SHREE VALLABH SANSKAR DHAM’S
SMT. SHOBHABEN PRATAPBHAI PATEL
DAY BOARDING SCHOOL
KILLA- PARDI, DIST.- VALSAD, GUJARAT -396125
Work and energy - Notes
STD – IX Sub.- SCIENCE
Introduction
For doing work, energy is required.
• In animals, energy is supplied by food they eat.
• In machine, energy is supplied by fuel.
Why sometimes not much work is done inspite of working hard?
Example:
→ A man is completely exhausted in trying to push a rock (wall), but work
done is zero as wall is stationary.
→ A man standing still with heavy suitcase may be tired soon but he does no
work in this situation as he is stationary.
Work is said to be done when:
(i) a moving object comes to rest.
(ii) an object at rest starts moving.
(iii) velocity of an object changes.
(iv) shape of an object changes.
Condition of Work done:
(i) Force should be applied on the body.
(ii) Body should be displaced.
Work = Force × Displacement
W=F×S
→ Work is a scalar quantity.
, Unit of Work:
→ Unit of work is Newton metre or Joule.
Negative, Positive and Zero Work
→ Work done by a force can be positive, negative or zero.
(i) Work done is positive when a force acts in the direction of motion of the
body.
Example: A child pulls a toy car with a string horizontally on the ground.
(ii) Work done is negative when a force acts opposite to the direction of motion
of the body.
Example: When we kick a football lying on the ground, the force of our kick
moves the football. Here direction of force applied & motion of football is same
so work done is positive. But when football slows due to force of friction acting
in a direction opposite to direction of motion of football, thus work done is
negative.
(iii) Work done is zero when a force acts at right angles to the direction of
motion.
Example: The moon moves around the earth in circular path. Here force of
gravitation acts on the moon at right angles to the direction of motion of the
moon. So work done is zero.
Example: A coolie lifts a luggage of 15 kg from the ground and put it on his
head 1.5 m above the ground. Calculate the work done by him on the luggage.
SMT. SHOBHABEN PRATAPBHAI PATEL
DAY BOARDING SCHOOL
KILLA- PARDI, DIST.- VALSAD, GUJARAT -396125
Work and energy - Notes
STD – IX Sub.- SCIENCE
Introduction
For doing work, energy is required.
• In animals, energy is supplied by food they eat.
• In machine, energy is supplied by fuel.
Why sometimes not much work is done inspite of working hard?
Example:
→ A man is completely exhausted in trying to push a rock (wall), but work
done is zero as wall is stationary.
→ A man standing still with heavy suitcase may be tired soon but he does no
work in this situation as he is stationary.
Work is said to be done when:
(i) a moving object comes to rest.
(ii) an object at rest starts moving.
(iii) velocity of an object changes.
(iv) shape of an object changes.
Condition of Work done:
(i) Force should be applied on the body.
(ii) Body should be displaced.
Work = Force × Displacement
W=F×S
→ Work is a scalar quantity.
, Unit of Work:
→ Unit of work is Newton metre or Joule.
Negative, Positive and Zero Work
→ Work done by a force can be positive, negative or zero.
(i) Work done is positive when a force acts in the direction of motion of the
body.
Example: A child pulls a toy car with a string horizontally on the ground.
(ii) Work done is negative when a force acts opposite to the direction of motion
of the body.
Example: When we kick a football lying on the ground, the force of our kick
moves the football. Here direction of force applied & motion of football is same
so work done is positive. But when football slows due to force of friction acting
in a direction opposite to direction of motion of football, thus work done is
negative.
(iii) Work done is zero when a force acts at right angles to the direction of
motion.
Example: The moon moves around the earth in circular path. Here force of
gravitation acts on the moon at right angles to the direction of motion of the
moon. So work done is zero.
Example: A coolie lifts a luggage of 15 kg from the ground and put it on his
head 1.5 m above the ground. Calculate the work done by him on the luggage.
