Determine the stopping distances for an automobile going a constant initial speed? Mastering Physics?

Question

Determine the stopping distances for an automobile going a constant initial speed of 94 km/h and human reaction time of 0.30 s , for an acceleration a=−3.6m/s^2.

Express your answer using two significant figures.
D= ________ m

Determine the stopping distances for an automobile going a constant initial speed of 94 km/h and human reaction time of 0.30 s, for an acceleration a=−5.8m/s^2.

D=_________ m

Answer 1

(1)

Step 1:

Initial speed of automobile is .

Human reaction time is  sec.

Acceleration is .

.

From Linear motion equations : .

Substitute  and  in the formula.

Find the time, when the automobile stops moving. 

When the automobile is stopped, .

 sec.

Step 2:

Find the distance traveled by the automobile, when  sec.

From Linear motion equation : .

Substitute ,  and  in the formula.

 

Find the distance covered during reaction time. 

.

Find stopping distance by adding results in (1) and (2).

Stopping distance is .

Solution:

Stopping distance is .

Answer 2

(2)

Step 1:

Initial speed of automobile is .

Human reaction time is sec.

Acceleration is .

.

From Linear motion equation : .

Substitute and   in the formula.

When the automobile is stopped, .

sec.

Step 2:

Find the distance traveled by the automobile, when  sec.

From Linear motion equation : .

Substitute ,  and  in the formula.

.

Find the distance covered during reaction time. 

.

Find stopping distance by adding results in (1) and (2).

Stopping distace is .

Solution:

Stopping distace is .