Module 3: Conceptual Questions

The SI unit for power is the watt (W), where

1 W=1 J/s

The power of many devices such as electric motors is also often expressed in horsepower (hp), where

1 hp=746 W

Acceleration of a rocket is a = (ve/m)(Δm/Δt)−g. So, a rocket's acceleration depends on three main factors and those are:

(1) The greater the exhaust velocity of the gases, the greater the acceleration. (2) The faster the rocket burns its fuel, the greater its acceleration. (3) The smaller the rocket's mass, the greater the acceleration.

Economic well-being is dependent upon energy use, and in most countries higher standards of living, as measured by GDP (Gross Domestic Product) per capita, are matched by higher levels of energy consumption per capita. The United States obtains its energy from renewable sources, mostly hydroelectric power by only about

10%

The energy included in the basal metabolic rate is divided among various systems in the body, with the largest fraction going to the liver and spleen, and the brain coming next. The percent of food calories that are used to sustain basic body functions included in the basal metabolic rate is:

75%

The efficiency Eff of a machine or human is defined to be Eff = Wout / Ein, where Wout is useful work output and

Ein is the energy consumed.

Newton's second law of motion in terms of momentum states that the net external force equals the change in momentum of a system divided by the time over which it changes. In symbols, Newton's second law of motion is defined to be Fnet = Δp / Δt where

Fnet is the net external force, Δp is the change in momentum, and Δt is the change time.

The SI unit for work and energy is

Joule

The translational kinetic energy of an object of mass m moving at speed v is

KE=(1/2)mv^(2)

Work Wnc done by a nonconservative force changes the mechanical energy of a system. In equation form, Wnc=ΔKE+ΔPE or, equivalently, (where i and f denote initial and final values)

Kei + Pei + Wnc = Kef + PEf

Power is the rate at which work is done, or in equation form, for the average power P for work W done over a time t,

P=W/t

The conservation of momentum principle is written as:

Ptot = constant Or ptot=p'tot(isolated system), where ptot is the initial total momentum and p'tot is the total momentum some time later.

During projectile motion and where air resistance is negligible, momentum is conserved in the horizontal direction because

The horizontal forces are zero.

When only conservative forces act on and within a system, the total mechanical energy is constant. In equation form, Or KE+PE=constant KEi+PEi=KEf+PEf where i and f denote initial and final values.

This is known as the conservation of mechanical energy.

The work-energy theorem states that the net work Wnet on a system changes its kinetic energy, which is expressed as:

Wnet = (1/2)mv^(2)−(1/2)mv0^(2)

When a force acts on an object and the object is displaced

Work is done

When all forms of energy are considered, conservation of energy is written in equation form as KEi+PEi+Wnc+OEi=KEf+PEf+OEf where OE is

all other forms of energy besides mechanical energy.

Commonly encountered forms of energy include electric energy, chemical energy, radiant energy, nuclear energy, and thermal energy.Energy is often utilized to do work, but it is not possible to convert

all the energy of a system to work.

The human body converts energy stored in food into work, thermal energy, and/or chemical energy that is stored in fatty tissue. The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate, and the corresponding rate when at rest is called the

basal metabolic rate (BMR)

An elastic collision is one that conserves

internal kinetic energy.

A collision in which the objects stick together is sometimes called perfectly inelastic because

it reduces internal kinetic energy more than does any other type of inelastic collision.

The potential energy of a spring is PEs=(1/2)kx^2, where

k is the spring's force constant and x is the displacement from its undeformed position.

The SI unit for momentum is

kg⋅m/s

In elastic collision

kinetic energy and momentum are conserved

The change in gravitational potential energy, ΔPEg is ΔPEg=mgh, where

m is mass, h is height and g is the acceleration due to gravity.

In symbols, linear momentum p is defined to be p=mv, where

m is the mass of the system and v is its velocity.

Linear momentum (momentum for brevity) is defined as the product of a system's

mass multiplied by its velocity.

A non-conservative force is one for which work depends on the path. Friction is an example of a non-conservative force that changes

mechanical energy into thermal energy.

The relative use of different fuels to provide energy worldwide has changed over the years, but fuel use is currently dominated by oil, although

natural gas and solar contributions are increasing.

Work done against gravity in lifting an object becomes potential energy of the

object-Earth system.

A conservative force is one for which work depends

only on the starting and ending points of a motion, not on the path taken.

Point masses are

structureless particles that cannot spin.

The work done by a force is zero if

the displacement is either zero or perpendicular to the force.

The work done is positive if

the force and displacement have the same direction, and negative if they have opposite direction.

An inelastic collision is one in which

the internal kinetic energy changes (it is not conserved).

An isolated system is defined to be one for which

the net external force is zero (Fnet=0).

Conservation of momentum applies only when

the net external force is zero.

The law of conservation of energy states that the total energy is constant in any process. Energy may change in form or be transferred from one system to another, but

the total remains the same.

Newton's third law of motion states that to every action,

there is an equal and opposite reaction.

The net work Wnet is the work done by the net force acting on an object. Work done on an object

transfers energy to the object.

When an object descends without friction, its gravitational potential energy changes into kinetic energy corresponding to increasing speed, such that

ΔKE= −ΔPEg

Impulse, or change in momentum, equals the average net external force multiplied by the time this non-constant force acts during that time and this Impulse is expressed as:

Δp=FnetΔt.