SP2: motion & forces Flashcards
scalar
a quantity with size/magnitude only
examples of scalar quantities
speed, mass, distance
vector
a quantity with both size and direction
examples of vector quantities
force, velocity, displacement
magnitude
the size of a physical quantity
displacement vs distance
distance: how far an object moves displacement: difference from object’s starting point to final point
what is speed?
the distance an object travels per unit of time
typical speed for walking
1.5 m/s
typical speed for running
3 m/s
typical speed for cycling
6 m/s
typical speed for cars
13-30 m/s
typical speeds for trains
50 m/s
typical speeds for aeroplanes
250 m/s
speed of sound in air
330 m/s
how to calculate average speed in a journey
total distance/total time
different ways to measure speed:
-ruler & stopwatch
-light gates (connected to timer & size of object measured with ruler)
-video analysis (distance moved from frame to frame observed on a ruler in video & time between frames)
what is the most accurate way to measure speed?
light gates
what is velocity?
speed in a given direction
what does a horizontal line on a distance-time graph represent?
stationary object
what does a sloping line show on a distance time graph?
the object is moving
what is acceleration?
the rate at which velocity changes
what is acceleration measured in?
m/s²
acceleration in freefall
10 m/s²
typical acceleration when running
2 m/s²
typical acceleration of a family car
1-7 m/s²
typical acceleration of a motorbike
1-10 m/s²
typical acceleration of a rollercoaster
30-60 m/s²
typical acceleration of a formula one car
49 m/s²
what does a sloping line mean on a velocity/time graph?
the object is accelerating
what does a horizontal line on a velocity-time graph represent?
constant velocity
how to find the distance travelled on a velocity time graph?
area under the graph
how to find out acceleration on a velocity time graph?
draw a tangent on a curved line & measure the gradient
motion equation
v² - u² =2ax
v = final velocity
u = initial velocity
a = acceleration
× = displacement
how can the motion equation be used?
can be rearranged to find initial velocity, final velocity, acceleration & displacement
what happens when an object moves in a circle at a constant speed?
it’s direction constantly changes, this change in direction causes a change in velocity, this change in velocity results in acceleration
how does acceleration happen in
circular movement?
if a resultant force acts on it (centripetal force) for the object moving in a circle, which acts towards the middle of the circle
gravitational attraction
provides the centripetal force that keeps satellites in orbit around planets and planets orbiting around the sun
newton’s first law
an object remains in the same state of motion unless a resultant force acts on it
resultant force
a single force that has the same effect as all the forces acting on the object
what is uniform speed?
motion at a constant speed
uniform motion example (car)
car travels at constant velocity when driving force from engine is balanced by resistant forces like friction & air resistance
what is non-uniform motion?
motion with a change in speed, direction, or both
non-uniform motion example
at the start of a run, a runner experiences less air resistance than their thrust so they accelerate
newton’s second law
force = mass x acceleration
(triangle = FMA)
how does acceleration change because of mass and resultant force?
-the acceleration of an object increases if the resultant force on it increases
-acceleration increases if the mass of the object decreases
intertial mass
how difficult it is to change the velocity of an object
inertial mass formula
force/acceleration
what sign do estimations need?
≈
practical: investigating force, mass & acceleration
- cut an interrupt card to a known length (such as 10 cm) and attach it to an air track glider
- set up the equipment as shown in the diagram. make sure that the air track is level, and that the card will pass through both gates before the masses hit the floor
- set the data logging software to calculate acceleration
- Use scales to measure the total mass of the glider, string and weight stack. record this value.
- attach the full weight stack (6 × 10g
masses) to the end of the string - make sure the glider is in position and switch on the air blower. the glider should accelerate. remove one weight and attach it to the glider using blu-tack. this will keep the total mass constant
- repeat steps 6-7 removing one weight from the stack each time. remember to attach each weight to the glider as it is removed from the weight stack
newton’s third law
for every action there is an equal and opposite reaction
forces acting on a cat sat on the ground
(gravitational forces)
the cat pulls the earth up the earth pulls the cat down
(these forces are equal in size and opposite in direction)
weight equation
mass × gravitational field strength
what is weight measured in?
N (newtons)
what is gravitational field strength measured in?
N/kg
what is the relationship between weight and mass?
-they are directly proportional
-the greater the mass of the object, the greater its weight is
what is gravity on earth?
10 N/kg
what is momentum?
-the product of mass and velocity
-a vector quantity
how to calculate momentum (p)?
mass × velocity
what is conservation of momentum?
total momentum before an event = total momentum after the event (as long as no outside forces are acting on that system)
what happens when a resultant force acts on a moving object?
momentum changes
the purpose of seatbelts
-stops tumbling
-strong enough to stop you before you hit steering wheel
-slightly flexible to increase time taken for change in momentum, reducing the chance of injury
-spreads the force over a wider area of the body, reducing the pressure on any one particular part
the purpose of an airbag
increases time for heads momentums to reach zero, reduces force
crumple zone
-area at the front of the car made to be crushed in the case of a crash and to take a lot of the impact
-increases time taken to change momentum of drivers & passengers in a crash & reduces the forces involved
what is stopping distance? (equation)
thinking distance + braking distance
what is thinking distance?
the time it takes for a driver to apply the brakes after realising they need to
what is braking time?
the time it takes for the brakes to stop the vehicle
what is the average reaction time?
0.2-0.9 seconds
what can reaction times be affected by?
tiredness, drugs, alcohol, distractions
what can braking distances be affected by?
-poor roads & weather conditions
-poor vehicle conditions
-more mass in the vehicle
how does a car brake?
- force applied to the brakes of a vehicle
- work done by the friction between the brakes and the wheel
- reduces the kinetic energy of the vehicle & slows it down
- causes the temperature of the brakes to increase
faster vehicle equals…
greater braking force needed to stop it & greater deceleration
what can large decelerations lead to?
brakes overheating and/or loss of control
work done
force x distance
KE
1/2 x mass x v²
are kinetic energy and work done the same?
yes
