First slide

Work

Question

A particle of mass 100 gm thrown vertically upwards with a speed of 5 m/s. The work done by the force of gravity during the time, the particle goes up is

Moderate

A

0.5 J
B

-0.5 J
C

-1.25 J
D

1.25 J

Solution

$W o r k d o n e b y g r a v i t y = - V e$
$f r o m w o r k e n e r g y t h e o r e m, w o r k d o n e = c h a n g e i n K E i n i t i a l v e l o c i t y = 5 m / s f i n a l v e l o c i t y = 0 m a s s m = \frac{100}{1000} k g W = K . E_{f i n a l} - K E_{i n i t i a l} W = 0- \frac{1}{2} m v^{2} W = - \frac{1}{2} \frac{100}{1000} 5^{2} W = - 1.25 j o u l e H e r e g r a v i t y i s o p p o s i t e t o t h e d i s p l a c e m e n t a n d h e n c e t h e w o r k d o n e b y g r a v i t y i s e q u a l t o n e g a t i v e o f p t e n t i a l e n e r g y w h e n t h e b o d y m o v e s u p .$

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