Physics 101 Chapter 7 Kinetic Energy and Work 1-15 جامعة الامير مقرن

Physics 101 Chapter 7 Kinetic Energy and Work
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7-1 Kinetic Energy
⚫ Energy is the ability to do work. ⚫ Energy is required for any type of motion ⚫ Energy:
o Is a scalar quantity assigned to an object or a system of objects.
o Can be changed from one form to another. o Is conserved in a closed system, that is the total amount of energy of all types is always the same.
⚫ Kinetic energy: ⚫ The energy an object has because of its motion. ⚫ For an object with speed v
o The faster an object moves, the greater its kinetic energy. o Kinetic energy is zero for a stationary object
⚫ The unit of kinetic energy is a joule (J)
Example Energy released by 2 colliding trains with given weight and acceleration from rest:
Find the final velocity of each locomotive: K = 1/2 * m * v ^ 2
v ^ 2 = v_{0} ^ 2 + 2a(x - x_{0})
v2 = 0 + 2(0.26 m/s²) (3.2 × 103m), v = 40.8m / s = 147km / h
Weight = mg
1.2 × 106 N 9.8 m/s² Convert weight to mass: m = 1.22 × 105 kg.
Find the kinetic energy:
K = 2(1/2 * m * v ^ 2) = (1.22 * 10 ^ 5 * kg) * (40.8m / s) ^ 2 = 2 * 10 ^ 8 * J
(Answer)
7-2 Work and Kinetic Energy
⚫ Changes in kinetic energy is a result of energy transfer to or from the object
⚫ In a transfer of energy via a force, work is:
o Done on the object by the force
o Throwing a baseball does work o Pushing an immovable wall does not do work
The force increases the kinetic energy of the object انتقلت
The force has transferred energy to the object from you.
Positive work is done on the object by the force.
The force decreases the kinetic energy of the object.
The force has transferred energy from the object to you.
Negative work is done on the object by the force.
Work W is energy transferred to or from an object by means of a force acting on the object.
Energy transferred to the object is positive work.
Energy transferred from the object is negative work.
⚫ Start from force equation and 1-dimensional velocity: F_{x} = m*a_{x}
v^ 2 * = v_{0} ^ 2 + 2a_{x}*d .
⚫ Rearrange into kinetic energies:1/2 * m * v ^ 2 - 1/2 * m * v_{0} ^ 2 =F x d
⚫ The left side is now the change in energy ⚫ Therefore work is:
W = F_{x}*d
For an angle φ between force and displacement:
⚫ As vectors we can write: W = vec F * vec d
⚫ Notes on these equations: W = Fd * cos phi
o Force is constant
o Object is particle-like (rigid) o Work can be positive or negative
Work is a scalar quantity.
Sl unit for work is the joule.
F cos is the component of the force along the direction of the displacement.
All the work is done by the force component along the displacement.
The force component perpendicular to the displacement does zero work.
A force does positive work when it has a vector component in the same direction as the displacement and it does negative work when it has vector component in the opposite direction.
A force does zero work when it has no component along the displacement
⚫ For two or more forces, the net work is the sum of the
works done by all the individual forces
⚫ Two methods to calculate net work:
 We can find all the works and sum the individual work terms.  We can take the vector sum of forces (F net ) and calculate the
net work once
⚫ The work-kinetic energy theorem states: ΔΚ = Kf - Ki
⚫ (change in kinetic energy) = (the net work done)
⚫ Or we can write it as: Kf = Ki + W
⚫ (final KE) = (initial KE) + (net work)
⚫ The work-kinetic energy theorem holds for positive
and negative work
Example If the kinetic energy of a particle is initially 5 J:
o A net transfer of 2 J to the particle (positive work)
• Final KE = 7 J o A net transfer of 2 J from the particle (negative work)
• Final KE = 3 J
Checkpoint 1
A particle moves along an x axis. Does the kinetic energy of the particle increase, de-crease, or remain the same if the particle's velocity changes (a) from -3 m/s to -2 m/s and (b) from 2m/s to 2 m/s? (c) In each situation, is the work done on the particle positive, negative, or zero?
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