Work energy and power and materials

How is work done defined in physics

Work done = force x distance moved in the direction of the force

What are the units of work done

Nm or joule

1j = 1Nm

How is energy defined

Work done = energy transferred

How do you calculate work done is the force applied is at an angle to the direction in which the object moves

W= Fx cos θ

Derive an expression relating power to constant speed v

Power = force x velocity

Define power in words

Power is the rate of work done

Give two alternative units for power

(J/s) or (W)

1W = 1J/s

What is gravitational potential energy

The capacity for doing work as a result of an object's position in a gravitational field

Why does Ep=mgh only work for objects in a uniform field

This equation assumes that the gravitational field is uniform, meaning the force of gravity is constant throughout the objects motion

Give an example in which GPE and KE are exchanged

As an object falls it's GPE decreases and it's KE increases

mgh = 1/2 mv2

What is kinetic energy?

the energy an object has due to its motion

How it's kinetic energy calculated

Kinetic energy = 1/2 x mass x velocity 2

What are the 2 general classifications of energy for systems with mass

* kinetic energy is the energy due to the moment of an object

* Gravitational Potential energy is the energy due to the position of an object in the Earth is gravitational field

State the principle of conservation of energy

In a closed system, energy cannot be created or destroyed but only transferred from one form to another.

What does the term potential mean

hidden or stored

Give two examples of potential energy

* Gravitational potential energy- energy of an object due to its position in a gravitational field

* Elastic potential energy- energy stored in an object as a result of reversible change in its shape

What is energy

the capacity to do work

J

Is energy scalar or vector?

scalar- magnitude but no direction

What is a tensile force

A force produces an extension of the object.

what is a compressive force

A force that reduces the length of an object.

State hooke's

The extension of the spring is directly proportional to the force applied. This is true as long as the elastic limit of the spring is not exceeded

what does spring constant (force constant) tell you about the spring

This is a measure of the stiffness of a spring. A spring with a large force constant is difficult to expand and you would refer to it as a stiff spring

Explain what elastic limit means

The maximum stress that a material can withstand without undergoing permanent deformation

explain what plastic deformation means

The permanent change in shape of a material when subjected to a stress or force beyond its elastic limit

how can you determine the elastic potential energy stored in a stretched elastic material from the force extension graph

Area under a force - extension graph = work done

The work done on the spring is transferred to elastic potential energy within the spring. This energy is fully recoverable because of the elastic behaviour of the spring

if you double the extension of spring what happens to the energy stored

E is directly proportional to extension squared so doubling the extension quadruples the energy stored

sketch a force extension curve for a metal

sketch a force extension curve for a rubber band

sketch a force extension curve for polythene

what is a hysteresis loop

The loop formed by the loading and unloading curves

how can you determine the thermal energy lost to the surrounding from a force extension graph

The area under a force extension graph is equal to work done. More work is done when stretching a rubber band than is done when its extension decreases again. Thermal energy is released when the material is loaded then unloaded, represented by the area inside the hysteresis loop

describe the molecular structure of a rubber band

rubber bands do not obey Hooke's law. The rubber band will return to its original length after the force is removed - elastic deformation - but the loading and unloading graphs are both curved and are different.

define tensile stress in words

the force applied per unit cross sectional area of the wire

tensile stress equation

force/cross sectional area

units of tensile stress

units pascals

define tensile strain in words

the fractional change in the original length of the wire

tensile strain equation

Extension / original length

what are the units of tensile strain

no units

what is meant by ultimate tensile strength

this is the maximum stress that a material can withstand when being stretched before it breaks

define young's modulus in words

within the limit of proportionality, stress is directly proportional to strain. The ratio of stress to strain for a particular material is constant and is known as its Young Modulus, E

state the equation for young modulus

tensile stress/ tensile strain

what are the units of young's modulus

Nm^-2 or Pa

(same as stress)

sketch a typical stress strain graph for a ductile material

sketch a stress strain graph for a brittle material

sketch a stress strain graph for rubber

rubber shows elastic behaviour

sketch a stress strain graph for polythene

Polythene shows plastic behaviour

what is a polymeric material

materials that consist of long molecular chains. These behave differently depending on their molecular structure and temperature

what is meant by limit of proportionality

the value of stress or force beyond which stress is no longer proportional to strain

what is a yield point on a stress strain graph

where the material extends rapidly. This part of the curve is typical of mild steel but may be absent from other ductile materials

whats the difference between ultimate tensile strength and breaking strength

the material eventually snaps as its breaking point. The stress value at the point of fracture is known s the breaking strength of the material