Mechanics definitions

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  • Created by: Rchilds
  • Created on: 07-02-17 13:51
Newton
The net force (is a newton) when a 1 kg mass has acceleration of 1 ms-2
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Joule
The work done is 1 Joule when a force of 1 N moves 1 m in the direction of the force
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Watt
A unit of power. 1 Watt is 1J of energy transferred per second.
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Kilowatt-hour
The (electrical) energy transferred when a 1kW device is on for 1 hour
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Scalar
A quantity with magnitude (size) but no direction Some examples: speed, distance travelled, density, mass, energy, time, power
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Vector
A quantity with magnitude (size) and direction Some examples: displacement, velocity, acceleration, force
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Displacement (s)
The overall distance travelled in a particular direction
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Instantaneous speed
The speed of an object at a given moment of time
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Average speed
Total distance travelled divided by the total time taken
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Speed
The rate of change of distance
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Velocity
Rate of change of displacement
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Acceleration
Rate of change of displacement
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Weight
The gravitational force acting on an object in a gravitational field
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Drag
The resistive or a frictional force that an object travelling in a fluid experiences. It is affected by speed of balloon, surface area, texture of balloon, temperature of air / density of air / viscosity (of air)
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Friction
The resistive force an object feels when moving (or the is a force 'trying' to move it if stationary) and in contact with another object. It increases on rougher surfaces and when an object becomes heavier
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Tension
The pulling force in a long, thin object like a string or wire. It is constant thought-out the object. Due to the inter-molecular attraction within the object
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Newton's First Law
A body will remain at rest or continue to move with constant velocity unless acted upon by a force
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Newton's Second Law
A body will remain at rest or continue to move with constant velocity unless acted upon by a force
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Newton’s Third Law
When one body exerts a force on another the other body exerts an equal (in magnitude) and opposite (in direction) force on the first body. They must be: Equal in size; Opposite in direction; Same type of force; Act on different bodies
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Linear Momentum (ρ)
Mass x velocity; a vector quantity.
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Conservation of Linear Momentum
Total momentum is constant/conserved for a closed system/provided no external forces
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Net force on a body (F)
The vector sum of all the individual forces acting on the body, equal to the rate of change in momentum
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Impulse of a force
Force x time for which the force acts/ duration of collision
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Perfectly elastic collision
A collision in which (momentum and) kinetic energy are conserved.
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Inelastic collision
A collision in which (momentum is conserved and) kinetic energy is not conserved.
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Terminal velocity
A falling object’s velocity when the net force on it is zero. i.e. weight = drag, so acceleration is zero (as F=ma)
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Density (ρ)
mass/volume or ‘density is mass per unit volume’
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Pressure
force/area ‘force per unit area’
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Centre of gravity
A point where the entire weight of an object appears to act
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Centre of mass
a point representing the mean position of the matter in a body or system (mass is equally distributed around it)
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Couple
A pair of equal and opposite co-planar forces (produce a rotation only as they are not on the same line of action).
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Moment of a couple
one of the forces x perpendicular distance between the forces
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Moment of a force
force x perpendicular distance from point / pivot
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Principle of moments
For a ridged body in equilibrium the sum of clockwise moments about a point = sum of anticlockwise moments about the same point
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Conditions for equilibrium
Net force = 0; Net moment = 0
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Thinking distance
The distance travelled (by the car) from when the driver sees a problem and the brakes are applied
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Braking distance
The distance travelled by the car after the brakes are applied until the car stops
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Stopping distance
thinking distance + braking distance
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Work done (W)
force x distance moved in the direction of force (it is equal to the energy transferred)
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Conservation of Energy
The total energy of a closed system remains constant as energy cannot be created or destroyed (it can only be transferred into other forms)
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Power (P)
work done/ time or the rate of doing work
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Efficiency
The percentage of energy that is transferred usefully by a device. It is always less than 100% due to heat losses.
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Stress (σ)
The force per cross-sectional area
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Strain (ε)
The extension per unit length
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Tensile force
Two or more forces applied to a material that act away from their surfaces causing it to stretch
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Compressive force
Two or more forces that have the effect on reducing the volume of the object on which they are acting or reducing the length.
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Hooke’s Law
The extension α applied force (as long as the elastic limit is not exceeded)
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Extension(x)
The change in length of an object when subjected to a tension.
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Force constant (k)
The force per unit extension.
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Young’s Modulus (ε)
Young modulus = stress/strain As long as the elastic limit is not exceeded (i.e.in the linear region of stress against strain graph/Hooke’s law is obeyed)
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Braking stress
Maximum stress material can withstand before fracture
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Elastic limit
The point at which elastic deformation becomes plastic deformation.
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Elastic deformation
The material will return to its original shape when the stress/ deforming force is removed.
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Plastic deformation
The material is permanently deformed when the stress / deforming force is removed
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Brittle materials
A material that experiences no plastic deformation, it brakes at its elastic limit. e.g. concrete, glass
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Polymeric materials
A material made of many smaller molecules bonded together, often making a tangle knot of chains e.g. rubber. These materials often exhibit very large strains e.g. 300%
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Ductile materials
Material with a very large plastic deformation region so they can be stretched out (without braking) and retain the new shape e.g. copper for wires.
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Elastic stain energy
The energy stored on an object due to an external force doing work on it by stretching or compressing
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pico (p)
10^(-12)
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nano (n)
10^(-9)
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micro (µ)
10^(-6)
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milli (m)
10^(-3)
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centi (c)
10^(-2)
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kilo (k)
10^3
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mega (M)
10^6
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giga (G)
10^9
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tera (T)
10^12
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What does the gradient of a displacement-time graph tell you?
Velocity
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What does the gradient of a velocity-time graph tell you?
Acceleration
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What does the area underneath a velocity-time graph tell you?
displacement
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What does the gradient of a force (y axis)-extension (x-axis) graph tell you?
Force constant
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What does the area underneath a force (y axis)-extension (x-axis) graph tell you?
Elastic strain energy stored
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What does the gradient of a stress-strain graph tell you?
Young's modulus
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What does the area underneath a force-time graph tell you?
Impulse (or change in momentum)
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Other cards in this set

Card 2

Front

The work done is 1 Joule when a force of 1 N moves 1 m in the direction of the force

Back

Joule

Card 3

Front

A unit of power. 1 Watt is 1J of energy transferred per second.

Back

Preview of the back of card 3

Card 4

Front

The (electrical) energy transferred when a 1kW device is on for 1 hour

Back

Preview of the back of card 4

Card 5

Front

A quantity with magnitude (size) but no direction Some examples: speed, distance travelled, density, mass, energy, time, power

Back

Preview of the back of card 5
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