Reader Questions on Friction

Here are some questions readers have asked concerning friction:

• Tires and friction
• Surface area of a clamp
• Torque on wheel and slippage

Tires and friction

I have been searching the web for an answer to a question and I ran across your discussion about friction and wheels. I am a beginning high school physics teacher and we just did a friction lab in which the students constructed Ff = u*Fn and we found no relationship between friction and surface area. I know this is true. The problem is many students then ask “Why do race cars put on wider tires? The drag racers have really wide tires for more traction. Isn’t that due to surface area?” I haven’t been successful in finding an answer for them. Could you help me out if you have a moment? Thank you.

Stefan Bradshaw
Student Teacher
Gilbert High School

Answer

Stefan —

With wheels or tires, there is not only sliding friction when the brakes are applied, when you go around a corner, or when you apply torque on the wheel to accelerate, but there is also what is called rolling friction. Rolling friction is not of concern here.

The friction in tires is a little more complex than dragging an object. If a tire was smooth or a wooden wheel was used, the standard rule for friction would hold. But most rubber tires have tread, and the number of edges on those treads in the direction of motion increase the friction for stopping or accelerating. So we have the friction of the rubber on the road, which is only dependent on the coefficient of friction and the weight, plus the “bite” of the edges. The number of edges is related to the area or width of the tire on the road.

Now race cars usually don’t have treads on their tires. They use rubber that is soft and almost sticky to the road. This is a form of molecular friction, and it is related to the surface area on the road. When a race car slides, it is more controlled with this type of tire than with a tire with treads, which may skid suddenly.

It would be good for the students to take a look at the different types of tires, including those used in snow and those good for rain. Observation is valuable.

Surface area of a clamp

I have a question regarding friction. An issue came up at work involving a three point chuck for a lathe. Some associates of mine were wondering if the surface area of the jaws of the chuck in contact with the object had any effect on frictional forces. Any help would be greatly appreciated.

Thanks, Jon

Answer

Jon —

The pressure or force at the point of contact is the major factor in friction. In theory surface area does not effect friction, but in reality it is a factor. The roughness of the surfaces in contact is more of a factor than surface area.

Look at disk brakes in a car, as opposed to the old drum brakes. The disk brake pads have a smaller surface area than brake drum pads, but they do as well if not better in stopping the car. It is the pressure and the materials used that are what make the brakes work.

I hope this helps you.

Torque on wheel and slippage:

If I have a wheel and know the weight on the wheel, the coefficient of friction, and the torque of the wheel shaft, how can I calculate if the wheel will slip or if it will roll?

Thanks, J. Woodward

Answer

This is a tricky problem.

If you push on a block of wood, you must overcome friction to get it to slide. That is not the way a wheel works. Consider pushing near the top of a high object. The friction at the ground–on the bottom–will keep it from sliding, and the object will tip over. If the object is round like a wheel, it will continually “tip over” and start rolling. See the drawings below:

Block slides if push overcomes friction.

Block tips forward when pushed near its top.

Wheel rolls forward when pushed near its top.

In other words, the resistive force of friction must be greater than the force at the top of the wheel. Although you know the torque, you also need to know the radius of the wheel to determine that force. If the force is less than the resistive force of friction (weight or wheel times coefficient of friction), then the wheel will roll.

Torque rolls wheel if friction great enough.

If your torque is large enough to overcome the friction, the wheel will slip. This is what happens when you step on your automobile accelerator too hard. Your wheels start to spin. When you are driving in icy conditions, it is very easy to overcome the friction, since the coefficient is so small.

High torque overcomes friction and causes wheel to spin.

I hope this helps you.

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Friction