P10 Flashcards
Force and motion
Describe three methods for a practical investigating Newton’s second law (the relationship between force applied and acceleration of an object, as well as the relationship between mass and the acceleration)
FORCE/ACCELERATION:
method 1
- gather equipment
[] chalk
[] metre rule
[] trolley
[] pulley
[] string
[] 10 10g masses
[] desk
[] phone
- on the desk, use the metre rule and chalk to draw markings along the long edge of the desk every 10cm
- attach a pulley to the end of the desk
- attach the trolley to a string and connect to the pulley
- FOR EACH MASS USED, TAKE 2 REPEAT MEASUREMENTS AND CALCULATE A MEAN FOR INCREASED ACCURACY
- place the 10 10g masses on the end of the string
[] hold the trolley at the start line on the desk
- have someone else use a phone camera to record when you let the trolley go (start recording as soon as let go)
[] if there are any zero errors in terms of the recording, account for these
- play back the video and record at which times in seconds the trolley passed each chalk line on the desk, pausing the video if you need to
- repeat with 80g to 20g
[] MUST PLACE MASS FROM PULLEY ON TROLLEY AFTER DONE; eg. at the end of the 100g, place two 10g masses on the trolley
[] this is because mass should be a control variable
- use (2xdistance(m))/time to get acceleration
- plot results (force on y-axis, acceleration on x-axis)
- should find that force and acceleration are proportional to one another
method 2
same thing as before but use light gates lol
MASS/ACCELERATION
same thing as method 1 but instead of changing force on pulley, keep force constant and continually add mass to the trolley
- should find that these two are inversely proportional
Give the equation for force (with units)
force (N) = mass (kg) x acceleration (m/s^2)
State Newton’s second law of motion
- an object’s mass is inversely proportional to its acceleration
- the resultant force on an object is directly proportional to its acceleration
What is inertia(l mass) (definition + equation for inertial mass) ?
- inertia is the tendency of a resting object to stay at rest, and an object in motion to stay in UNIFORM motion
inertial mass = force (N)/acceleration (m/s^2)
INERTIAL MASS IS A MEASURE OF THE DIFFICULTY TO CHANGE THE OBJECT’S VELOCITY
When is acceleration negative/positive ?
negative
- deceleration; resultant force (acceleration) is in the opposite direction to its velocity
positive
- acceleration; resultant force (acceleration is in the same direction as the object’s velocity)
What is the difference between weight and mass ?
- weight = the force acting on an object due to gravity
- mass = the quantity of matter in the object
How many newtons are in 1 kg ?
10N
Give the equation for weight (with units)
weight (N) = mass (kg) x gravitational field strength (N/kg)
Give the acceleration due to gravity on Earth
9.8 m/s^2
Give the gravitational field strength of Earth
9.8 N/kg
What measurement tool can measure weight ?
newton meter
Why do cars have a top speed ?
terminal velocity; the point at which the frictional force is equal and opposite to the driving force/weight
Explain terminal velocity using the example of a car
- at first, very little air resistance so the driving force is able to overcome air resistance
- as such the char accelerates
- as the car accelerates, air resistance on the car increases
- at a point, the car has a constant velocity and can no longer accelerate as the air resistance acting on the car is equal to the driving force, meaning the resultant force is 0
What is the frictional force on an object in a fluid ?
drag force
What does the braking force required to stop a vehicle’s movement depend on ?
- speed of vehicle at the time of applying brakes
- vehicle’s mass
What is stopping distance ?
- the shortest distance a vehicle can safely stop in
- stopping distance = thinking distance + braking distance
[] thinking distance is essentially the driver’s reaction time; how long it takes for them to actually realise the need to stop and then hit the brakes
[] braking distance is the distance travelled by the vehicle whilst the brakes are applied
Give the equation for final velocity squared using acceleration, initial velocity and distance
v^2 = u^2 + 2as
What equation should be used to calculate braking force ?
f = ma
Give some key factors affecting stopping distance
- tiredness
- alcohol
- certain drugs
- age
- high speed
- road conditions (ice/rain)
- worn brakes/tyres
- distractions to the driver
Give the equation for kinetic energy using mass and velocity
kinetic energy (J) = 1/2 x mass (kg) x velocity^2 (m/s)
Describe the energy transfers that take place when the brakes are applied to a car’s wheels and why they may have an adverse effect on the car in excess
- when brakes are applied, they press against the moving wheels, creating friction
- due to friction, kinetic energy from the wheels is transferred as thermal energy
[] as the car thus loses kinetic energy, it slows - if too much heating occurs, the breaks could overheat, wearing them down and making them less effective in future
Describe the relationship between a vehicle’s speed and both its stopping distance and the braking force needed to stop it
- higher speed = greater stopping distance
- higher speed = greater braking force needed
[] large braking force = possible overheating of brakes and possible loss of control of the vehicle
Calculate the braking force needed for a car of 1000kg that decreases in velocity from 30 m/s to 0 m/s in 10s
f = m x a
a = (u - v)/t
a = (30 - 0)/10
a = 3 m/s^2
f = 1000 x 3
f = 3000N
How much larger should the stopping distance be on wet roads and on icy roads ?
wet roads = x2
icy roads = x10
Is momentum a vector or scalar quantity ?
vector
Give the equation for momentum (with units)
momentum (kg m/s) = mass (kg) x velocity (m/s)
How much momentum does an object at rest have ?
NONE - its momentum is 0 kg m/s
Calculate the momentum of a car of 1000kg and with a velocity of 0 m/s
momentum (kg /s) = mass (kg) x velocity (m/s)
momentum = 1000 x 0
momentum = 0 kg m/s
Calculate the momentum of a car of 1000kg and with a velocity of 20 m/s
momentum (kg /s) = mass (kg) x velocity (m/s)
momentum = 1000 x 20
momentum = 20 000 kg m/s
What is the law of conservation of momentum ?
in a CLOSED SYSTEM, total momentum before an event is equal to the total momentum after an event
What are the two types of events in regards to momentum ?
collision and explosion
A van with mass 2000 kg is moving with a velocity of 30 m/s. It then collides with a stationary car of 800 kg, and both continue moving forwards together. Calculate the velocity of the car and the van TOGETHER
y = velocity here
mom. before = mom. after
((2000 x 30) + (800 x 0)) = (2000 + 800) x y
60 000 kg m/s = 2800 x y
60 000/2800 = y
y = 21.4 m/s (1 d.p)
A van with mass 1400 kg is moving with a velocity of 20 m/s. It then collides with a stationary car of 1000 kg, and both continue moving forwards together. Calculate the velocity of the car and the van TOGETHER
v = velocity
mom. before = mom. after
((1400 x 20) + (1000 x 0)) = (1400 + 1000) x v
28 000 = 2400 x v
28 000/2400 = v
v = 11.7 m/s (1 d.p)
A 600 kg cannon recoils at a speed of 0.5 m/s when a 12 kg cannon ball is fired from it. Calculate the velocity of the cannon ball
v = velocity
(mass of A x velocity of A) = -(mass of B x velocity of B)
600 x 0.5 = -(12 x v)
300 = -12 x v
300/-12 = v
v = -25 m/s
NOTE: DO NOT FORGET THE MINUS; IT REPRESENTS MOMENTUM IN THE OPPOSITE DIRECTION
Give the equation for the force acting on an object when there is a change in momentum
force (N) = (mass x (u - v))/time (s)
OR
force = change in momentum (kg m/s)/time (s)
A man with a mass of 100kg is a passenger in a car travelling at 30 m/s. The car stops in 1s. Calculate the force acting on the man
force = change in momentum/time
change in momentum = mass x (u-v)
change in mom. = 100 x (30 - 0) = 3000 kg m/s
force = 3000/1
force = 3000N
Why are rapid changes in momentum dangerous ?
leads to huge forces acting on the person
How can you reduce the forces acting on a person as a result of rapid change of momentum ?
make the IMPACT TIME LONGER
- this spreads out the change in momentum over a longer period of time
[] thus the impact forces are reduced
Give some key safety features and devices that reduce impact force on people
- safety helmets
- sidebars on car doors
- crumple zones on cars (rear and front)
- seatbelts
- cushioned surfaces at playgrounds
- air bags
How do helmets and cushioned floors in playgrounds reduce impact forces ?
- lengthen impact time
- cushioned surfaces give cushioning effects to an impact
[] safety helmets also have cushioning inside for the same effect
How do seatbelts and air bags reduce impact forces ?
spread force out over a larger area and so it is felt less by the person
How do side impact bars and crumple zones reduce impact forces ?
give way in an impact and thus lengthen impact time
Describe the forces acting on a skydiver from the moment they jump out of the plane to when they hit the ground
- when first jumps, the only force acting is their weight so the skydiver accelerates towards the ground
- as their velocity increases, air resistance increases
- at a point, the upwards force of air resistance is EQUAL AND OPPOSITE to the downwards force of weight, and the skydiver is at terminal velocity
- this terminal velocity is much too fast, and if they hit the ground at this speed, they would die
- when the skydiver opens their parachute, their surface area is greatly increased and so the upwards force of air resistance greatly increases also
- the air resistance is now greater than the force of weight, and so the skydiver decelerates
- as the skydiver loses velocity, air resistance also decreases until it is equal and opposite to the skydiver’s weight again
- now they are at a new, lower terminal velocity that it is safe to land at
- when they land, there is no force of air resistance and weight and normal contact force are the only forces acting on them
What is an elastic material ?
a material that returns to its original shape/length/size when the deforming forces acting on it are removed
What is elastic deformation ?
deformation of an elastic material when 2+ forces act on it
- if only one force acted on it, the forces would be unbalanced and the object would simply move instead of being deformed
Give the equation for the force needed to elastically deform an elastic object (with units)
force (N) = spring constant (N/m) x extension (m)
Calculate the force required to extend a spring by 0.04m if the spring constant is 200 N/m
f = ke
f = 200 x 0.04
f = 8N
Describe a method for an investigation into Hooke’s Law (the relationship between force and extension of a spring)
- gather equipment
[] clamp stand
[] two bosses
[] metre rule
[] spring
[] 5 1N weights - set up clamp stand and bosses so that metre rule hangs vertically and the top of the spring’s first spiral is in line with the 0cm mark on the rule
- measure and record the unstretched length of the spring
- hang a 1N weight on the spring and measure and record the new length
- repeat with the other 4 weights
- calculate the extension produced by each cumulative weight (eg. 1N, 2N, 3N, 4N, 5N) by subtracting the unstretched length of the spring from each measurement
- plot extension in metres against weight in Newtons
- should find that there is a linear relationship (graph is a straight line through 0)
What is Hooke’s Law ?
the extension of a spring is DIRECTLY PROPORTIONAL to the force applied to it, as long as the LIMIT OF PROPORTIONALITY is NOT exceeded
What is an elastic object’s limit of proportionality ?
the point at which the force applied is too large and the object is inelastically deformed
