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Push a wheelchair forward, or stop a moving ball. In one case your force helps the motion, in the other it fights it. Work can carry a sign.
- Positive work: displacement is in the same direction as the force — e.g. pushing a wheelchair, the force and displacement point the same way.
- Negative work: displacement is opposite to the force — e.g. a goalkeeper stopping a ball pushes against the ball's motion.
- Work has no direction ; it is just a number carrying a + or − sign .
- Positive work — work done by a force when the displacement is in the same direction as the force.
- Negative work — work done by a force when the displacement is in the direction opposite to the force.
A girl lifts a dumbbell and slowly lowers it. When does she do positive work and when negative?
Lifting up: her force (up) is in the direction of displacement (up) → positive work .
Lowering down: her force (up, holding it) is opposite to the displacement (down) → negative work .
A goalkeeper's hand moves back 15 cm while applying 200 N to stop a ball. How much work does she do on the ball?
The force opposes the ball's motion, so the displacement is taken as negative.
\( W = F \times s = 200\ \text{N} \times (-0.15\ \text{m}) = -30\ \text{J} \). The goalkeeper does −30 J of work on the ball.
- The ball and the goalkeeper apply equal and opposite forces on each other, so both do work on each other: the ball does positive work on the goalkeeper, while the goalkeeper does negative work on the ball.
- 1. Weightlifter holding a barbell steady: no work is done on the barbell, because there is no displacement (even though holding it is tiring).
- 2. Friction on coins sliding on a rough surface: friction acts opposite to the motion, so the work done by friction is negative .
NCERT Question 1 — State whether True or False
Five statements about work, joule, kinetic/potential energy and energy forms — mark each True or False.
View the answer →NCERT Question 2 — Fill in the blanks
Complete the blanks on work, the joule, kinetic energy, potential energy and power.
View the answer →NCERT Question 10 — A ball of mass 2
A 2 kg ball is thrown up at 20 m sā»¹. Sign of work by gravity up/down; and work by air resistance if it reaches 19.4 m (g = 10).
View the answer →