   1. What minimum coefficient of static friction do shoes need to have to keep a person from slipping?
   2. Wood, leather, and rubber have all been used as materials for the soles of shoes. Pick any one of these materials.
      1. Identify a surface that people can walk on using a sole made from this material because they won't slip.
      2. Identify a surface that people can't walk on using a sole made from this material because they will slip.
      Provide a reference for the source of your information and summarize your findings in a table like the one below.

      Sole material: wood, leather, rubber (select one)

      	can walk / won't slip	can't walk / will slip
      µs
      material
      reference

The Physics Hypertextbook © 1998–2013 Glenn Elert

No condition is permanent.