4 dymanics II: forces Flashcards
effects of a force on the motion of a body
a body at rest moves
a moving body increases in speed
a moving body decreases in speed
a moving body changes direction
what does zero acceleration imply
zero acceleration implies that the object can be stationary or moving with constant velocity. however, even though acceleration is zero, it does not mean there are no forces acting on it. it means resultant force is zero
if the resultant for is zero
we say the forces acting on the object are balanced
state newton’s first law of motion
newton’s first law of motion states that every object will continue in its state if rest or uniform motion in a straight line unless a resultant force acts on it
what is the inertia of an object
the inertia of an object refers to the reluctance of the object to change its state of rest or motion, due to its mass
____ is the property that resists the change in motion (inertia)
mass
the larger the mass of the object, the harder it will be for the object to:
start moving
slow down
move faster
change direction
if the resultant force acting on an object is not zero
we say the forces acting on the objects are unbalanced
state newton’s second law
newton’s second law of motion states show when a resultant force acts oh an ohiedt of a constant mass, the object will accelerate in the direction of the resultant force
what is newton’s second law in symbols
F=ma
F: resultant force (N)
m: mass of object (kg)
a: ax elevation of object (in m/s^2)
newtons edmond law of motion tells us that:
- a resultant force (F) on an object produces acceleration (a)
- doubling the resultant force (F) in an object doubles its acceleration (a)
- whether the same resultant force (F), doubling the mass (m) halves the acceleration (a)
newton’s third law indicates that
forces occur in pairs
state newton’s third law
newton’s third law of motion states that if body A exerts a force Fab on body B, then body B will exert an equal opposite force Fba on body A
what characteristics does newton’s third law of motion tell us about forces
- fours always occur in pairs. each pair is made up of an action and a reaction force
- action and reaction forces are equal in magnitude
- action and reaction forces act in opposite directions
- action and reaction forces act on different bodies
friction
friction is the contact force that opposes or tends to oppose motion between surfaces in contact
what does friction do
friction opposes motion between surfaces in contact and is the result of irregularities of the surface
positive effects of friction
we can walk without slipping
moving objects are able to slow down when needed
negative effects of friction
cars are less efficient by up to 20%
moving parts in engines, motors and machines suffer wear and tear
ways to reduce negative effects of friction
using wheels
using ball bearings
using lubricants and polished surfaces
using air cushions
using wheels
being circular shapes, wheels greatly reduce the friction
using ball bearings
being spherical shaped, ball bearings are used to reduce friction between moving parts of machines, cars and line skates.
ball bearings are placed between moving parts so that they can roll around and prevent moving parts from rubbing against each other
using lubricants
applying a layer of lubricant (like oil) can greatly reduce friction
lubricants are frequently used between the parts of an engine helping to prolong its life
polished surfaces
polishing a surface removes surface irregularities and can also reduce between surfaces in contact
using air cushion
friction between two surfaces is reduced when there is a thin layer of cushion between the two surfaces
ways to enhance positive effects of friction
using treads
using parachute
using chalk
using treads
tyres are designed with treads (grooves that quickly channel water from underneath the tyres)
this improves the grip of the tyres on wet roads, thus preventing skidding
using parachutes
air resistance is a type of friction in air
a skydiver in midair varies air resistance to change his speed
using chalk
rock climbers use chalk powder on their hands right absorb perspiration and improve their grip
when can an object free fall
an object can only free fall if the only force acting on it is its own weight (eg when an object falls in a vacuum)
characteristics of objects falling WITH air resistance
it always opposes motion of moving objects
it increases the speed of the objects
it increases with the surface area (or size) of the objects
it increases with the. density of air
what is terminal velocity
when air resistance acting against an object equals its weight, the object starts to travel at a constant speed known as terminal velocity. (this means that the object has zero acceleration)
