Question

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1
20 km/hr west relative to student 2
2
12 km/hr east relative to ground
what is the westward rate of motion of student 1 relative to the ground?
○ 32 km/hr
○ 20 km/hr
○ 12 km/hr
○ 8 km/hr

Explanation:

Step1: Define reference frames

Let east be positive, west be negative.
Let $v_{1/2}$ = velocity of student 1 relative to student 2 = $-20$ km/hr, $v_{2/g}$ = velocity of student 2 relative to ground = $12$ km/hr, $v_{1/g}$ = velocity of student 1 relative to ground (unknown).

Step2: Use relative velocity formula

The relative velocity formula is $v_{1/g} = v_{1/2} + v_{2/g}$
Substitute values:
$v_{1/g} = -20 + 12$

Step3: Calculate the result

$v_{1/g} = -8$ km/hr. The negative sign means direction is west.

Answer:

D. 8 km/hr