Question

What was the initial speed of a car if its speed is 40 m/s after 5 seconds of accelerating at -4 m/s²?

A. 50 m/s
B. 60 m/s
C. 25 m/s
D. 20 m/s

Answer 1

`\huge\boxed{\sf V_i=60 \ m/s}`

Step-by-step explanation:

Given Data:

Final speed =
`V_f` = 40 m/s

Time = t = 5 s

Acceleration = a = -4 m/s²

Required:

Initial velocity =
`V_i` = ?

Formula:

`\displaystyle a = (V_f-V_i)/(t)`

Solution:

Put the givens in the above formula

`\displaystyle -4=(40 - V_i)/(5) \\\\Multiply \ -5 \ to \ both \ sides\\\\-4 * 5 = 40 - V_i\\\\-20 =40-V_i\\\\Subtract \ 40 \ to \ both \ sides\\\\-20-40=-V_i\\\\-60\ m/s=-V_i\\\\60 \ m/s = V_i\\\\V_i=60 \ m/s\\\\\rule[225]{225}{2}`