# friction

## what is friction

Friction is a force that opposes motion between two surfaces that are touching. Friction can work for or against us. For example, putting sand on an icy sidewalk increases friction so you are less likely to slip. On the other hand, too much friction between moving parts in a car engine can cause the parts to wear out. Other examples of friction are illustrated in Figure 13.7. You can see an animation showing how friction opposes motion at this URL: http://w

## why friction occurs

Friction occurs because no surface is perfectly smooth. Even surfaces that look smooth to the unaided eye appear rough or bumpy when viewed under a microscope. Look at the metal surfaces in Figure 13.8. The metal foil is so smooth that it is shiny. However, when highly magnified, the surface of metal appears to be very bumpy. All those mountains and valleys catch and grab the mountains and valleys of any other surface that contacts the metal. This creates friction.

## factors that affect friction

Rougher surfaces have more friction between them than smoother surfaces. Thats why we put sand on icy sidewalks and roads. The blades of skates are much smoother than the soles of shoes. Thats why you cant slide as far across ice with shoes as you can with skates (see Figure 13.9). The rougher surface of shoes causes more friction and slows you down. Heavier objects also have more friction because they press together with greater force. Did you ever try to push boxes or furniture across the floor? Its harder to overcome friction between heavier objects and the floor than it is between lighter objects and the floor.

## static friction

Static friction acts on objects when they are resting on a surface. For example, if you are walking on a sidewalk, there is static friction between your shoes and the concrete each time you put down your foot (see Figure 13.12). Without this static friction, your feet would slip out from under you, making it difficult to walk. Static friction also allows you to sit in a chair without sliding to the floor. Can you think of other examples of static friction?

## friction produces heat

You know that friction produces heat. Thats why rubbing your hands together makes them warmer. But do you know why the rubbing produces heat? Friction causes the molecules on rubbing surfaces to move faster, so they have more heat energy. Heat from friction can be useful. It not only warms your hands. It also lets you light a match (see Figure 13.10). On the other hand, heat from friction can be a problem inside a car engine. It can cause the car to overheat. To reduce friction, oil is added to the engine. Oil coats the surfaces of moving parts and makes them slippery so there is less friction.

## rolling friction

Rolling friction is friction that acts on objects when they are rolling over a surface. Rolling friction is much weaker than sliding friction or static friction. This explains why it is much easier to move boxes on a wheeled dolly than by carrying or sliding them. It also explains why most forms of ground transportation use wheels, including cars, 4-wheelers, bicycles, roller skates, and skateboards. Ball bearings are another use of rolling friction (see Figure

## types of friction

There are different ways you could move heavy boxes. You could pick them up and carry them. You could slide them across the floor. Or you could put them on a dolly like the one in Figure 13.11 and roll them across the floor. This example illustrates three types of friction: static friction, sliding friction, and rolling friction. Another type of friction is fluid friction. All four types of friction are described below. In each type, friction works opposite the direction of the force applied to a move an object. You can see a video demonstration of the different types of friction at this URL: (1:07).

## fluid friction

Fluid friction is friction that acts on objects that are moving through a fluid. A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases. If youve ever tried to push your open hand through the water in a tub or pool, then youve experienced fluid friction between your hand and the water. When a skydiver is falling toward Earth with a parachute, fluid friction between the parachute and the air slows the descent (see Figure 13.14). Fluid pressure with the air is called air resistance. The faster or larger a moving object is, the greater is the fluid friction resisting its motion. The very large surface area of a parachute, for example, has greater air resistance than a skydivers body.

## sliding friction

Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than static friction. Thats why its easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write with a pencil and when you put on your bikes brakes.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

Friction occurs because

``````-->  a. all surfaces have some roughness.

b. surfaces in contact generate heat.

c. chemical reactions take place when surfaces touch.

d. none of the above
``````

Friction is greater when surfaces are

``````-->  a. rougher.

b. smoother.

c. smaller.

d. two of the above
``````

Which factors affect friction?

``````a. roughness of the surfaces

b. area of the surfaces

c. force of weight pressing on the surfaces

-->  d. two of the above
``````

Rubbing your hands together makes them warmer because

``````-->  a. friction causes molecules to move faster.

b. rubbing causes chemical reactions.

c. rubbing causes skin cells to release enzymes.

d. none of the above
``````

If you pick up and carry a piece of heavy furniture, which type of friction do you have with the floor?

``````-->  a. static friction

b. lifting friction

c. sliding friction

d. rolling friction
``````

Which way of moving a box produces no friction?

``````a. sliding the box across the floor

b. rolling the box on a dolly

c. picking up the box and carrying it

-->  d. none of the above
``````

Why do ball bearings reduce friction in a wheel?

``````a. Ball bearings prevent the wheel from sliding on the road.

b. Rolling friction is less than sliding friction.

c. Ball bearings roll instead of slide.

-->  d. two of the above
``````

Why is it easier to slide a heavy box over a floor that it is to start it sliding in the first place?

``````a. The box is lighter when it is sliding.

b. The box has less mass when it is moving.

c. The box has no friction when it is stationary.

-->  d. The box has less friction when it is sliding.
``````

Which statement about rolling friction is false?

``````a. It would be hard to ride a bike without it.

b. It occurs when ball bearings are used.

-->  c. It is stronger than sliding friction.

d. It is weaker than static friction.
``````

Fluid friction is greater when the object moving through the fluid is

``````a. larger.

b. smaller.

c. faster.

-->  d. two of the above
``````

A skydiver uses a parachute to

``````a. increase air resistance.

b. cushion the landing.

c. slow the descent.

-->  d. two of the above
``````

Which type of friction occurs between a paddle and the water?

``````a. static friction

b. sliding friction

-->  c. fluid friction

d. rolling friction
``````

Friction is never useful.

``````a. true

-->  b. false
``````

Too much friction can cause parts to wear out.

``````-->  a. true

b. false
``````

Friction can cause scrapes on the skin.

``````-->  a. true

b. false
``````

Friction is a force that opposes motion.

``````-->  a. true

b. false
``````

Some surfaces are so smooth that they have no friction.

``````a. true

-->  b. false
``````

The force of friction between surfaces is always useful.

``````a. true

-->  b. false
``````

You use friction when you strike and light a match.

``````-->  a. true

b. false
``````

It takes more force to slide than to roll a heavy object.

``````-->  a. true

b. false
``````

Sliding friction is greater when the sliding object is heavier.

``````-->  a. true

b. false
``````

Friction works in the same direction as the force applied to move an object.

``````a. true

-->  b. false
``````

Sliding friction is stronger than static friction.

``````a. true

-->  b. false
``````

Rolling friction is weaker than sliding friction.

``````-->  a. true

b. false
``````

When a dolly is stationary, there is rolling friction between the wheels and ground.

``````a. true

-->  b. false
``````

Static friction prevents you from sliding out of your chair to the floor.

``````-->  a. true

b. false
``````

The brakes on a bike create rolling friction.

``````a. true

-->  b. false
``````

type of friction between ice skates and ice

``````a. friction

b. static friction

c. air resistance

d. fluid

-->  e. sliding friction

f. fluid friction

g. rolling friction
``````

any substance that can flow and take the shape of its container

``````a. friction

b. static friction

c. air resistance

-->  d. fluid

e. sliding friction

f. fluid friction

g. rolling friction
``````

force that opposes motion between any two surfaces

``````-->  a. friction

b. static friction

c. air resistance

d. fluid

e. sliding friction

f. fluid friction

g. rolling friction
``````

type of friction between shoes and pavement

``````a. friction

-->  b. static friction

c. air resistance

d. fluid

e. sliding friction

f. fluid friction

g. rolling friction
``````

type of friction between a parachute and air

``````a. friction

b. static friction

-->  c. air resistance

d. fluid

e. sliding friction

f. fluid friction

g. rolling friction
``````

type of friction between roller skates and concrete

``````a. friction

b. static friction

c. air resistance

d. fluid

e. sliding friction

f. fluid friction

-->  g. rolling friction
``````

type of friction between an object and a gas or liquid

``````a. friction

b. static friction

c. air resistance

d. fluid

e. sliding friction

-->  f. fluid friction

g. rolling friction
``````

## diagram questions

No diagram questions associated with this lesson