Mechanics and Materials Flashcards
Vector
Any physical quantity that has a direction as well as a magnitude.
Scalar
Any physical quantity that is not directional.
Equilibrium
Forces are balanced, at rest or moving at a constant velocity.
The moment of a force
F x d = force x the perpendicular distance from the pivot. Units (Nm).
The principle of moments
The sum of the clockwise moments = the sum of the anticlockwise moments.
The centre of mass
The point of a body through which a single force on the body has no turning effect.
Uniform
The centre of mass is exactly at the centre of the body.
A couple
A pair of equal and opposite forces acting on a body, but not along the same line.
Displacement
Distance in a given direction.
Speed
Change of distance per unit time.
Velocity
Change of displacement per unit time.
Acceleration
Change of velocity per unit time.
Suvat equation excluding s
v = u + at
Suvat equation excluding a
s = (u + v) t / 2
Suvat equation excluding v
s = ut + 1/2 at^2
Suvat equation excluding t
v^2 = u^2 + 2as
Projectile
Any object acted upon only by the force of gravity. The acceleration of the object is always equal to g, no horizontal component to the acceleration so the horizontal velocity is constant. Motions in the horizontal and vertical directions are independent of each other.
Newton’s first law of motion
Objects either stay at rest or move with constant velocity unless acted on by a force.
Newton’s second law of motion
F = ma
Weight
W = mg
Inertia
The resistance to change of motion. Mass is a measurement of inertia.
Drag force
Experienced by any object moving through a fluid.
Terminal speed
As drag force increases, resultant force equals zero. Zero acceleration and constant velocity.
Thinking distance
The distance travelled by a vehicle in the time it takes the driver to react.
Braking distance
The distance travelled by a car in the time it takes to stop safely, from when the brakes were first applied.
Stopping distance
Thinking distance + braking distance
Momentum, p
Mass x velocity
Impulse / change of momentum
Force x time
Newton’s third law of motion
When two objects interact, they exert equal and opposite forces on each other.
The principle of conservation of momentum
For a system of interacting objects, the total momentum remains constant, provided no external resultant forces act on the system.
Elastic collision
One where there is no loss of kinetic energy.
Inelastic collision
Occurs when the colliding objects have less kinetic energy after the collision than before the collision.
Principle of conservation of energy
Energy cannot be created or destroyed.
Work done
Force x distance moved in the direction of the force
Kinetic energy
Ek = 1/2 m v^2
Change of gravitational potential energy
ΔEp = m g Δh
Definition of power
The rate of transfer of energy.
Power equations
P = ΔE /Δt = ΔW /Δt
Density of a substance
Mass per unit volume.
Hooke’s law definition
The force needed to stretch a spring is directly proportional to the extension of the spring from its natural length.
Hooke’s law equation
F = k ΔL, where k is the spring constant.
Elasticity of a solid material
Its ability to regain its shape after it has been deformed or distorted and the forces that deformed it have been released.
Tensile deformation
Deformation that stretches an object.
Compressive deformation
Deformation that compresses an object.
Tensile stress
Tension / Area
Unit of stress
Pascal (Pa) equal to 1 N m^-2
Tensile strain
ΔL /L
Young modulus
Tensile stress / Tensile strain
Elastic limit
The point beyond which the wire is permanently stretched and suffers plastic deformation.
Brittle material
Snaps without any noticeable yield.
Ductile material
Can be drawn into a wire.
Elastic energy stored in a stretched wire
1/2 T ΔL
μ
μ = density x cross sectional area = weight/unit length.
