Work, Energy, and Power

Flashcards on Work, Energy, and Power.

Work Done

Work is said to be done when a force produces a displacement of an object.

Work Done

A scalar quantity, calculated by Force x Distance moved in direction of force. Measured in Joules (J) or Newton Metre (Nm).

Work Done Equation

W = F • S = FS Cosθ

Work Done When θ=90°

When force and displacement are perpendicular (θ=90°), no work is done (W=0J).

Work Done by Force at an Angle

Only the component of force parallel to the displacement does work.

Work-Energy Theorem

The net work done by the forces on an object equals the change in its kinetic energy.

Work-Energy Theorem Equation

W = ½ m v2 - ½ m u2

Variable Force

A force which changes with position of body in magnitude, direction or both.

Work Done by Variable Force

The work done equals = ∫ Area of strip PQRS = Total area ABCD = Total area under curve CD and x-axis from xi to xf

Change in bullet Kinetic Energy Example

Δ(K.E.) of the bullet = 1/2{0.02(500)2 – 0.02(400)2} Therefore, Δ(K.E.) of the bullet = 900 J

Mechanical Energy Equation

ME = K.E + P.E

Law of Conservation of Mechanical Energy

Energy can neither be created nor destroyed but it can only be changed from one form to another form. Thus energy of an isolated system is constant.

Freely Falling Body at Point A

At point A, the body is at rest, Potential energy(P.E)=mgh and Kinetic energy (K.E)=0, so Total mechanical energy(M.E)=P.E+K.E=mgh.

Linear Momentum

P= mv

Relation between kinetic energy and Linear momentum

K.E= P2/2 m

Kinetic Energy of Exploding Objects

When a body explodes, the lighter mass has greater kinetic energy than heavier one.

Conservative Force

A conservative force is a force for which the total work done in moving a particle between two points is independent of the path taken.

Conservative Force Examples

Gravitational force, force in elastic spring, electrostatic force between two electric charges, and magnetic force between two magnetic poles.

Non-Conservative Force

A non-conservative force is one for which work depends on the path taken.

Non-Conservative Force Examples

Friction, fluid resistance.

Elastic Collision

An elastic collision is a collision where total linear momentum and total kinetic energy are conserved.

Elastic Collision Example

Elastic collision between atomic and sub-atomic particles. •Nature of force involved during the interaction is conservative.

Inelastic Collision

The interaction between two bodies is said to be inelastic collision if the Kinetic energy of the system is not conserved.

Inelastic Collision Examples

Collision between two buses, collision between two stones

Perfectly Inelastic Collision

When two colliding objects practically stick together and hence move with same common velocity after collision is called Perfectly Inelastic Collision.

Perfectly Inelastic Collision Example

When a bullet is fired in a wooden block, the bullet embeds itself into the wood and the system (wood and bullet) moves as a single body after collision

Elastic Collision in 1-D Equation

m1(u1+v1 ) (u1– v1 )= m2 (v2 + u2)(v2 – u2)

Inelastic One Dimensional Collision Equation

For inelastic collisions the equation for conservation of momentum is : m1u1 + m2u2 = (m1 + m2) v Since both the objects stick, we take final velocity after the collision as v. Now v shall be: v= (m1u1 )/ m1 + m2 0

Coefficient of Restitution

Ratio of relative velocity of separation and relative velocity of approach.

Power Equation

Power= work (W) / time (t) = F. S/ t = F. v

Power Unit

Watt (Joule per second)