## Topic outline

• ### Friction

##### Q44. Why do we slip when we step on a banana peel?

Ans. The inner side of banana peel being smooth and slippery reduces the friction between the sole of our shoes and the surface of road. Thus, we slip on it.

##### Q45. Why the sole of our shoes is grooved?

Ans. The grooves are made in the soles of shoes to increase friction with the ground so that the shoes get a better grip on the floor and we can walk safely.

##### Q46. Why do car wheels often spin on icy roads?

Ans. A car spins on icy road because the treads of the car can no longer maintain the proper friction to keep it moving. Due to ice, the friction reduces and the car spins.

##### Q47. Why a pencil eraser loses tiny pieces of rubber each time we use it?

Ans. When we use a pencil eraser, friction between the eraser and the paper rubs off some rubber particles from the eraser. Thus, the eraser loses tiny pieces of rubber due to friction.

##### Q48. Why a vehicle slows down when brakes are applied?

Ans. When brakes are applied, the brake pads press against the discs of the rotating car wheels. This produces friction between brake pads and the discs, making the wheels to slow down and ultimately stop.

##### Even a child can pull attaches and other pieces of luggage fitted with rollers. Why is it so?

Ans. Rolling reduces friction. It is always easier to roll than to slide a body over another. That is the reason it is convenient to pull the luggages fitted with rollers.

##### Q50. What enables us to fix nails in a wall and knots to be tied?

Ans. When we hammer a nail in the wall, it is the friction between the surface of nail and wall which holds the nail tightly in the wall. Without friction, nails could not be fixed in a wall to hold things.

Friction enables knots to be tied in the ropes.

##### Q51. What is a spring balance?

Ans. Spring balance is a device used for measuring the force acting on an object. It consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force.

##### Q52. How does a bicycle stop when its brakes are applied?

Ans. We deliberately increase friction by using brake pads in the brake system of bicycles and automobiles. When we are riding a bicycle, the brake pads do not touch the wheels. But when we press the brake lever, these pads arrest the motion of the rim due to friction. The wheel stops moving.

##### Q53. When we strike a matchstick against the rough surface, it catches fire. Give reason.

Ans. When we strike a matchstick against the rough surface, then the friction between the head of the matchstick and rough surface produces heat. This heat burns the chemicals present on the head of the matchstick due to which the matchstick catches fire.

##### Q54. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Ans. When a book slides on the writing desk, a frictional force acts between the book and the surface of the desk. The direction of frictional force on the book is opposite to the direction of its motion and acts in upward direction.

##### Q55. How lubricants work?

Ans. When oil, grease or graphite is applied between the moving parts of a machine, a thin layer is formed there and moving surfaces do not directly rub against each other. Interlocking of irregularities is avoided to a great extent. Thus, Movement becomes smooth.

##### Q56. Explain why the sliding friction is less than the static friction.

Ans. Friction is caused by the interlocking of irregularities in the two surfaces. When the object starts sliding, the contact points on its surface do not get enough time to lock into the contact points on the floor. So, the sliding friction is slightly smaller than the static friction and we find it somewhat easier to move the object already in motion than to get it started.