mechanics Flashcards
when a object is in equilibrium, what are the forces doing?
all forces acting on it are balanced and cancel each other out. there is no resultant force acting on the object
how do you find a missing magnitude of a force when a particle is in equilibrium and three forces are acting (one vertical another horizontal, one diagonal)?
draw the forces tip to tail, then use Pythagorus to work out the missing length.
how to resolve three forces at different angles for a body in equilibrium
sum of the vertical components = 0
sum of the horizontal components=0
work these out using trigonometry
what’s a good way of investigating equilibrium?
force boards- you can apply forces to an object in different directions and when at equilibrium, the object will be in the middle.
what are the two main types of friction?
contact- between solid surfaces
fluid-liquid or gas
what are the three factors that effect fluid friction?
the thickness (viscosity) of the fluid
force increases as speed increases
the larger the area pushing against the fluid, the greater the resistive force
what do frictional forces convert kinetic energy into?
heat
what is lift force?
an upward force on an object moving through a fluid
when does a lift force occur?
when the shape of an object causes the fluid flowing over it to change direction
when does terminal speed occur?
when frictional forces equal the driving force
an object will reach it at some point if there’s a driving force that is constant and a resistive force that increases with speed
what will happen to the forces if a car speeds up with a constant driving force?
the air resistance on it will increase as speed increases until its equal to the driving force, it will then be travelling at maximum speed and terminal velocity
what are the two main ways of increasing a car’s maximum speed?
increase driving force eg. increase engine size
reduce frictional force eg. more streamlined
how could you investigate terminal speed in a fluid?
put elastic bands around the tube of viscous liquid at fixed distances using a ruler
drop a ball bearing into the tube
use a stopwatch to record the time it reaches each band
repeat this, use a strong magnet to remove the ball bearing from the tube
calculate and average time and use this and distance to work out average velocity
as soon as a skydiver leave the plane what happens and when do they reach terminal velocity?
they accelerate downwards until air resistance equals their weight
they will then travel at terminal velocity
this velocity is too large to land safely so just before hitting the ground open parachute
this immediately increases air resistance so its bigger then weight
this slows them down until weight equals air resistance again
then reaches a lower terminal velocity which is small enough to survive the landing
what’s newton’s first law of motion?
the velocity of an object will not change unless a resultant force acts against it
what’s newton’s second law?
the acceleration of an object is proportional to the resultant force acting on it
F=ma
what’s newton’s third law?
if object A exerts a force on object B, then object B will exert and equal and opposite force on object A
how can you find change in velocity from an acceleration time graph?
area under the graph
how do you find acceleration from a velocity time graph?
the gradient
how do you find displacement from a velocity time graph?
area under the line
whats an ultrasound position detector?
a type of data-logger that automatically records the distance from the sensor multiple times every second
what are some advantages of using data loggers over traditional methods for drawing graphs?
more accurate data, eliminates human reaction time
much higher sampling rate then humans
you can see the data displayed in real time
if an object is accelerating, what will the displacement-time graph look like?
curved line with an increasing gradient
when is linear momentum conserved?
assuming there is no external forces, always
what is conserved in an elastic collision?
momentum and KE
during an inelastic collision, what happens to the KE and momentum?
some of the KE is transferred into other forms during the collision
linear momentum is conserved
what is impulse?
the product of force and time, its equal to change in momentum
how can you work out impulse from a force-time graph?
its the area underneath
what do crumple zones on cars do?
the parts on the front and back of the car crumple up and deform plastically on impact. this causes the car to take longer to stop, increasing impact time and decreasing the force on passengers. some of the cars KE goes to charging shape of vehicle so less is transferred to passengers
what do seat bents do?
stretch slightly, increasing the time taken for the wearer to stop which reduces the force acting on the chest
what do air bags do?
slow down passengers more gradually and prevent them hitting hard surfaces in the car
what’s the definition of a JOULE
one joule is the work done when a force of 1 newton moves an object through 1 metre
what is power?
the rate of doing work, the amount of energy transferred from one form to another per second
what’s the definition of a WATT
a rate of energy transfer equal to 1 joule per second
what does the area under a force-displacement graph tell you?
the work done
what is the principle of conservation of energy?
states that energy cannot be created or destroyed. energy can be transferred from one form to another but the total amount of energy in a closed system won’t change
how do you work out elastic potential energy?
E=1/2(k)(extension)^2
where k= stiffness of object
how can you investigate efficiency of an electric motor as it lifts a mass through a vertical distance?
as the mass is raised, use an ammeter to measure current. you can then find electrical energy using E=ItV. t is the time taken to move the mass, V is the power supply voltage. you can then find gpe.
efficiency= mgh/ItV
what is the centre of mass?
single point where you can consider all of its weight to act
experiment to find the centre of mass of a flat object
hand the object freely from a point eg. its corner
draw a vertical line downwards from point of suspension
hand the object at a different point and draw another vertical line down
centre of mass is where these two lines cross
how can you find the resultant vector of two vectors?
draw a tip to tail diagram then measure the length and angle of the resultant
what’s the gravitational field strength on the moon?
1.6
what is moment?
the turning effect of a force around a turning point
what is the principle of moments?
states that for a body to bee in equilibrium, the sum of clockwise moments about any point = the sum of anticlockwise moments
what are couple forces?
a pair of forces of equal size which act parallel to each other but in opposite directions
produces a turning effect (moment)
what is free fall?
when there’s gravity acting on an object and nothing else
-the motion of an object undergoing the acceleration of g
what was Galileo’s free fall experiment
he believed that all objects fall at the same rate
he measured the time a ball took to roll down a smooth groove in an inclined plane
by rolling the ball along different fractions of the total length of the slope, he found that distance travelled was proportional to the square of the time taken
why do all objects fall at the same rate?
the acceleration is independent of the mass so it makes no difference what the mass is
how can you determine g using freefall? RP3
-drop a ball bearing onto a trap door using an electromagnet
-measure the height from the bottom of the ball bearing to the trapdoor
-flick the switch to simultaneously start the timer and disconnect the electromagnetic, releasing the ball bearing
-the ball bearing falls, knocking the trap door down and breaking the circuit which stops the timer
-repeat 3 times and find an average time then repeat but dropping the ball from different heights
-plot a graph h-t^2 to find g
what’s the most significant source of error when determining g using freefall?
random error for the value taken for height. uncertainty of the ruler 1mm
and there’s a small systematic error if there’s a delay in the switch or timing mechanism
A spherical particle Q is fired at a right angle to the surface of an identical block.
Q has the same mass as P and is travelling at the same speed as P when it strikes the surface of the block.
Q is made from a less dense material than P.
Compare the distance travelled by Q with that travelled by P as they are brought to rest.
(3 marks)
-Q has a larger volume (for the same mass and KE) /
-Q has a larger surface area (for the same mass and KE) ✔
-Q will experience a greater resistive force (at any given speed) / Q will displace more matter per unit distance✔
-Q will do more work per unit distance / Q will transfer more of its kinetic energy per unit distance / Q will experience a greater deceleration✔
Must have Q will travel a shorter distance for all 3 marks.
average speed of humans walking
1-2m/s
