Question

A Dodge Stealth is driving at 70 mph on a highway. It passes a BMW going the same direction. The BMW is moving 7 mph backward relative the Dodge. What is the velocity of the BMW? Let the positive direction be forward.
a. 63 mph


b. 77 mph


c. −63 mph


d. −77 mph

Answer 1

A Dodge Stealth is driving at 70 mph on a highway. It passes a BMW going the same direction. The BMW is moving 7 mph backward relative the Dodge. 63 mph is the velocity of the BMW

Answer: Option A

Explanation:

Given is the speed of the dodge stealth as 70 mph passing by the BMW moving with 7 mph relative to Dodge. Thereby, to calculate the velocity of BMW, relative velocity concept has to be employed.  

As we know that the relative velocity tells us about relative velocity of mobile reference system as the difference or sum of the initial reference system.

Therefore,

velocity of the BMW =  $v_{b}$

Velocity of the dodge = $v_{d}$

Velocity of the BMW with respect to dodge = $V_{b d}$

As per the formula,

                     $v_{b d}=v_{b}-v_{d}$

        $v_{b} = v_{d} + v_{b d} = 70 + (-7) = 70 - 7 = 63 \mathrm{mph}$