Physics Reviewer

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PART II. MECHANICS

WORK, ENERGY, and POWER

WORK

Work is a physical quantity, a measure of the amount of change to which a force gives rise when it acts on something.

The work done by a constant force “F” acting on an object while it undergoes a displacement “x” is equal to the magnitude of the force contact Fx in the direction of the displacement multiplied by the magnitude “x” of the displacement

Work is a scalar quantity with no direction.

Joule is the SI unit of work. One joule is the work done by a force of 1N acting through a distance of 1 meter.

1 joule = 1J – 1 N. m

WORK DONE AGAINST GRAVITY

The work needed to lift an object of mass “m” against gravity. The force of quantity in the object is simply its weight w= mg.

W = mgh work done against gravity

POWER

Power is the rate of doing work. In doing work, like pushing a car along the road there is two factors involved:

The size of the force F

The bigger the force, the greater the amount of work done.

The distance (x) you push the car

The further you push it, the greater the amount of work

Thus, the bigger the force, and the further it moves, the greater the amount of work done.

If the amount of work W is done in a time interval t, the power involvement is

Force, Speed and Power

How much power is delivered when a constant force ‘F” does work on object moving at the constant velocity “V”. If ө is he angle between F and V, then

The principle of Conservation of energy states that the total amount of energy in a system isolate from the rest of the universe always remains constant although energy transformations from one form to another may occur within the system.

Work done by a conservative force (such a gravity) is independent of the path taken; such a force can give rise to a potential energy. Work done by a non-conservative force (such as friction) varies with the path taken and is dissipated, such a force cannot give rise to a potential energy.

MOMENTUM AND IMPULSE

LINEAR MOMENTUM (P)

The linear momentum of an object is the product of its mass and velocity. Linear momentum is a vector quantity having the direction of the object’s velocity.

P = mv p = linear momentum

IMPULSE

The impulse of a force is the product of the force and the time during which it acts. Impulse is a vector quantity having the direction of the force. When a force acts on an object that is free to move, its change in momentum equals the impulse given it by the force.

CONSERVATION OF MOMENTUM

The law of conservation of momentum states that when the vector sum of the external forces acting on a system of particles equals zero, the total linear momentum of the system remains constant.

∑p = p1 + p2 + p3 ……….

The thrust with a rocket is the force that results from the expulsion of exhaust gases.

Elastic and Inelastic Collision

In an elastic collision, kinetic energy is conserved

mv = 0 = m1v1 = m2v2

m1v1 = – m2v2

w1v1/g = w2v2/g

w1v1 = -w2v2

In an inelastic collision, KE is not conserved. A completely inelastic collision is one in which the objects stick together on impact, which results in the maximum possible KE loss. Linear momentum is conserved in all collisions.

ENERGY TRANSFER

Energy transfer is maximum when the colliding objects have the same mass.

COEFFICIENT OF RESTITUTION

The coefficient of restitution is the ratio between the relative speeds of two colliding objects after and before they collide. It equals to 1 for an elastic collision and to 0 for a completely inelastic collision.

CIRCULAR MOTION AND GRAVITATION

An object travelling in a circle at constant speed is said to be undergoing uniform circular motion. The velocity of an object in circular motion continually changes in direction, although its magnitude may remain constant. The centripetal force is the force perpendicular to the velocity of an object moving along a curved path. The centripetal force is directed forward the center of curvature of the path. The acceleration that causes the objects velocity to change is called centripetal acceleration, and it points forward the center of the objects circular path.

NEWTONS LAW OF UNIVERSAL GRAVITATION

Newton’s law of universal gravitation states that every object in the universe attracts other object with a force directly proportional to both their masses and universally proportioned to the square