Mechanics Flashcards
Scalars
quantities that only have a magnitude
Vectors
quantities that have both direction and magnitude
Distance
how far you have travelled
Displacement
is the distance travelled in a particular direction
Speed
is the rate of change of distance (distance/time)
Velocity
is the rate of change of displacement (displacement/time)
Instantaneous Speed
means the rate of change of distance at one particular time
Average Speed
means the speed over a period of time
Acceleration
is the rate of change of velocity (change in velocity/time)
Displacement - time graphs
gradient = velocity
Velocity - time graphs
gradient = acceleration
area below the line = displacement
Acceleration - time graphs
area below the line = change in velocity
Projectile Motion
a projectile is an object moving through the air under the influence of only one force (gravity). The trajectory it will follow is a parabola as:
- the VERTICAL component of velocity is constantly changing (due to gravity)
the HORIZONTAL component of the velocity is constant (assuming no air resistance)
How does projectile motion differ in reality with air resistance?
- the range is shorter
- the maximum height is lower and is reached earlier
- the gradient is steeper on the way down
Projectiles that are launched horizontally
initial vertical velocity = zero
horizontal velocity = launch velocity, v
Projectiles that are launched above the horizontal
initial vertical velocity = v sinθ
horizontal velocity = v cosθ
Projectiles that are launched at an angle below the horizontal
initial vertical velocity = v sinθ
horizontal velocity = v cosθ
5 Key Points of Projectile Motion
- at the highest point, velocity = 0
- acceleration is always downwards at 9.81 ms-2
- horizontal velocity is zero
- range = horizontal velocity x time of flight
- problems may be solved using kinematic equations or conservation of energy
What does the Magnitude of Fluid Resistance (drag) depend on?
- the velocity of the object relative to the fluid
- the size and shape of the object
- the fluid used (its viscosity)
- fluids are either liquids or gases
What is terminal velocity?
when an object falls under the influence of gravity, it will initially accelerate at 9.81 ms-2
however, the magnitude of air resistance increases as the objects velocity increases
so it will continue to accelerate but less than 9.81 ms-2
once the magnitude of the upwards air resistance = downwards gravitational force, the object will no longer accelerate, as forces are balanced and it is at terminal velocity
Mass (m)
is the amount of matter in an object. It is measured in kg and does not change with a change in position
Weight (W)
is the force of gravity acting on an object. It is measured in newtons (N). It will change if an object changes its position.
* always is straight down
Normal Reaction Force
when an object pushes down on a surface, the surface pushes back on the object. The normal reaction force is always perpendicular to the surface the object is on.
Balanced Forces
the object will remain at rest or will continue to move in a straight line at a constant speed
Unbalanced Forces
the object will accelerate in the direction of the net force
Newtown’s First Law
“an object continues in uniform motion in a straight line or at rest unless a resultant external force acts.”
*if resultant force = zero, object is said to be at translational equilibrium
Newton’s Second Law
“the resultant force on an object is proportional to the acceleration providing the mass of the object remains constant.”
Newton’s Third Law
“for every action on one object there is an equal but opposite reaction on another object.”
i.e. forces come in pairs (and will be of the same type)
Resolving forces
resolving means to break down a force into two components
the components can be found in a number of ways including pythagorus, trigonometry or scale diagrams
Resultant Forces
a resultant force is the sum of all the forces acting on an object
Resolving Forces of an object on an incline
the weight force can be resolved into two forces:
- parallel to the slope (mg sinθ), this will = friction if speed is constant
- perpendicular to the slope (mg cosθ), = normal reaction
Solid Friction (Ff)
is the force that opposes the relative motion of the surfaces of two solid objects.
two types:
- static friction: occurs when there is no relative motion between the surfaces. A force is applied to an object resting on a surface, but the force is not large enough to move the object.
- dynamic friction: occurs when there is relative motion between the surface. Once the force applied to the object exceeds the maximum value of static friction force the object will begin to slide. The magnitude of the dynamic friction force is less than the maximum static value and is a constant.
Definition of Energy
it is the “capacity to do work”
Kinetic Energy
is the energy an object has because it is moving
Gravitational Potential Energy
the energy an object has because of its height above the ground
Elastic Potential Energy
this is the energy stored when an object is either stretched or compresses
Hooke’s Law
the extension of a spring is proportional to the applied force
F = k x change in X
X = extension or compression (m)
K = spring constant (Nm-1)
Law of Conservation of Energy
energy can be converted from one type to another. It cannot be created or destroyed.
Work
is said to be done when a force moves an object through a displacement
therefore it is a transfer of energy
Power
is the rate at which work is done or energy is transferred
Linear Momentum (p)
is defined as the product of mass and velocity. It is a vector
Law of Conservation of Momentum
the total linear momentum of a system remains constant provided no resultant external force acts
* therefore total pbfore = total pafter
Elastic Collsion
no kinetic energy is lost during the collision (in reality, only occurs between sub-atomic particles)
Inelastic Collision
kinetic energy is lost (converted to heat and sound)
however, total amount of energy still remains constant
Impulse
is the change in momentum
Explosions
typically a stationary system will break apart into two pieces. As with collisions the total momentum is conserved and will be equal to zero.
