35 terms

# Forces and motion / Mechanics (OCR Physics A)

Key words and definitions for Unit G481 (Mechanics) of OCR Physics A (AS Level). Created by Rob Cowen (https://www.youtube.com/cowenphysics).

#### Terms in this set (...)

newton
The force which gives a mass of 1kg an acceleration of 1ms^-2.
joule
The work done by a force of 1 N moving an object 1 m in the direction of the force.
watt
The power transferred by one joule per second.
displacement
the distance moved by an object in a particular direction
instantaneous speed
the speed at the precise instant that it is measured (e.g. a car's speedometer)
average speed
the speed calculated during a period of time
speed
rate of change of distance
velocity
rate of change of displacement
acceleration
rate of change of velocity
density
mass / volume
torque of a couple
one of the forces x perpendicular distance between the forces.
moment of a force
force x perpendicular distance from pivot.
work done by a force
force x distance moved in the direction of the force
power
the rate of work done
force constant of a spring
force per unit extension
scalar
a quantity with magnitude only
vector
a quantity with both magnitude and direction
terminal velocity
the velocity of a falling object when weight = drag
centre of gravity
a point where the entire weight of an object appears to act
thinking distance
the distance travelled by the car in the time taken by the driver to react
braking distance
the distance travelled by the car after the brakes are applied until the car stops
stopping distance
thinking distance + braking distance
principle of conservation of energy
Energy cannot be created or destroyed; it can only be transferred into other forms
principle of moments
For an object in equilibrium, the sum of the clockwise moments about a point = the sum of the anticlockwise moments about the same point
Hooke's law
extension of a spring is proportional to the force applied (as long as the elastic limit is not exceeded)
stress
= force / area
strain
= extension / original length
Young modulus
= stress / strain, as long as the elastic limit is not exceeded
ultimate tensile strength
the breaking stress of a material
elastic deformation
material returns to original shape when the force is removed
plastic deformation
material does not return to original shape when the force is removed
Archimedes' Principle
the upthrust is equal to the weight of the fluid displaced
Newton's First Law
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Newton's Second Law
The net force on an object is proportional to the rate of change of momentum
Newton's Third Law
For every action force there is an equal and opposite reaction force
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