Question

is static or not moving.

1. _______________ friction is the resistance on a moving object.
2. _______________ friction is caused by a combination of the resistance due to the roughness of the surface and the molecular attractions trying to hold the surfaces together.
3. once an object is moving, the amount of friction is ____________ than when it was standing still.
4. _______________ friction is primarily caused by the stickiness of the surfaces.

reducing friction

friction can be a nuisance, because it changes kinetic energy into heat which is usually wasted. friction also tends to wear away at the surfaces, causing damage.

we can reduce friction by oiling (\lubricating\) the surfaces. this means that the surfaces no longer rub directly on each other, but slide past on a layer of oil. it’s now much easier to move them.

a slippery material such as grease, oil, or graphite holds the surfaces of a bearing apart and thus greatly reduces friction. even wax or soap will act as a lubricant. nails or screws will go into wood if wax is applied to lubricate them. lubrication is essential for reducing friction and protecting bearing surfaces of machines.

fill in the blank.

1. _______________ is essential for reducing friction and protecting bearing surfaces of machines.
2. name three slippery materials used for lubrication. _____________ ___________ _____________

Explanation:

Question 1 (Fill in the blank)

From the text under "Reducing Friction", it says "Lubrication is essential for reducing friction and protecting bearing surfaces of machines." So we can directly get the answer from this sentence.

Looking at the text: "A slippery material such as grease, oil, or graphite holds the surfaces of a bearing apart... Even wax or soap will act as a lubricant." We can pick three from grease, oil, graphite, wax, soap. Let's choose grease, oil, and graphite (or any other three from the mentioned slippery materials).

Answer:

Lubrication

Question 2 (Name three slippery materials)