3.2 FORCES IN ACTION Flashcards
define resultant force
is a single force which has the same effect as the sum of all the forces acting on a body
what is the formula for resultant (or net) force? and what law is this?
f = ma
where f = resultant force
m = mass of object
a = acceleration
form of Newton’s second law when mass is constant
when can you use f = ma?
when mass is constant (e.g not a rocket)
what is the unit of force and define it?
newtons, N
1 newton is the force required to accelerate a mass of 1kg by 1ms^-2 in the direction of the force
if the forces on an object are balanced the object is said to be in…?
equilibrium
no resultant force, no acceleration
what is the formula for weight force?
w = mg
where w = weight force
m = mass of object
g = acceleration due to gravity
what are the four fundamental forces of nature?
-the gravitational force
the force between two masses, has infinite range
-the electromagnetic force
holds atoms and molecules together, has infinite range
-the weak force
responsible for radioactive decay, acts over very short ranges
-the strong force
responsible for holding subatomic particle together in the nucleus and is the strongest force of all nature
define tension
is the force experienced by any rope, string, cable or wire that is being pulled, hung, rotated or supported
define normal contact force (or reaction force)
is the force that acts perpendicularly (at right angles) to the point of contact of a body and the surface with which it is in contact
define upthrust
the upwards force that a liquid or gas exerts on a body floating in it due to the fluid displaced
define friction
the force of friction occurs between two surfaces in contact with one another and resists the motion at the point of contact
what are free body force diagrams and what should they contain?
simplified labelled drawing where the object is modelled as a point, it should contain…
the body on which the forces act
the direction of application of each force
the type of each force
the size of each force
if you drop two objects of different masses which will hit the ground first? (ignoring air resistance)
both hit the ground at the same time because acceleration is independent of mass, objects fall at the same rate
what is drag?
the frictional force experienced by an object travelling through a fluid (can be liquid or gas)
what factors affect drag for an object travelling through air?
cross sectional area of the object
the density of the fluid
what is the relationship between drag caused by air resistance and the cross sectional surface area of the object?
drag ∝ cross sectional area
directly proportional
what is the relationship between drag caused by air resistance and velocity of the object?
drag ∝ velocity^2
directly proportional to square of velocity
what is terminal velocity?
the velocity at the point during freefall when the object stops accelerating as the weight force is balanced by the air resistance (becomes equal), object falls at constant velocity
outline an investigation to determine the terminal velocity in fluids
- Clamp tube with a stands, fill tube with glycerol
- Wrap elastic bands, or draw marks using a marker pen, around the tube of viscous liquid at set intervals measured by the ruler
- Hold a ball bearing directly above the surface of the liquid, and then release it from rest
- Record the time it reaches each elastic band or marker line by using the lap feature on the stopwatch
- Calculate the time taken to travel between consecutive bands and calculate the average speed at each time
- Repeat the procedure several times over a wide range of readings
When the ball bearing reaches terminal velocity, the distance it has travelled between each time interval will be the same, and so its velocity can be determined
Use the equation speed = distance / time to find the average velocity of the ball bearing between each set of bands or markers
Plot a graph of velocity against time and draw a line of best fit
The maximum velocity is where the graph plateaus and has zero gradient
This is the terminal velocity
what does it mean for an object to be in equilibrium?
objects are in equilibrium when all the forces acting on them in the same plane (coplanar forces) are balanced - there is zero net or resultant force, in terms of motion, the object is either stationary or is travelling at constant velocity
what is a moment?
a turning effect of a force
what is the definition/equation of a moment?
the moment of force (or turning moment) is the product of a force and the perpendicular distance of its line of action or axis
moment = force x perpendicular distance from pivot
Moment = F x d
what is the principle of moments?
the principle of moments states that for an object to be in rotational equilibrium, the sum of the clockwise moments must be equal to the sum of the anticlockwise moments
(moments must be balanced or the object will turn)
what is a couple?
a couple occurs when two equal, anti-parallel forces act to produce a rotation (no linear motion occurs)
what is the definition/equation of a torque of a couple?
the turning moment (torque) due to a couple is the product of one of the forces and the perpendicular distance between them
torque of couple = size of one of the forces x perpendicular distance between them
T = F x d
what are examples of objects that experience moments?
muscles, bones and joints act as LEVERS, an effort force acts against a load force by means of a rigid object rotating around a pivot
define the centre of mass
the centre of mass of an object is the single point at which all of the mass of the object can be assumed to be situated
define the centre of gravity
the centre of gravity of an object is the single point through which the entire weight of the object can be thought to act
for a symmetrical body of constant density where is the centre of mass?
at the centre of the body
outline an investigation to determine the centre of mass or centre of gravity of an object
make three holes in random places on the edge of the shape
hang the shape from first hole by placing through pin, let it swing until comes to rest
use a plumb line to draw a vertical line down from the point of suspension to the bottom of the shape
repeat these steps but with the different holes
where the lines intersect is the centre of mass
what does a wide base and a low centre of mass mean for an object?
it has more STABILITY
how does an object become unstable?
if an object is titled so that the centre of mass lies outside the width of the base of the object it will become unstable and will topple and fall
what is neutral stability and give an example?
neutral stability is when the centre of mass can never fall outside the with of the base of the object (that is touching the ground), for example objects that can roll like a football
what does resolving a force mean?
splitting it into its horizontal and vertical components using trig
what does coplanar forces mean?
forces that act in the same plane
what does a closed loop mean in vector triangles?
there is no resultant force, acceleration is zero
what does a velocity-time graph look like for terminal velocity?
line starts increasing with high gradient and slowly flattens out (curves to gradient of zero)
what does a velocity-time graph look like for terminal velocity of a skydiver with a parachute being opened?
line starts increasing with high gradient and slowly flattens out (curves to gradient of zero)
then once parachute opened steep drop to lower terminal velocity, continues straight with zero gradient
what is the formula for density?
ρ = m / v
the mass per unit volume
what is the symbol of density?
ρ , rho
how do you calculate the density of an irregular shape? (E.g not a cuboid, sphere or something you can calculate the volume of)
measure the object’s mass using scales
then calculate the volume by putting the object in a cylinder of water of known volume (cm^3) and work out the volume of water displaced (this will be equal to the volume of the object) - can use a displacement can
what is Archimedes’ principle?
Archimedes’ principle states that the upward buoyant force (upthrust) exerted on an object immersed in a fluid, is equal to the weight of the fluid the object displaces
what is the formula for pressure exerted by a fluid at any depth?
p = hρg
where p = pressure
ρ = density of fluid
h = depth of fluid
g = acceleration due to gravity
how do you derive p = hρg?
density = m / v
therefore m = ρ X v
for a cylinder/column shape volume is equal to h X A (where h is the height and A is the cross sectional area)
therefore m = ρhA where m is the mass of the fluid
the weight of this mass is equal to mg, so weight force = ρhAg
pressure = force / area so ρhAg / A gives you pressure = hρg
what happens when a force is applied through the centre of a mass of a body?
the body will accelerate in the direction of the applied force in a STRAIGHT LINE WITH NO ROTATION
what is the formula for pressure for solids, liquids and gases?
pressure = force / area
measured in pascals
the force is perpendicular to the area
