03 Forces And Motion Flashcards
Distance time graphs: straight line
Straight line - represents constant speed
Distance time graphs: the slope of the straight line
The slope of the straight line represents the magnitude of the speed
Distance time graphs: a steep slope
A slope slope represents large speed
Object is covering more distance per time
Distance time graphs: a shallow slope
A shallow slope means the object is moving at a small speed
Distance time graphs: flat horizontal line
A flat horizontal line menas the object is stationary (not moving)
Distance time graphs: changing speed
Represented by a curve
- if the slope is increasing the speed is increasing (accelerating) object accelerated from starting position
- if the slope is decreasing the speed is decrease (decelerating)
Distance time graphs: calculating speed
Gradient of a line
Change in y / change in x
Average speed equation
Average speed = distance moved/time taken
Practical: investigate the motion of everyday objects like a tennis ball
Independent variable = distance -> use tape measure
Dependent variable = time -> use stopwatch
- measure a height using tape measure
- drop tennis bal, which is the distance moved by object
- use stop watch to measure how long it takes
- repeat and take avg
- use equation s=d/t
Errors - human reaction time 0.25 (use data logger), measurements taken at eye level, use light gate to measure time
Acceleration equation
Change in velocity/time taken
Unit for acceleration
M/s^2
Velocity time graphs: an increasing slope
An increasing slope(positive gradient) shows increasing velocity
Object is accelerating
Velocity time graphs: decreasing slope
A decreasing slope (negative gradient) represents decreasing velocity
Object is decelerating
Velocity time graphs: straight line
A straight line represents constant acceleration/velocity
Velocity time graphs: slope of a line
A slope of the line represents the magnitude of acceleration
Steep slope - large acceleration, object speed changes very quickly
Gentle slope - small acceleration, object speed changes very gradually
Velocity time graphs: calculating speed (acceleration)
Calculate gradient
Change in y / change in x
Velocity time graphs: finding the distance
D = v x t
For triangles 1/2 x b x h
(Area under graph)
What quantity is force
Vector - both direction and magnitude
Vector quantity
Has both direction and magnitude (size)
Scalar quantity
Has magnitude only
Describe the magnitude and direction of two arrows pointing in opposite directions
- same magnitude (size)
- heads show its going in opposite directions so the vectors are acting in the opposite direction
Final speed equation
(Final speed)^2 = (initial speed)^2 + (2 + acceleration + distance)
Friction is a force that
Opposes motion
If an object has a 50N force acting north and a 50N force acting south what is the resultant force
50N - 50N = 0N
Scalar quantities examples
Distance, speed, time, mass, temp, pressure , KE, GPE, work done, power, current, resistance
Vector quantities
Displacement, velocity, acceleration, force, weight, momentum
Normal reaction force
When an object rests on a solid it feels a reaction force at 90 degrees to surface
Gravitation force
Also known as weight
Drag and air resistance
Always acts in opposite direction to motion
Increases if speed increases
Particles of air collide with the object moving through it and slows it motion
Friction
Acts in the opposite direction to motion
Thrust
Reaction force
Occurs when mass is pushed out the back of something, causing it to move forward
E.g. rockets, letting go of a balloon, jet engine
Upthrust
Can only occur in fluids, reason things float
The more fluid the object displaces the greater the upthrust
Electrostatic force
Force between unlike charges
Tension
When a pull force is exerted on each end, tension acts across the length of the objects
Force equation
Mass x acceleration
Weight equation
Mass x gravitational field strength
Stopping distance
The total distance travelled during the time it takes to stop in an emergency
Stopping distance formula
Stopping distance = thinking distance + braking distance
Main factors affecting a vehicles stopping distance
- speed (brakes need to do more work to being vehicle to stop)
- mass (the more mass the more distance it will travel as it comes to a stop)
- road conditions (wet or icy roads makes brakes less effective)
- reaction times (increases thinking distance)
Thinking distance and its main factors
Thinking distance is the distance travelled in the time it takes the driver to react to an emergency and prepare a stop
Main factors: speed of car, reaction time of driver (human avg reaction time is 0.25)
Reaction time is increased by
- tiredness
- distractions (e.g. using a mobile phone)
- intoxication (e.g. consumption of alcohol or drugs)
Describe forces acting on falling object and explain why they reach terminal velocity
- initially thrust/weight is much higher than drag
- so the car/person accelerates
- as velocity increases drag increases
- as drag increases the resultant force (thrust minus drag) overall decreases (f=ma)
- so acceleration decreases
- when drag = thrust/weight the resultant force is 0
- so the car/person travels at constant velocity
- this is terminal velocity (final velocity the car can reach)
Velocity equation
Velocity = distance / time
Practical: investigate how extension varies with applied force for helical springs, metal wires and rubber bands (force and extension)
- measure the spring/band with no mass added with a ruler and record this initial length
- add 100 g mass to the hanger of the spring/band
- record the mass and extension of spring/band
- add another 100 g
- record new mass and extension
- repeat until all masses have been added
- remove masses and repeat again 3x
- use equation w = M x g
- plot graph with
Errors of force and extension practical
- wait a few seconds for the spring to fully extend
- take measurements of the ruler at eye level to avoid parallax error
- make sure spring doesn’t go past its limit of proportionality otherwise it stretches too far (no longer obeys Hookes law)
Hookes law
The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality
Hookes law: if the force doubles..
.. extension will double
Hookes law: if the force halves
.. extension also halves
Limit of proportionality
The point where the relationship between force and extension is no longer directly proportional to
Which part of a force-extension graph is associated with Hookes law
The initial linear region
Elastic behaviour
Ability of a material to recover its original shape after the forces causing the deformation have been removed
Deformation is a change in the original shape of an object
Elastic deformation
When the object does return to its original shape after deforming forces are removed
- not permanent
E.g. rubber bands, fabrics, steel springs
Inelastic deformation
The object does not return to its original shape after deforming forces are removed
- permanent
E.g. plastic, clay, glass
Directionally proportional
Linear and passes through (0,0)
What happens at limit of proportionality
Changes in shape are permanent and can’t return to (0,0) as we pass elastic limit
Linear to non-linear
What doesn’t obey Hookes law and why
Rubber bands -> non linear
What doesn’t obey Hookes law and why
Rubber bands -> non linear
Gravitational force
Attractive only
- affects objects with mass
Non contact
Electrostatic force
Attractive or repulsive
- affects objects with charge
Non contact
Magnetic
Attractive or repulsive
- affects objects with poles or magnetic materials
Non contact
