SP15: forces and matter Flashcards
how many forces must be acting on a stationary object for it to change shape?
more than one
what is bending?
when one side of an object is compressed and the other is stretched
what is stretching?
pulling an object’s ends apart
what is compressing?
pushing an object’s ends together
what is a change in shape called?
distortion
what does an elastic material do?
return to its original shape when force is removed
examples of elastic materials:
-rubber bands
-rubber gloves
-tennis ball
-spring
-diving board
what does an inelastic material do?
keep its new shape when force is removed
examples of inelastic materials:
-clay
-banana
-Uncooked pastry
when does extension happen?
when an object increases in length
when does compression happen?
when an object decreases in length
hooke’s law
force (N) = spring constant/k (N/m) x
extension (m)
what is spring constant? (k)
a measure of the stiffness of a spring
what is the limit of proportionality?
the point beyond which hooke’s law is no longer true when stretching a material
what is elastic limit of a material?
-the furthest amount it can be stretched/ distorted without being able to return to its previous shape
-once a material has gone past its elastic limit, its distortion is said to be inelastic (the object does not return to its original length when the force is removed
what does a higher spring constant mean?
a stiffer/less stretchy spring
what is the relationship between extension and force?
-they are directly proportional
-as force increases, extension increases
-this works until the limit of proportionality is exceeded
what is the gradient of a line on a force-extension graph?
spring constant
what does a graph showing force and extension for a rubber band look like?
an s shape
what does a graph showing force and extension for a spring look like?
a straight line through the origin
directly proportional
a straight line
linear relationship
a straight line through the origin
non-linear relationship
not a straight line
what is done when a spring is extended or compressed?
work is done
when is elastic potential equal to the work done?
inelastic distortion hasn’t happened
how can you calculate the elastic potential energy stored when a spring is extended or compressed?
1/2 × k × (extension)?
investigating force and extension with a spring or rubber band:
(method)
- secure a clamp stand to the bench
- use bosses to attach two clamps to the clamp stand
- attach the spring to the top clamp
- measure and record the unaffected length of the spring or rubber band
- hang a 100g (1N) mass carrier from the spring or rubber band
- measure and record the new length of the spring/rubber band
- add a 100g slotted mass to the carrier
- measure and record the new length of the spring/rubber band
- repeat step 7 until you have added
a total of 1000g/1kg - record your results in a table
investigating force and extension with a spring or rubber band:
(drawing the table)
- for each result, calculate the extension: extension = stretched length - original length
- plot a line graph with extension on the x axis, and force on the y axis
- draw a suitable line or curve of best fit
- identify the range of force over which the extension of the spring is directly proportional to the weight hanging from it
- for the region where extension is proportional to force, find the gradient of the line (the spring constant, k)
- work done = force × distance moved. here, the work done in extending the spring is given by the area under the line on the graph
- The energy transferred to a spring’s elastic store is given by the equation (e = 1/2 × k × ×²)
- compare the area under the line, from the origin up to a point, with the calculation of the energy stored in the spring for that extension
investigating force and extension with a spring or rubber band:
(hazards & control measures)
1.
hazard = equipment falling off table
consequence = bruise or fracture to feet
control measures = use a G-clamp to
secure the stand
2.
hazard = masses falling to floor if the
spring fails
consequence = bruise or fracture to feet
control measures = gently lower load onto
spring and step back
investigating force and extension with a spring or rubber band:
(variables)
independent: masses
dependent: length of spring or rubber band
control: amount of masses used
what is pressure?
force per unit area
pressure exerted by different shoes
1.
-flat shoes spread the force over a large area, reducing the pressure
-snow shoes have a much larger area than feet to spread the force over a larger area and reduce the pressure on the snow - this stops people sinking into the snow
2.
-high heeled shoes transfer the force through a much smaller area, causing a much greater pressure
-it will hurt more if a person steps on someone’s foot in high heels than if they were wearing flat shoes
-this is also why it hurts so much to accidentally step on a plug or stones barefoot
what is the relationship between area and pressure?
inversely proportional: as area increases, pressure decreases
what does an object in a fluid experience pressure from?
-the fluid itself
-atmospheric pressure
what angle to a surface is the force caused by pressure in a fluid?
a right angle (90)
how do you calculate pressure?
force (N) / area (m²)
what is the relationship between depth and pressure in liquids?
pressure increases as the depth increases
pressure acting on a dam
-the pressure acting on a dam at the bottom of a reservoir is greater than the pressure acting near the top
-this is why dam walls are thicker at the bottom
why can air and liquid cause pressure?
they have weight
pressure in both air and liquids comes from _____ directions
all
what is pressure in a liquid due to?
the weight of the column of water above
what is a greater density in liquid due to?
greater weight of fluid for the same volume
what direction does pressure in a liquid act in?
all directions
how to calculate pressure caused by a column of liquid (Pa):
height of column (m) × density of liquid (kg/ms) × gravitational field strength (N/kg)
key features of the earth’s atmosphere
-it is thin compared to the size of the earth
-it becomes less dense as the altitude increases
what causes atmospheric pressure?
air molecules colliding with a surface
atmospheric pressure at sea level
100,000 Pa
when does atmospheric pressure decrease?
with increasing altitude
why does atmospheric pressure decrease with altitude?
as the altitude increases:
-the number of air molecules decreases
-the weight of the air decreases
-there is less air above a surface
what is upthrust?
-the resultant upwards force caused by a greater pressure on an objects bottom surface than on its top surface
-acts on an object that is partly or completely submerged
what is upthrust force equal in size to?
the weight of the fluid displaced by the object
how upthrust affects if an object floats, sinks or rises:
-if upthrust is equal to the weight of the object, it with float
-if the upthrust is less than the weight of the object, the object will sink
-if the upthrust is larger than the weight of the object, the object will rise
the relationship between density and upthrust for a sinking object:
-if the density of the object is greater than the density of the fluid, the object can never displace enough fluid to create an upthrust that will hold its weight up so it sinks
is seawater or
freshwater denser?
seawater
