When a body is projected vertically upwards with a velocity 10 m/s, its speed after 1 seconds is (g =10m/s2) (A) 20 m/s (B) zero (C) 10 m/s (D) 15 m/s
Answers
Question-
When a body is projected vertically upwards with a velocity 10 m/s, its speed after 1 second is (g =10m/s₂)
Options-
(A) 20 m/s
(B) zero
(C) 10 m/s
(D) 15 m/s
Answer-
v = final velocity
u = initial velocity
g = gravitational acceleration
s = displacement.
As the body moves with the velocity of 10 m/s, and the time passed is 1 second, the displacement covered by the body will be=
s = v/t
s = 10 ms⁻¹/1s
s = 10m
Now we have to use the third equation of motion.
v² - u² = 2as
⇒ v² - u² = -2gh (Acceleration is gravitational acceleration)
⇒ v² = -2gh + u²
⇒ v² = -2 × 10 m/s² × 10 + (10 m/s)²
⇒ v² = 100 m/s² - 100 m/s²
⇒ v² = 0
⇒ v = 0 m/s
We can also use another method which is a shortcut method. You don't need the application of equations in this method.
What you need to simply understand, is that the speed of the body will always be less than 10 m/s. This is because there is gravity involved. As we know, the gravity of the earth exerts an attractive force on any body. It is also given in the question to take g = 10m/s². So the speed MUST be less than the initial speed. The options in the question are-
(A) 20 m/s
(B) zero
(C) 10 m/s
(D) 15 m/s
The initial velocity was 10 m/s. Now we can easily tell from the question that the answer is 0. This is because 0 is the only speed less than 10 m/s in the options of the given question. This trick is very helpful for competitive examination as it saves our time.