What is Friction?
First, let’s familiarize ourselves with “normal force”. The normal force is the component exactly perpendicular to where the two surfaces touch. When you stand on the ground, the normal force points up to the sky because the two touching surfaces are the bottoms of your feet and the ground. When you push a box up a ramp, the normal force is perpendicular to the ramp:
In the standard model of friction, we consider friction to be proportional to the normal force. Frictional force F equals mu times N:
The proportionality constant, μ, changes depending on if the object is in motion or is standing still. We use the coefficient of static friction when the object is not in motion.
Suppose we have a very heavy box we need to push across the ground. We push lightly, and the box doesn’t move. So we start pushing harder and harder until suddenly the box starts moving! In this example, the box doesn’t move at first because static friction resists the change in motion. As we push harder, static friction increases. However, we reach a point where the box suddenly starts moving. At this point, our model switches to kinetic friction, which is a constant, to describe the frictional forces.
This is our standard model of friction. Static friction is directly proportional to the normal force, and it increases until the object begins to move. Kinetic friction is a constant, and it is less than or sometimes nearly equal to the maximum static friction.
Friction eats up energy you would normally convert into useful energy. We see this in static friction: instead of an object moving as soon as you apply force, friction eats up that energy and resists the change. Then, kinetic friction takes away energy as an object moves. If you slide across the floor, eventually you come to a stop because kinetic friction resists your motion and slows you down.
There may be times when you want to minimize kinetic friction. If you want to slide further across the floor, you can buff the floor with wax to make the floor more slippery. Another option is to spray your feet with cooking oil to create a buffer between the floor and your feet. In this case, the cooking oil acts as a lubricant and decreases the coefficient of kinetic friction.
Another way we can lower friction is by using ball bearings and grease in wheels. The ball bearings help the wheel rotate more smoothly and the grease creates a more slippery surface for the balls to roll over.
Sometimes, we actually want to increase kinetic friction! Let’s say you are developing a new kind of abrasive substance for sandpaper. The abrasive substance creates a high coefficient of friction between a test surface and the paper. We can even do an experiment to pick the most abrasive substance from a range of options. Using the same amount of paper and block of a given mass for each, attach a spring scale to the block and pull until the block starts to move. The maximum read from the scale is correlated with static friction. Then you can pull the block at a constant velocity to measure the kinetic friction.
Mass (g) 400
|B||263||153||0.66||0.38||<----- – Highest coefficient of kinetic friction|
|D||288||136||0.72||0.34||<----- – Highest coefficient of static friction|
Now, our model isn’t perfect. We are assuming kinetic friction is always constant. For low speeds, this is mostly true, but once you start going very fast, kinetic friction becomes dependent on velocity. One such example is skydiving. You are flying extremely fast through the air, gaining speed from the moment you jump, but, eventually, your speed is constant. This is called terminal velocity, and it happens because kinetic friction is proportional to the square of the velocity.
Now that we’ve covered the basics of what friction is, here’s an interesting example to test your knowledge.
Engineers who designed the Mars rover Curiosity were very worried about Mars’s atmosphere as the rover was landing on Mars. Why?
- The rover was going so fast that the kinetic friction from the molecules in the atmosphere could heat up the rover and blow it up.
- The normal force doesn’t exist on Mars so friction is infinite.
- They weren’t worried because the atmosphere doesn’t affect the rover’s landing.
- Gravity is less on Mars so the atmosphere doesn’t matter, and they were worried over nothing.
The answer is A. While gravity on Mars is less compared to gravity on Earth, the atmosphere on Mars is still thick enough to cause a fast-moving object to convert energy to heat because of kinetic friction. Engineers designed a special nose cone to protect the rover from the extreme heat. They also used a very specially designed parachute to slow down the rover to decrease the energy loss due to kinetic friction. This was particularly tricky due to the thin atmosphere! The parachute needs enough atmosphere to help slow down, which is really hard to do when there isn’t much of an atmosphere to start with!
Thanks for watching, and happy studying!