Question

An object is acted upon by the forces $\vec { F } _ { 1 }$= $4 \hat { i }$ N and $\vec { F } _ { 2 }$ =

( $\hat { \boldsymbol { i } } - \hat { \boldsymbol { j } }$ )N. If the displacement of the object is D =

( $\hat { i } + 6 \hat { j } - 6 \hat { k }$ )m, the kinetic energy of the object

• A. Remains constant
• B. Increases by 1J
• C. Decreases by 1 J
• D. Decreases by 2J

Answer 1

Answer: (C)

Decreases by 1 J

Answer 2

The work done on the object W = $\vec { F } _ { 1 } \cdot \vec { D } + \vec { F } _ { 2 } \cdot \vec { D }$ where is the displacement vector.

∴ W = 4(1) + (1) (1) + (−1) (6) = −1J

From work energy theorem W = KE_f – KE_i = −1J

⇒ Kinetic energy decrease by 1J. Hence, the correct choice is (3)