A boy lifts a dumbbell and lowers it down. When the boy does positive work. Class 9

A boy lifts a dumbbell and lowers it down. When the boy does positive work. Class 9

Question 1.

A boy lifts a dumbbell and lowers it down. When the boy does positive work and negative on the dumbbell. Class 9

Answer:

Boy applies force = Weight of dumbbell to lift it upwards.

When it is lifted upward, force applied is in the direction of displacement. Thus work done is positive. When it is lowered downward, force applied to hold it is in the opposite direction of displacement. Thus work done is negative.

Question 2.

A goalkeeper moved his hand back by 15 cm as he stopped a ball while applying a force of 200 N. What is the work done by this force on the ball in stopping it? Class 9

Answer:

When goalkeeper stops the moving ball, force applied is in direction opposite to the direction of motion of ball. Thus work done by person on the ball is negative. Displacement is taken negative because ball moves in a direction opposite to the direction of force. W = 200 N × (- 0.15) = - 30 J.