Dynamics (Review) Flashcards
What is a force?
Anything that changes or tends (you may apply force to an object, but fail to change its state of rest/motion) to change an object’s state of motion/rest (in other words, forces cause acceleration [can also change the shape and direction of an object]). Force is a vector because it has acceleration (also, scalar x vector = vector). All forces can be grouped into two: contact forces and field forces.
Equation: F = ma
*Only works when there’s only one force present (as opposed to Fnet)
Unit: Newton or Kgm/s^2
What are contact forces? What are field forces?
Contact forces result from physical contact between two objects (FUNT - normal reaction force, friction, tension, upthrust [air resistance is debatable]). Field forces do not involve physical contact and can act from a distance (GEMA - gravitational force, electrostatic force, magnetic force, and air resistance [we’ll list air resistance here]).
Adding forces
Opposite forces have opposite signs, and the sum of all forces acting on an object is known as net or resultant force (forces in different directions can’t be added).
Depicting forces in free-body diagrams…
As a vector, force can be represented by arrows (the length tells us the magnitude of the force and the direction is shown by where it points to).
What are unbalanced forces?
Unbalanced forces are two unequal, opposite forces.
Types of forces
GENMUFTA
Gravitational force (force due to gravity)
Electrostatic force (forces between charges)
Normal reaction force (force between two objects in contact)
Magnetic force (between magnets)
Friction (opposes motion)
Tension (involved during pull or push [rope, spring, elastic, etc.])
Upthrust (upward force exerted by a liquid)
Air resistance (opposes motion in air)
What are balanced forces?
Forces on an object are said to be balanced if the net (resultant) force is zero
For the net force to be zero:
- Vertical forces must be equal
- Horizontal forces must be equal
Forces you need to remember (as illustrated in free-body diagrams)
Friction (F [can be lowercase]) always opposes motion and is at point of contact (center or not).
Upthrust (U) is the force that keeps objects floating in the water, so arrows are drawn from the surface of the water upwards (towards [and past?] the object)
Air resistance (R [same as normal, but that doesn’t really matter]) - for simplicity (or aesethetics), we normally connect it and gravity’s arrows.
Free-body diagrams
Free-body diagrams show us the key/all forces acting on AN object.
NOTE:
We’ll get a sentence/scenario: if it says at rest, we know our arrows must be the same length (you must indicate this)
For an object resting on a ramp, gravity should be longest (we can guess as to the length)
They will tell you if say air resistance should be considered
Ropes and such are not considered separate objects? Also, tension would be longer for objects attached to the ceiling BECAUSE the object is attached to the ceiling
The vertical forces must balance out for an object to be moving in a horizontal direction
If it’s a car, each wheel has normal reaction force, and we can say W = 2R
Terminal velocity is when velocity is constant
Newton’s First Law
States that an object at rest tends to stay at rest, and an object in motion tends to stay in motion, unless acted on by an external unbalanced force (in other words, for an object to accelerate, there must be an unbalanced force applied to it); also called the Law of Inertia.
What is inertia?
The tendency of an object to preserve its state of motion. Three types: inertia of rest (tendency to resist change in state of rest), inertia of motion (tendency to resist change in state of uniform motion), and inertia of direction (tendency to resist change in direction of motion).
What is equilibrium?
An object is said to be equilibrium if it is at rest or moving with constant velocity (acceleration is zero, so net force is zero): in other words, the object doesn’t accelerate and its net force is zero.
Necessary conditions:
Object’s speed may be constant (zero acceleration).
Object may be at rest.
Object’s net force must be zero.
Newton’s Second Law
States that the acceleration of an object is directly proportional to the net force applied to it and (two parts) inversely proportional to the mass of the object.
a = Fnet/m
a is directly proportional to Fnet (both in numerator)
a is inversely proportional to m (m is in denominator)
NOTE:
*In those three-part formula pyramids, Fnet would be at the top (m and a would be in the bottom)
If the problem gives you a value in grams, you have to convert it to kg
Newton’s Third Law
States that for every action, there’s an equal and opposite reaction: when two objects interact, they exert some force on each other (these forces are equal and opposite in magnitude, and are called action-reaction pairs: action-reaction pairs always act on different objects [so normal reaction force and weight are not an action-reaction pair]).
Action-reaction pair ex.
With a moving car colliding with a wall, the action would be the force exerted by the car on the wall and the reaction would be the force exerted by the wall on the car.
What is mass?
Mass is the amount of matter in an object and it is measured using a digital beam balance or lever: it is an intrinsic value, it never changes, and can never be zero.
Unit: kg
Remember: def, measurement, intrinsic/extrinsic, changeability, ability to be zero.