Because the Earth attracts (pulls) every object towards itself. That pull is the gravitational force.
A dropped object takes a straight vertical path downwards (Fig. 5.11a) because that is the direction in which the Earth pulls it — towards the Earth. An object thrown vertically upwards moves up straight, slows down, stops momentarily at the top, and then takes a straight vertical path downwards (Fig. 5.11b). Going up, its speed keeps decreasing; coming down, its speed keeps increasing. Both changes are produced by the same downward pull. Motion of this kind, under the influence of the gravitational force, is called vertical motion.
Q2.
Is there any force which acts on them? What exerts this force?
Answer
Yes — the gravitational force, and it is exerted by the Earth.
The gravitational force exerted by the Earth is also called the force of gravity, or simply gravity. Two things about it are worth fixing in mind:
It acts without contact with the object it attracts, so it is a non-contact force.
It is always an attractive force, unlike magnetic force or electrostatic force, which can be either attractive or repulsive.
Q3.
Does the Earth pull every object with equal force?
Answer
No. Different objects are pulled with different forces, and Activity 5.9 shows it directly.
Hang a spring from a nail and suspend a pencil box from it — the spring stretches. Change the object and the stretch changes. Since the stretch is produced by the Earth's pull on the hanging object, an unequal stretch means an unequal pull. The force with which the Earth pulls an object towards itself is what we call the weight of that object, so this is the same as saying that different objects have different weights.