Chapter 4-Forces in motion Definitions Flashcards
Force
A push/pull on an object
units: N
F=ma
Weight
The gravitational force on an object
units: N
W=mg
Centre of mass
A point through which any externally applied force produces straight line motion but no rotation
Centre of gravity
An imaginary point at which the entire weight of an object appears to act
(can find COM using plumline experiment)
Plum-line
A string with a weight used to provide a vertical reference line
Free-body diagram
A diagram that represents the forces acting on a single object
Normal contact force (R)
The force exerted by a surface on an object, which acts perpendicularly to the surface.
units: N
Friction
The force that arises when two surfaces rub against each other
acting in opposite direction to motion
( if object on rough surface)
f= mui* R
Drag
the resistive force on an object travelling though a fluid (same as friction)
depends on: -speed, shape and texture of object
-density of fluid travelling through
larger cross-sectional area–> greater drag
(drag) directly proportional to (speed^2)
in air–> drag= air resistance
Tension
The force within a stretched cable or rope
Upthrust
an upwards buoyancy force act8ng on an object when it is in a fluid
Terminal velocity
The constant speed reached by an object when drag force( and upthrust) is equal and opposite to weight of object.
acceleration =0
resultant force=0
drag=weight
Moment
= force* perpendicular distance of the line of action. of force from axis or point of rotation
=Fd
units:Nm
PRINCIPLE OF MOMENTS
For a body in rotational equilibrium, the sum of anticlockwise moments about a point= sum of clockwise moments about same point.
Couple
A pair of equal and opposite forces acting on a body but not in same straight line
Torque (of a couple)
The product of one of the forces of a couple and the perpendicular distance between the forces
Torque= Fd
Triangle of forces
Three forces acting on a point in equilibrium, represented by sides of triangle
resolve horizontally and vertically
Density
The mass per unit volume of a substance
units: kgm-3
density= m/V
(experiment to determine density)
Fluid
A substance that can flow, including liquids and gases
Pressure
The force exerted per unit cross-sectional area
units: Pascals ( Pa) or Nm-2
P=F/A
in fluids–> P=hdensity9.81
ARCHIMEDES’ PRINCIPLE
The upthrust on an object in a fluid is equal to the weight of fluid it displaces.
upthrustobject will SINK
upthrust>weight –>object will FLOAT
