An object is dropped without initial speed from a height of 20 m above ground and falls
with a constant acceleration of 10 m. s-2.
a. How long does it take the object to
reach the ground?
b. What is its speed upon reaching
ground?

a. t = 2 s

b. vf = 20 m/s

Explanation:

a.

We can the second equation of motion to find out the time to reach the ground:

where,

h = height = 20 m

vi = initial velocity = 0 m/s

t = time taken to reach the ground = ?

a = acceleration = 10 m/s²

Therefore,

t = 2 s

b.

Here, we will use the first equation of motion to find the final speed:

vf = 20 m/s

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.