The kinetic energy | POINT classical mechanics
The kinetic energy
A person needs energy to do any work (work). For example, when a person pushes a swing, the chemical energy stored in his body is converted into another form of energy that causes the swing to move.
Energy: The body's ability to do work
The unit of measurement for energy is the joule (the same unit of measurement for work) and is equivalent to N.m or km.m²/s²
and its dimensional formula is M.L/T²
The Kinetic Energy:
When work is done to move an object, this work is acquired by the object in the form of energy called kinetic energy.
kinetic energy: is the energy that the object possesses as a result of its movement.
Examples:
A person running
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waves breaking on a beach
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water flowing through a dam
an electron orbiting the nucleus of an atom
Deduction of the kinetic energy of a body:
If a force F acts on a stationary body of mass m and it moves with a constant acceleration a until its speed reaches Vf after covering a displacement d, then:
Vf² = Vi² + 2ad
as Vi = 0
so Vf² = 2ad , d = Vf²/2a
so Fd = . F/a . Vf²
as F/a = ½m
so Fd = ½mVf²
(1) The kinetic energy of an object is a scalar quantity. because It is the product of two scalar quantities: the mass of the object and the square of its velocity.
(2) In the opposite figure, the work done by the car to move from position A to position B: W = ½mv² - ½mv² = ½m(v² - v²) = Δ(ΚE)
(3) If the work done on an object is:
Positive: The kinetic energy of the object increases by the amount of work done, and the object's speed increases.That is, the net force acting on the object is in the same direction as its motion.
Negative: The kinetic energy of the object is transferred by the amount of work done, and the object's speed decreases.That is, the net force acting on the object is in the opposite direction to its motion.
If equal to zero, the kinetic energy remains constant, which indicates that the object's speed remains a constant amount.That is, the net force acting on the object is zero.
Life Applications:
It is clear from the relationship Fd = ½mv² =KE that the work done on a body in the form of kinetic energy is directly proportional to the square of the speed at which it is moving. So if
• A car is moving at a speed of 30 km/h when the brake pedal is pressed, it travels a distance of 1m before stopping.
• The same car is moving at a speed of 60 km/h when the brake pedal is pressed with the same force used in the first case, it travels a distance of 4d before stopping, where d∝v²