Chapter 3: Work, Energy, and Momentum Flashcards Preview

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Flashcards in Chapter 3: Work, Energy, and Momentum Deck (63):
1

Energy

A property or characteristic of a system to do work / make something happen

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Kinetic Energy

The energy of motion

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Objects with mass and some form of velocity will have:

Kinetic Energy

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Kinetic Energy Equation

KE = ½mv2

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The SI Unit for Kinetic Energy is:

Joules (J)

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If velocity doubles, kinetic energy will:

quadruple (assuming the mass is constant)

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Potential Energy

An object that has mass and the potential to do something.

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Potential Energy Equation

PE = mgh

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Total Mechanical Energy is:

The sum of an object's potential and kinetic energies.

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Total Mechanical Energy (E) Equation

E = PE + KE

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The First Law of Thermodynamics states:

Energy is never created or destroyed. It is merely transferred from one system to another.

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When the work done by nonconservative forces is zero (when there are no nonconservative forces acting on the system), the total mechanical energy of the system:

remains constant. (E = PE + KE = Constant)

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When nonconservative forces such as friction and air resistance are present, total mechanical energy:

is not conserved

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Work Done by Nonconservative Forces (W') Equation

W' = ΔE = ΔKE + ΔPE

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The work done by nonconservative forces such as air resistance and friction is exactly equal to:

the amount of energy 'lost' from the system. NOTE: the energy was not 'lost', it was just transferred out of the system and into another.

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Mechanical Energy is conserved when:

No nonconservative forces (friction, air resistance, etc. are present).

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Work is:

a process in which energy is transferred from one system to another when something exerts forces on or against something else.

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Work (W) Equation

W = FdcosΘ

(F = force applied; d = displacement through which the force is applied; Θ = the angle between the applied force vector and the displacement vector)

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In the Work equation,  Θ is:

the angle between the displacement and force vectors

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Power

The rate at which energy is transferred from one system to another.

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Power (P) Equation

P = W / t (where W = work and t = time)

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SI Unit of Power

Watts (W)

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The Work-Energy Theorem states:

the net work done on or by an object will result in an equal change in the object's kinetic energy.

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Work-Energy Theorem Equation

Wnet = ΔK = Kf - Ki

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Momentum is defined as:

a quality of objects in motion. It is defined as the product of an object's mass times its velocity. Therefore, it is a vector quantity.

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Momentum (p) Equation

p = mv

(where m = mass and v = velocity)

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For two or more objects, the total momentum is equal to:

the vector sum of the individual momenta

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Inertia

the tendency of objects to resist changes in their motion and momentum

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Impulse is defined as:

the change in an object's momentum. It is a vector quantity.

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For a constant force applied through a period of time, impulse and momentum are related by this equation:

I = FΔt = Δp = mvf - mvi

(where I = impulse, t = time; p = momentum; m = mass; v = velocity)

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The longer the time of change in momentum (Impulse), the smaller the:

Force necessary to achieve the impulse. (Example - Crush zones in cars. The longer amount of time the car crushes, the smaller the force felt on the occupancy zone by the change in momentum - the change in momentum occurs over a longer period of time, and thus the force felt is smaller)

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Conservation of momentum means that:

the momentum after a collision is the same as the momentum before the collision.

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Conservation of momentum occurs when:

No nonconservative forces (friction, air resistance, etc. are present).

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Conservation of Momentum Equation for elastic and inelastic collisions

mavai + mbvbi = mavaf + mbvbf

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The three types of collisions for which momentum is conserved:

Completely Elestic Collisions; Inelastic Collisions; Completely Inelastic Collisions

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Completely Elastic Collisions occur when:

Two or more objects collide in such a way that both kinetic energy and momentum are conserved. They do not stick together.

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Conservation of Kinetic Energy in a Completely Elastic Collision Equation

½mavai2 + ½mbvbi2 = ½mavaf2 + ½mbvbf2

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In completely elastic collisions:

kinetic energy and momentum are BOTH conserved

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Inelastic Collisions occur when:

a collision results in the decrease of kinetic energy of the system through the production of sound, heat, light, etc.

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In inelastic collisions:

momentum is conserved, but the final kinetic energy is LESS THAN the initial kinetic energy

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Inelastic Collision Equation

½mavai2 + ½mbvbi2 > ½mavaf2 + ½mbvbf2

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Completely Inelastic Collisions occurs when:

objects collide and stick together rather than bouncing off each other and moving apart.

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In completely inelastic collisions:

momentum is conserved, but the final kinetic energy is LESS THAN the initial kinetic energy

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Conservation of Momentum Equation for completely inelastic collisions

mavai + mbvbi = (ma + mb)(vf)

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In completely elastic collisions, what is conserved?

KE and momentum

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In completely inelastic collisions, what is conserved?

momentum; KE is lost

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In inelastic collisions, what is conserved?

momentum; KE is lost

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Mechanical Advantage

Any device (such as an inclined plane) that allows for work to be accomplished through a reduced applied force is said to provide mechanical advantage.

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Mechanical Advantage Equation

Mechanical Advantage = Fout / Fin

(Fout = the force exerted on an object by a simple machine; Fin = the force actually applied on the simple machine)

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Mechanical Advantage is the ratio of:

the force exerted on an object by a simple machine (Fout) to the force actually applied on the simple machine (Fin)

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Mechanical Advantage comes at the expense of:

increasing the distance over which the work is done

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Mechanical Advantage: load

the load is the weight of an object (mg) in a mechanical advantage situation

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Mechanical Advantage: effort

the force applied to the simple machine in mechanical advantage

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Mechanical Advantage: Load Distance

the distance an object is moved by a simple machine in mechanical advantage

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Mechanical Advantage: Effort Distance

In a pulley system, the length of rope that must be pulled in order to move the object the load distance. Effort distance is longer than load distance.

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Efficiency of a Simple Machine Equation

Efficiency = Wout / Win = (load)(load distance) / (effort)(effort distance)

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Efficiencies are expressed as:

Percentages

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The efficiency of a machine gives a measure of:

the amount of work put into the system that "comes out" as useful work

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The six devices considered to be classic simple machines:

inclined planes, wedges, pulleys, axle and wheel, lever, and screws

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In an idealized pulley (one that is massless and frictionless), the work put into the system is equal to:

the work that comes out of the system. Thus, the pulley has a 100% efficiency.

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Center of Mass

The point that acts as if the entire mass was concentrated at that point.

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Center of Mass Equation

X = m1x1 + m2x2 + m3x3 + .. / m1 + m2 + m3 ..

(where m1, m2, and m3 are the masses of three different objects and x1, x2, and x3 are there positions along the x-axis)

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Center of Gravity Equation

X = w1x1 + w2x2 + w3x3 + .. / w1 + w2 + w3 ..

(where w1, w2, and w3 are the weights of three different objects and x1, x2, and x3 are there positions along the x-axis)