Dynamics Flashcards
Dynamics
Air resistance is included and other frictional forces
No SUVAT
W = mg: Weight = mass x gravitational field strength
F = ma: Force = mass x acceleration
Four Fundamental Forces
Gravitational Force
Electromagnetic Force
Strong Nuclear Force
Weak Interaction Force
Weight Force
The gravitational force acting on an object through it’s centre of mass - vertically downward
Frictional Force
The force that arises when two surfaces rub against each other
Opposite to the direction of motion
Drag Force
The resistive force on an object travelling through a fluid
Same as friction
Tension Force
The force within a stretched or compressed cable or rope
Up thrust Force
An upward buoyancy force acting on an object when it is in a fluid
Vertically Upward
Drag Force
A frictional force that resists the motion of an object through a fluid (liquid or gas)
Affected by:
Higher Velocity = Higher Drag
Cross-Sectional Area: Of the moving object. A more streamlined object will have a lower drag force (Not SA)
Roughness: Of the surface of the moving object
Viscosity: Of the fluid being moved through
Resultant or Net Force: F(R)
Vectors can be added including forces to find the resultant force
If vectors are at a right angle, we can use trigonometry and Pythagoras
If the vectors are not at right angles:
Resolve the vectors into their components
Use scale diagrams
Use sine and cosine rules
Newton’s Second Law
F refers to the resultant force of an object: Measured in Newtons, N
m is always measured in kg
a is always measured in ms^-2
F = ma
Acceleration and force are both vectors and are in the same direction
This version of newton’s second law applies to objects with a constant mass
Definition of One Newton
One Newton is the force that causes a mass of one kilogram to have an acceleration of one metre per second squared
Speed of light
Theory of special relativity (Einstein)
Mass increases (don’t say changes) if the object is travelling very close to the speed of light
F = ma is no longer valid
m = m(0) / √ (1 - v^2 / c^2)
Terminal Velocity
The graph shows velocity of an object dropped vertically through the air from rest
The gradient of the tangent to the velocity-time graph varies so acceleration is not constant
At t = 0s, there is no drag force as the object is not moving so the acceleration is 9.81ms^-2
A short time later, the drag force is increasing as the object gains speed. Weight > drag so acceleration is vertically downward but < 9.81
When drag is equal but opposite to weight then resultant force = 0N, so there is no acceleration. The object has reached it’s terminal velocity
Two forces acting in opposite directions, weight and drag F(R) = W - D
Gradient = Acceleration
Parachuting Graph
Initially, only weight acts so a = g vertically downward, No drag
Drag increases with increasing speed W > D
Acceleration < 9.81
1st Terminal Velocity: Weight = Drag, resultant force = 0N
acceleration = 0
Parachute opens:
Large drag force caused by open parachute
Drag > Weight, resultant force and acceleration is upward
Velocity is downward but decreasing
2nd Lower Terminal Velocity, W = D, F(R) = 0N, a = 0
Terminal Velocity Experiment
Place a ball bearing in a vertical cylinder, with washing detergent and insulating tape at regular intervals
Measure the time for the ball to pass each 10cm marker
Use v = x / t
Find the mean velocity at each time
Terminal Velocity Experiment: Error
Stopwatch - 0.01s
Diameter - Micrometre / Vernier Calliper - 0.01mm
Length - Metre Ruler - 1mm
Human Error with reaction time -> Use light gates
