forces Flashcards
what is a scalar quantity?
only has magnitude + isn’t direction dependent
what is a vector quantity?
has both magnitude + associated direction
how can a vector quantity be drawn + what does it show?
as an arrow (length represents magnitude)
what is a force?
push / pull acting on an object due to an interaction with another object
what are the two categories that all forces can be split into?
contact (objects touching) + non-contact (objects separated)
give three examples of contact forces
friction, air resistance + tension
give three examples of non-contact forces
gravitational, electrostatic + magnetic forces
is force a vector or a scalar quantity?
vector
give three examples of vector quantities
velocity, force + displacement
give three examples of scalar quantities
temperature, time, mass, speed + distance
what is weight?
force that acts on an object due to gravity + object’s mass
what quantities does weight depend on?
weight = mass x gravitational field strength
-object’s mass
-gravitational field strength at the given position in the field
what is the unit used for weight?
newton (N)
what is the unit used for gravitational field strength?
N/kg
what is meant by an object’s centre of mass?
single point where an object’s weight can be considered to act through
what piece of equipment can be used to measure an object’s weight?
calibrated spring-balance or newton-meter
what is the name given to the single force that is equivalent to all the other forces acting on a given object?
resultant force
what does it mean if a force is said to do ‘work’?
the force causes an object to be displaced through a distance
what is the equation used to calculate work done? give units
work done (J) = force (N) x distance (m)
what distance must be used when calculating work done?
distance that is moved along the line of action of the force
under what circumstances is 1 joule of work done?
when a force of 1N causes a displacement on 1m
how many newton-metres are equal to 1 joule of energy?
1Nm = 1 J
what occurs when work is done against frictional forces?
-rise in temperature of object
-kinetic energy is converted to heat
why does air resistance slow down a projectile?
-object does work against air resistance
-kinetic energy is converted to heat
explain the relationship between force applied + extension of an elastic object
extension is directly proportional to force applied, provided that the limit of proportionality isn’t exceeded
what is meant by an inelastic (plastic) deformation?
deformation which results in the object permanently stretched + object doesn’t return to its original shape when force is removed
what is elastic deformation?
object returns to original shape when force is removed
state the equation relating force, spring constant + extension. give units
force (N) = spring constant (N/m) x extension (m)
what type of energy is stored in a spring when it is stretched?
elastic potential energy
what can extension be replaced with in the equation for spring force?
compression
does a distance quantity require a specific direction? is it scalar or vector?
no specific direction is required so it is scalar
if an object moves 3m to the left and then 3m back to its initial position, what is the object’s total displacement?
-object has zero displacement
-displacement is vector quantity so it involves direction
-object starts + ends at same point
state typical value for speed of sound
330 m/s
what is a typical value for human walking speed?
1.5 m/s
what is a typical value for human running speed?
3 m/s
what is a typical value for human cycling speed?
6 m/s
state the equation linking distance, speed + time. give units
distance (m) = speed (m/s) x time (s)
why can an object travelling at a constant speed in a circle not have a constant velocity?
-speed is scalar
-velocity is vector which means it can only be constant if direction is constant
-in circular motion, direction is continuously changing
how can speed be calculated from a distance-time graph?
speed is equal to gradient of the graph
what must be done to calculate speed at a given time from a distance-time graph for an accelerating object?
-draw tangent to the curve at required time
-calculate gradient of tangent
state equation for average acceleration of an object. give units
acceleration (m/s²) = change in velocity (m/s) / time (s)
how can the distance travelled by an object be calculated from a velocity-time graph?
equal to area under graph
give an approximate value for the acceleration of an object in free fall under gravity near earth’s surface
9.8 m/s²
what can be said about the resultant force acting on an object when it is falling at terminal velocity?
-resultant force is zero
-when at terminal velocity, object is moving at constant speed + isn’t accelerating
state newton’s first law for a stationary object
if resultant force on a stationary object is zero, object will remain at rest
state newton’s first law for a moving object
if resultant force on a moving object is zero, object will remain at constant velocity
what can be said about braking forces + driving forces when a car is travelling at constant velocity?
braking forces are equal to driving forces
if an object changes direction but remains at a constant speed, is there a resultant force?
since there is a change in direction, there is a change in velocity + so there must be a resultant force
what is inertia?
tendency of an object to continue in its state of rest or uniform motion
state the defining equation for newton’s second law
resultant force = mass x acceleration
state newton’s second law in words
an object’s acceleration is directly proportional to resultant force acting on it + inversely proportional to its mass
what is inertial mass?
-measure of how difficult it is to change a given object’s velocity
-ratio of force over acceleration
state newton’s third law
whenever two objects interact, forces they exert on each other are equal + opposite
what is the stopping distance of a vehicle equal to?
sum of thinking distance + braking distance
for a given braking distance, if the vehicle’s speed is increased, what can be said about its stopping distance?
the stopping distance is increased with an increase in speed
give a typical range of value for human reaction time
0.2 secs - 0.9 secs
give three factors which can affect a driver’s reaction time
-tiredness
-drugs
-alcohol
give two factors which may affect braking distance
-adverse (wet/icy) road conditions
-poor tyre/brake conditions
describe the energy transfer that takes place when a car applies its brakes
-work is done by friction force between brakes + wheel
-kinetic energy of wheel is converted to heat + dissipated to surroundings through brake discs
to stop a car in a given distance, if its velocity is increased, what must happen to the braking force applied?
braking force must also be increased
state two consequences of a vehicle undergoing very large decelerations
-kinetic energy converted to heat is very high causing brakes to overheat
-loss of control of vehicle
state the equation used to calculate an object’s momentum
momentum = mass x velocity
unit for momentum
kg m/s
in a closed system, what can be said about the momentum before + after a collision?
total momentum before is equal to total momentum afterwards
state an equation linking change in momentum, force + time
force x time = change in momentum
what quantity is equal to force experienced in a collision?
rate of change of momentum
if an object’s change of momentum is fixed, what is the only way to reduce the force that the object experiences?
increase the length of time over which the change of momentum occurs
outline the basic steps of the practical
1) hang a spring from a clamp stand alongside a metre ruler
2) measure the spring’s initial length
3) add weights in intervals of 10N + record extension
4) plot graph of extension against force
what piece of apparatus is used to secure the clamp stand to the desk and why?
a g-clamp so the clamp doesn’t tip over whilst adding masses. the clamp produces a moment which counteracts the moment caused by the masses so the stand remains in equilibrium
how do you calculate the extension of the spring when you add masses?
subtract the extended length of the spring from the original unstretched length
what can you add to the spring to ensure that the extension measurements are accurate?
a pointer attached horizontally to base of spring + extending to metre ruler. this will make reading length easier
what piece of safety equipment should be used throughout this experiment and why?
safety glasses should be worn in case of spring breaking or becoming unattached + damaging your eyes
what other safety precautions should be taken?
avoid standing with feet underneath where weights are hanging in case they fall + wear appropriate footwear
what graph should you plot with your results? what would you expect it to look like?
extension against force (weight). you would expect it to be a straight line passing through origin + variables should be directly proportional
what is the name of the relationship between extension and force? state the relevant equation
hooke’s law
force = spring constant x extension
what type of energy is stored in the spring as it is stretched?
elastic potential energy
what equation can be used to calculate the elastic potential stored in the spring?
elastic potential energy = 0.5 x spring constant x extension²
how can the elastic potential stored in the spring be worked out graphically?
stored energy is equal to area under the force/extension graph
how can you use your apparatus and graph to work out the weight of an unknown object?
1) hang the object on the spring + record extension it produces
2) draw a line from extension on your graph until it meets your plotted line + then read off the corresponding weight
outline the basic steps of the practical which measures the effect of force on acceleration at constant mass
1) mark out 20cm intervals on a bench
2) attach a string to trolley + pass it over a pulley
3) attach varying masses to end of string + release them so trolley accelerates
4) time how long it takes to reach each interval
suggest a method for marking out 20cm intervals on the desk
place pieces of tape every 20cm from starting point of trolley + use a metre ruler to measure
what piece of equipment is used to record the times?
stopwatch with ability to record multiple times so you can record when it passes each interval
what safety precautions should be taken when releasing masses?
ensure you’re not standing underneath where they are hanging. a padded bucket could be placed underneath them to catch them when they drop
how would you expect the acceleration of the trolley to change as you add more weights to the end of the spring?
the more weights added, the greater the force that is accelerating the trolley + so the greater the acceleration
which of newton’s laws explains the relationship between force + acceleration? state the relevant equation
newton’s second law
force = mass x acceleration
outline the basic steps of the practical which measures the effect of mass on acceleration at constant force
1) mark out 20cm intervals on a bench
2) attach a string to trolley + pass over pulley
3) attach fixed mass to end of string + release it so trolley accelerates
4) time how long it takes to reach each interval
5) repeat with varying masses attached to trolley
how can you choose an appropriate quantity of masses to use as the fixed mass for this experiment?
carry out a preliminary experiment to find out what weight is needed to just accelerate the trolley from rest
what would be the consequence of using too large a fixed weight?
the trolley would accelerate too quickly for the timing at each interval to be accurate
how would you expect the acceleration of the trolley to change when you add masses onto the trolley?
acceleration will decrease as masses are added to trolley because:
acceleration = force / mass
suggest a way that the experiment could be changed to improve the timing of the trolley at each interval
a video camera could be used to record the experiment + then timings taken from the slowed down video afterwards. this reduces uncertainty from human reaction times. alternatively, light gates could be used
