Answer:

Useful/not useful

a. Useful

b. Useful

c. Not useful

d. Not useful

e. Useful

Increase/decrease friction

a. Increase

b. Decrease

c. Decrease

d. Decrease

e. Decrease

Explanation:

Useful or not useful?

a. Friction between the shoes and the floor is useful, because if there's no friction we might slip.

b. Friction between the bicycle tires and the road is useful, so the bicycle can move without skidding.

c. Friction in the axle of a bicycle wheel is not useful, because we need to oiling the axle so that the bicycle wheel is not easily damaged.

d. Friction between skis and the snow is not useful, because it can detain the movement of skiers.

e. Friction between brake pads and a bicycle wheel is useful, so that we can stop the bicycle when we are braking.

Increase or decrease friction?

a. Using a special rubber for climbing shoes can increase friction, with this climbers can be safer in climbing because it reduces slippage.

b. Rubbing smooth wax onto the bottom of downhill skis can decrease friction, it allows them to slide much faster when they ski downhill.

c. Oiling the axles of a bicycle can decrease friction, it allows your bicycle to move easily and smoothly.

d. Getting a bicycle's brakes wet can decrease friction, which can make it more difficult for the bike to brake.

e. Polishing a playground slide to make the surface smoother can decrease friction, because less friction allows sliding freely.