4. Mechanics and Material - definitions Flashcards
Breaking stress
The maximum stress an object can withstand before failure occurs
Brittle
A brittle object will show very little strain before reaching its maximum stress
Centre of mass
the single point of which all the mass of an object can be said to act
Conservation of energy
Energy cannot be created or destroyed - only transferred into different forms
Conservation of momentum
The total momentum of a system before and event must be equal to the total momentum the system after the event , assuming no external forces act
Couple
Two equal and opposite parallel forces that act on an object through different lines of action. It has the effect of rotation without translation
Density
The mass per unit volume of a material
Efficiency
The ratio of useful output to total input of a system
Elastic behaviour
If a material deforms elastically it will return back to its original shape when the deforming forces are removed. The object will not be permanently deformed
Elastic colision
An elastic collision is when the total kinetic energy of a system before the collision is equal to the total kinetic energy of the system after the collision
Elastic limit
The force beyond which an object will no longer deform elastically, and instead is deformed plastically. Beyond the elastic limit, when the deforming forces are removed the object does not return to its original shape
Elastic strain energy
The energy stored in an object when it is stretched. It is equal to the work done to stretch the object and can be found from the area under a force-extension graph
Equilibrium
For an object to be in equilibrium, the resulting forces an momentum acting upon it must be equal to 0
Hooke’s Law
The extension of an elastic object will be directly proportional to the force applied to it up to the limit of proportionality of the object
Impulse
The change in momentum of an object when a force acts on it. It is equal to the product of the force acting on it and the time over which it acts
Inelastic collision
A collision in which the total kinetic energy of the system before the collision is not equal to the kinetic energy of the system after the collision.
Moment
The product of an objects mass and distance
Newton’s first law
An object will remain in its current state of motion, unless acted on by an external force. An object requires a resultant force in order to accelerate
Newton’s second law
The sum of the forces acting on an object is equal to the rate in change of momentum of the object
Newton’s third law
Every action has an equal ad opposite reaction. If an object exerts a force on another object it must exert a force back equal in magnitude but opposite in direction
Plastic behaviour
If an object deforms plastically then it does not return to its original size when the deforming forces are removed
Principle of moments
:For an object to be in equilibrium, the sum of the
clockwise moments acting about a point must be equal to the sum of the anticlockwise moments acting about the point
Spring constant
The constant of proportionality for an extension of a string under force. The greater the spring constant the greater force needed to achieve a certain extension
Stiffness
A measure of how difficult it is to stress a given material
Tensile strain
The ratio of an objects extension to its original length
Tensile stress
The amount f force acting per unit area , its unit is Pascal
Terminal speed
The maximum speed an object reaches when the driving forces and resistive forces acting on it are equal