NCERT 5th Class (CBSE) Science: Force and Energy – Simple Machine

Question: List any two advantages and two disadvantages of friction.

Answer: The two advantages of friction are:

1. The friction between the tyres of vehicles and the road makes them roll forward. Thus, friction makes movement of vehicles possible.
2. Friction between the chalk and the black board makes it possible for the teachers to write on the blackboard.

The two disadvantages of friction are:

1. Friction makes it hard to slide heavy objects accross the floor.
2. Friction generates heat. Heat produced due to friction causes damage to machine parts like car brakes.

Question: What is a lever? Describe the three types with examples.

Answer: A lever is a rigid road arranged in such a manner that it can move freely around a fixed point. It consists of the following three parts:

1. Fulcrum: This is the fixed point around which the road moves.
2. Load: It is the object on which work is to be performed.
3. Effort: It is the force that needs to be applied on te road in order to perform a task.

First class lever:

1. Levers in which the fulcrum is located between the load and effort are called first class or class one levers.
2. Eg: a pair of scissors, a seesaw and a crowbar.

Second class lever:

1. Levers in which the load is located between the fulcrum and the effort are called second class or class two levers.
2. Eg: a wheelbarrow, a bottle opener and a nut cracker.

Third class lever:

1. Levers in which the effort is located between the fulcrum and the load are called third class or class three lever.

Question: What is a wheel and axle? Give examples.

Answer: A wheel and axle consists of a wheel rigidly attached to a rod (called axle). When one rotates the wheel, the axle also rotates.
Eg: The crane used to raise a bucket of water from a well, a screw driver, a door knob and the steering wheel of a car.

Question: What is a pulley? What are its different types? Give examples.

Answer: A pulley consists of a wheel with a grooved rim, with a rope, chain or belt running round it. The wheel rotates around a stationary axle. A pulley can be of two types fixed and movable.

Fixed pulley:

1. The axle is fixed to some kind of support like a hook or a wall. This pulley changes the destruction of the force and makes the work easier.
2. Eg: To lift a bucket of water from a well, to raise a flag to the top of a flagholder.

Movable pulley:

1. The axle is not fixed to any kind of support. It is free and movable. This pulley helps to lift heavy loads with less effort. It is usually used in combination with a fixed pulley.
2. Eg: Machine like a crane uses both fixed and movable pulleys to lift heavy loads.