6th Class Science Motion and Measurement of Distances NCERT Summary - Motion, Force, and Machines

Motion, Force, and Machines

MOTION

• When an object changes its position with time as compared to a stationary object, it is said to be in motion.
• In science, motions are classified as follows: (i) linear motion, (ii) random motion, (iii) circular motion, (iv) oscillatory motion.

(i)Linear motion: A bullet fired from a rifle, a boy sliding down a slope or a ball rolling on the ground or moving along a line are examples of linear motion.

Note: In a linear motion, an object may move along a straight line or a curved line.

(ii) Random motion: The motion of a fly, of a player on a football ground, or of a child at home are not along a fixed path. They keep on changing directions. Such motions are called random motions.

(iii) Circular motion: The Moon moves around the Earth. The Earth moves round the Sun. A bull moves around a central pole. These objects move along a circular path. Such motions are called circular motions.

(iv) Oscillatory motion: If a hanging object is taken to one side and then released, it starts moving like a swing. Such to-and-fro motion is called oscillatory motion.

SPEED

The speed of an object can be calculated by using the relation:

\[speed=\frac{Total\,\,\text{distance}\,\,travelled}{Time\,\,taken}\]

Example: If a train travels 120 km in 3 hours, its speed per hour will be

\[Speed=\frac{120}{3}\]

\[Speed=40\,Kilometre/hour\]

• The standard unit of distance is metre and the standard unit of time is second. Therefore, the unit of speed is metre/second or m/s or \[m{{s}^{-1}}\].
• For convenience, the speed of some objects is expressed in metres/minute. Cheetah is a fast animal. It can move at a speed of 1,700 metres/minute.

FORCE

• The push or pull applied on an object is called force. The direction in which the object is pushed or pulled is called the direction of force. The effect of force can bring three kinds of changes—

(i)   Change in speed

(ii)   Change in direction

(iii) Change in shape

(i) Change in speed: If a force is applied in the direction of motion of the object, its speed increases and if the force is applied in the direction opposite to the direction of motion of the object, its speed decreases.

For example: Hitting (applying force) a glass marble in motion with another marble from behind increases the speed of the moving marble. However, when the moving marble is hit (applied force) with another marble from the opposite direction, the speed of the moving marble decreases.

(ii)Change in direction: Force can change the direction of motion of a moving object.

For example: During a game of cricket, if a moving ball is hit by a bat, the direction of the ball changes; the smoke rising from an agarbatti changes its direction if we gently blow air on it.

(iii) Change in shape: When a force is applied on an object, it may undergo a change in shape.

For example: The shape of dough changes on applying force, the shape of sponge changes on pressing, the shape of an iron strip changes on hammering.

Types of Forces

(i)   Muscular force

(ii)   Magnetic force

(iii)  Electrostatic force

(iv) Gravitational force

(v) Frictional force

(i) Muscular force: The force exerted by the muscles is called muscular force. Both animals and human beings exert muscular force to do work.

(ii) Magnetic force: The force exerted by a magnet is called magnetic force. If a magnet is brought near small objects made of iron like nails or pins, the magnet pulls them towards it. It means that the magnet has applied a force on these objects.

(iii) Electrostatic force: The force exerted by an electrostatic charge is called electrostatic force. For example, take a piece of paper and tear it into tiny pieces. Place them over a notebook. Now take a plastic ball point pen. Rub the body of the pen with your dry hair for about a minute. Bring it near these tiny pieces of paper. They appear to move towards the pen. This is due to the force exerted by electrostatic charge acquired by the pen.

(iv) Gravitational force: If a ball or a stone is released from some height, it falls on the ground. The Earth pulls them down. This pull of the Earth is a force called force of gravity. The Earth exerts this force on all objects. As the Earth exerts a force on the bodies, all bodies exert a force on each other also. The force exerted by the bodies that possess mass is called gravitational force.

(v) Frictional force: If a ball is rolled on the floor, it stops after some time. This is because when the ball moves, a force acts on it which opposes its motion. This force is called frictional force. It acts between two surfaces in this exam (the ball and the floor) that are in contact.

Note: If a ball is rolled with a similar force on a cemented floor and then on a kaccha floor, the ball rolling on the cemented floor will travel a larger distance than the ball rolling on the kachcha floor as the cemented floor is smooth and the frictional former force between it and the ball is less. Since the kachcha floor is rough, the frictional force between the floor and the ball is greater. Thus, the frictional force depends on the smoothness of the surfaces in contact with each other.

Disadvantages and Advantages of Friction

• Disadvantages of Friction: When you step on a banana peel, you might slip because the banana peel reduces friction between the foot and the floor. Due to friction between the sole of a shoe and the floor, the shoe's wears out. Machine that rubs against each other wear out due to friction.
• Advantages of Friction: Without friction between a pencil or a pen and paper, writing and drawing would not be possible. It would be impossible to cut wood without friction between saw and a tree. The burning of a matchstick would not be possible without friction.

MACHINES

• With the use of machines, our work becomes easier and faster. In situations where a greater force is required we can do more work with the help of machines to perform by applying less force.
  • Generally, machines are of two kinds:

(i) Simple machines

(ii) Complex machines

(i) Simple Machines: A knife, a screw, a pair of tongs, a pulley are some examples of simple machines.

(ii) Complex machines: Complex machines are made up of a large number of simple machines. A bicycle, a sewing machine, a tractor are some examples of complex machines.