Friction

Question 1

A book starts sliding down a table. Show the direction of frictional force acting on it.

Answer 1

The frictional force will act upwards, opposite to the direction of the sliding book.

Question 2

You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult to walk on it?

Answer 2

It is difficult to walk on the soapy floor because soapy water reduces friction, making the floor slippery.

Question 3

Explain why sportsmen use shoes with spikes.

Answer 3

Sportsmen use shoes with spikes to provide better grip on the ground.

Question 4

Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Why will Seema have to apply a larger force?

Answer 4

The force of friction increases when two surfaces are pressed harder. A heavier box applies more pressure on the floor, resulting in greater friction, so Seema will experience more frictional force.

Question 5

Explain why sliding friction is less than static friction.

Answer 5

The sliding friction is less than static friction because of the interlocking of irregularities on the two surfaces. When the objects start sliding, the contact points on its surfaces don’t get enough time to lock into the contact points on the floor. So, the sliding friction is slightly less than static friction.

Question 6

Explain Friction as a Friend and a Foe

Answer 6

Friction as a Friend: Friction of force is very useful in several situations. If there was no friction between the wheels and the road, no vehicle would be able to move.

Friction as a Foe: Machine parts that come in contact with one another wear out because of friction. Similarly, the soles of our shoes wear out because of friction.

Question 7

Why Objects Moving in Fluids Must Have Special Shapes

Answer 7

When objects move through fluids, they have to overcome the friction acting on them. Efforts are therefore made to minimize the friction, so objects are given special shapes. The idea of such shapes comes from the body structure of birds and fishes, which have to move about in fluids all the time. Such shapes are called streamlining. Giving such shapes to ships, planes, and cars is called streamlining.

Question 8

What is the cause of friction?

Answer 8

The cause of friction is the irregularities on the two surfaces in contact.

Question 9

Name the force responsible for wearing out of bicycle tires.

Answer 9

Frictional force.

Question 10

List the names of five smooth surfaces and five rough surfaces from your surroundings.

Answer 10

Smooth surfaces:

Surface of a mirror
Floor of the room
Surface of bed soaps
Glazed tiles
Surface of an oily pot

Rough surfaces:

Soles of shoes
Surfaces of plywood
Unpainted board
Surface of a brick
Surface of a tire

Question 11

Describe some examples when heat is produced due to friction.

Answer 11

Friction, which is the force that opposes the relative motion between the two surfaces, also produces heat.

When we rub our hands vigorously for a few minutes, we can feel heat.

When we strike a matchstick against a rough surface, it catches fire.

Question 12

When the cutting edge of a knife is put against a fast rotating stone to sharpen it, sparks are seen to fly. Explain the reason.

Answer 12

Friction between the grinding stone and the cutting edge of the knife generates a large amount of heat. This intense heat causes small particles of metal to get heated up and burn, resulting in sparks flying.

Question 13

Name the two factors which the magnitude of the frictional force depends on.

Answer 13

Friction depends on the nature (smoothness) of the surfaces in contact.

Friction depends on how hard the two surfaces are pressed together.

Question 14

The smoothness of a sliding surface affects friction. Explain

Answer 14

Friction is caused by the irregularities on the two surfaces in contact. Even though surfaces that appear very smooth have a large number of minute irregularities, the irregularities on the two surfaces lock into one another. When we attempt to move one surface over the other, we must apply force to overcome this interlocking. On rough surfaces, there are more irregularities, and the force of friction is greater.

Question 15

What are the different types of friction? Explain

Answer 15

1. Static Friction:
   Static friction is the friction that prevents an object from starting to move. It acts when an object is at rest. This type of friction is the strongest because the irregularities of the surfaces are tightly interlocked, making it hard to get the object moving.
   Example: If you try to push a heavy box that’s sitting on the floor and it doesn’t move, static friction is acting between the box and the floor, resisting the motion.
2. Sliding Friction:
   Sliding friction occurs when an object slides over another surface. Once an object begins to move, static friction turns into sliding friction. This type of friction is weaker than static friction because the surfaces are already in motion, so the interlocking of the irregularities is less.
   Example: When you slide a book across a table, the friction that resists the sliding motion is sliding friction.
3. Rolling Friction:
   Rolling friction happens when an object rolls over a surface, like a wheel or a ball. It is usually much smaller than sliding friction because a rolling object has less surface area in contact with the ground. Rolling friction is useful in many situations, like in vehicles with wheels.
   Example: When a bicycle tire rolls on the road, the friction between the tire and the road is rolling friction.

Question 16

How do brakes stop moving cycles and automobiles?

Answer 16

Brakes stop a moving cycle or automobile by using friction. When brakes are applied, the brake pads or shoes create friction with the wheels or discs of the vehicle. This frictional force opposes the motion of the wheels, causing them to slow down and eventually stop.