Question

What was the initial speed of a car if its speed is 40 m/s after 5 seconds of

Answer 1

Answer:

$\boxed {\boxed {\sf C. \ 20 \ m/s }}$

Explanation:

We are asked to find the initial speed of a car.

We are given the final speed, the time, and the acceleration, so we will use the following kinematic equation:

$v_f=v_i+at$

We know the final speed is 40 meters per second, the acceleration is 4 meters per second squared, and the time is 5 seconds.

• $v_f$= 40 m/s
• t= 5 s
• a= 4 m/s²

Substitute the values into the formula.

$40 \ m/s = v_i+(4 \ m/s^2 * 5 \ s)$

Multiply inside the parentheses.

$40 \ m/s =v_i+ 20 \ m/s$

We are solving for the initial speed, so we must isolate the variable $v_i$.  

20 meters per second is being added to $v_i$. The inverse operation of addition is subtraction. Subtract 20 m/s from both sides of the equation.

$40 \ m/s - 20 \ m/s = v_i + 20 \ m/s - 20 \ m/s$

$40 \ m/s - 20 m/ s = v_i$

$20 \ m/s=v_i$

The initial speed of the car is 20 meters per second and choice C is correct.

Answer 2

We use the formula:
V= u + at

V- final velocity. U- intial velocity. A- acceleration. T- time

Note: velocity is speed in this question

Fill in values:
V= u + at
40= u + 4 x 5
40= u + 20
40 - 20 = u
u = 20m/s