Question

If a ball is thrown vertically upwards with $40m/s$ its velocity after two seconds will be:-
A. $10m/s$
B.$20m/s$
C.$30m/s$
D.$40m/s$

Answer 1

Here, we will be using the third equation of motion, i.e. $v = u + at$ . Here we will keep $a = - g = - 10m/{s^2}$ as the acceleration is due to the gravitational force applied by the earth.

Complete answer:
Velocity - The velocity of an object, with respect to a frame of reference, is the rate of change of its location and is a function of time. Velocity refers to a definition of the speed and direction of motion of an object (e.g. 60 km / h to the north). In kinematics, the branch of classical mechanics that explains the motion of bodies, velocity is a basic concept.
Acceleration - In mechanics, acceleration is the rate at which an object's velocity varies in relation to time. Accelerations are (in that they have magnitude and direction) vector quantities. The orientation of the acceleration of an object is provided by the orientation of the net force which acts on that object. Newton's Second Law defines the extent of the acceleration of an object.
In this case, the acceleration of the body will be due to the gravitational force of the earth, so the acceleration, in this case, will be equal to –g (the negative sign indicates that the gravitational acceleration will be in a direction opposite to the initial motion of the ball.
We know that the third equation of motion is
$v = u + at$
Here, $v =$ The final velocity of the ball
$u = 40m/s =$ The initial velocity of the ball
$a = - g = - 10m/{s^2}$
$t = 2\sec =$ Time
$v = 40 + \left( { - 10} \right)2$
$v = 20m/s$
So, the final velocity of the ball after 2 seconds will be $20m/s$ will be in the upwards direction.

Hence, option B is the correct option.

Note:
Both velocity and acceleration are vector quantities. Vector, a quantity that has both magnitude and direction in physics. An arrow whose direction is the same as that of the quantity and whose length is equal to the magnitude of the quantity is usually represented. If there is a magnitude and direction for a vector, it does not have a position.