ICSE Physics Class 8 Notes Chapter 4 Energy
→ When we are pushing a wall we are not doing any work as the position of wall is not change i.e. wall has not moved in the direction of force.
→ WORK : “is said to be done if on applying force on a body, the body moves (or changes it position) from it place in the direction of force.” W = F x d
Or
Work is said to be done by a force applied on a body, if it changes its size or shape.”
→ FACTORS AFFECTING THE AMOUNT OF WORK DONE :
W = F x d
- Magnitude of force applied.
- Distance moved by the body in the direction of force.
→ UNITOFWORK : W= F x d
S.l. unit W = 1 N X 1 m N m joule (J)
I kgf = 9.8N ___ is force on 1kg. F = mg
Workdone = 1 kgf x m = 9.8 Nm = 9.8 J = 1o J nearly
→ A cooli standing with a box on his head, does no work as distance moved is zero.
→ A coo li with a box on his head and walking is doing no work as force is acting vertically downward and direction of motion is at right angle.
→ ENERGY : “is capacity of doing work.”
“The work done on a body in changing its state is called energy.”
S.I. unit of energy = S.I. unit of work = (J)
→ JOULE : “A body is said to possess a energy of one joule. If a force of 1 Newton moves the body by a distance of 1 metre in the direction of force.”
→ MECHANICAL ENERGY : “The energy possessed by a body due to its state of rest or state of motion is called mechanical energy.
→ Potential energy and kinetic energy are mechanical energies.
→ POTENTIAL Energy (P.E.) : “Is energy possessed by body due to its state of rest or position.” P.E. = mgh
→ KINETIC Energy (K.E.) : “Is energy possessed by body due to its motion.”
K.E. = \(\frac {1}{2}\)mv2
→ GRAVITATIONAL POTENTIAL ENERGY : “When a stone or water is raised (lifted) from ground to a height, work is done against the force of gravity. This work is stored in the stone or water in the form of GRAVITATIONAL POTENTIAL ENERGY.”
→ A stretched bow, due to change in position possesses potential energy. When stretched bow is relreased the arrow comes in motion and due to motion possesses the kinetic energy and hits the body on which it strikes.
→ When a body at a hight, it possess P.E. = mph. When it falls, height decreases and speed increases
∴ its P.E. decreases and K.E. increases.
→ Powder : “Rate of doing work”. P = \(\frac {w}{t}\)
S.I. unit of power =
= JS-1 = watt