Mechanics Flashcards
Scalar quantity
Has no direction, only magnitude
For example, mass, speed, distance etc.
Vector quantity
Has a magnitude and direction
For example, velocity displacement force, et cetera
Forces in equilibrium
If an object is an equilibrium, all the forces acting on it are balanced and cancel each other out
An object in equilibrium can be at rest or moving with constant velocity
Inertia
A measure of how much an object resist a change in velocity
The larger the inertia of an object the larger applied force needed to change its velocity by given amount
Moments
A moment is the turning effects of a force around turning point
The moment of a force depends on the size of the force and how far the forces applied from the turning point
Principle of moments
The principle of Mum states that for a body to be an equilibrium the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point
If the objects are unbalanced, the object will turn
Moments in levers
In a lever and effort force acts against the load force by means of a rigid object rotating around a pivot
They increase distance from the pivot are force as applied so you need less force to get the same moment
Couples
A couple is a pair of forces of equal size which acts parallel to each other but opposite directions
A couple doesn’t cause any resultant linear force that does produce a turning effect
Size of the moment depends on the size of the forces and the distance between them
Centre of mass
The centre of mass of an object is the single points that you consider the whole weight to act through
The object will always balance around this point
Newton’s first law
The velocity of an object will not change unless a resultant force acts on it
Newton second law
The acceleration of an object is proportional to the resultant force acting on it
F = m x a
Newton’s third law
If an object A exerts a force on object B, then object B exerts an equal but opposite force and object A
Projectile motion equation
s = 0.5 gtt
Drag
Air resistance causes a drag force that acts in the opposite direction to motion and affects the trajectory of a projectile
Friction
A force that oppsoses motion
Two main types of friction
Contact and fluid friction
Contact friction
Happens between solid surfaces
Fluid friction
Fluid friction is known as drag, or fluid resistance or air resistance
Three things that affect fluid friction
-The force depends on the thickness or viscosity of the fluid
-the force increases as speed increases
- force depends on the shape of the object moving through it - the larger the area pushing against the fluid, the greater the resistance force
Three things to remember about frictional forces
-they act in opposite direction to the motion of the object
-They can never speed things up or start something moving
- they covert kinetic energy into heat
Lift
The upwards force on an object moving through a fluid. It happens when the shape of an object causes the fluid flowing over it to change direction
The force acts perpendicular to the direction in which the fluid is flowing
Terminal speed
Happens when frictional forces equal the driving force
An object will reach a terminal speed at some point if there’s a driving force that stays the same all the time and a frictional or drag force that increases with speed
Momentum
Linear momentum depends on its mass and velocity
Momentum = mass x velocity
The principle of linear momentum
Assuming no external forces act, linear momentum is always conserved
Meaning total linear momentum of two objects before they collide equals the total linear momentum after the collision
Elastic collisions
Where momentum is conserved and kinetic energy is conserved
No energy is dissipated as heat, sound, et cetera
Ek = 0.5 m vv
Inelastic collisions
Means some of the kinetic energy is conserved in other forms during the collisions
Linear momentum is always conserved in inelastic collisions
Impulse
Impulse is defined as the product of force and time
So the impulse on a body is equal to the change in momentum of that body and is measured in newtons
F△t = △(mv)
F △t = impulse in Ns
△(mv) = change in momentum
Impulse equation
F△t = △(mv)
F △t = impulse in Ns
△(mv) = change in momentum
Calculating Work done
W = Fs
W = Work done
F = force causing motion in newtons
s = Distance moved in metres
Power and Work
P = △W/△t
P = power in watts
△W = work done in joules
△t = time in seconds
Forces at an angle
W = Fs cos(theta)
Theta = angle at which the force acts from the direction of motion
Equation for moving objects for power
P = Fv
P = power in watts
F = for causing the motion in newtons
v = speed in the direction of motion ms^-1
If working at angle :
P=Fv cos(theta)
The principle of conservation of energy
Energy cannot be created or destroyed
Energy can be transferred from one form to another, but the total amount of energy in a closed system will not change
Equation for elastic potential energy
E = 0.5k (△L)^2
E = elastic potential energy in joules
k = stiffness in an object in Nm
△L = extension of the material
Transfers between kinetic and potential energy
0.5mvv = mg △h
