Section 4: Forces & Energy Flashcards
What are the 8 types of energy stores?
- Kinetic
- Gravitational potential
- Elastic potential
- Electrostatic
- Thermal
- Chemical
- Magnetic
- Nuclear
What are the 4 types of energy transfer?
- Mechanical - a force doing work
- Electrical - work done by moving charges
- Heating
- Radiation - e.g. light or sound
What is a system?
A single object or a group of objects
What is conservation of energy?
Energy can be transferred usefully, stored or dissipated but not created or destroyed
What is a closed system?
When no energy or matter is transferred in or out of the system - there is no net change in total energy
What are the energy transfers for an arm throwing up a ball?
Chemical energy store of the arm —> kinetic energy store of ball and arm
BECAUSE WORK DONE MECHANICALLY BY FORCE EXERTED BY ARM
What are the energy transfers for a ball falling?
Gravitational potential energy store of ball —> kinetic energy store of the ball
BECAUSE WORK DONE MECHANICALLY BY GRAVITATIONAL FORCE
What are the energy transfers for a kettle boiling water?
Mains —> thermal energy store of kettles heating element —> thermal energy store of water
BECAUSE ENERGY WAS TRANSFERRED ELECTRICALLY FIRST THEN TRANSFERRED BY HEAT
What is the equation for efficiency?
Efficiency = useful energy transferred/total energy supplied
Why is no device 100% efficient?
In all systems, energy is dissipated (wasted) to a store that’s not useful
What does the width of an arrow show in a sankey diagram?
Energy supplied to that transfer (to the useful transfer and to the wasted transfer)
Steps to drawing a sankey diagram?
- Create a key
- First relate the TOTAL energy supplied to the key and draw that width
- Coming horizontally from the total energy line, draw an arrow the size of the USEFUL energy that’s related to the key
- Coming vertically from just after the total energy line and just before the useful energy arrow, draw an arrow the size of the WASTED energy that’s related to the key
What are 2 ways to increase the efficiency useful energy transfers?
- Applying lubricant
- Thermal insulation
What is the process of lubrication?
- The frictional force acts between moving gears, so energy is wasted and dissipated in an unwanted energy transfer
- Apply lubricant
- Frictional force is reduced so less energy is wasted
What does thermal insulation do?
Reduce unwanted energy transfers by heating
What are the 2 ways to decrease how quickly a building cools?
- Increase thickness of it’s walls
- Make the walls out of material with a lower thermal conductivity
As a materials thermal conductivity increases, what happens to the rate at which it transfers energy by conduction?
The rate increases
What is conduction?
When vibrating particles transfer energy to neighbouring particles
What is work done the same as?
Energy transferred
When is work done on an object?
When a force moves an object from one to another
What is work done measured in?
Joules (J)
If a force does work on a box and energy is transferred to the box’s kinetic energy store, what work does the box do?
The box does work against the frictional forces - causes temperature of the box to increase
What is power?
Rate of energy transfer (or rate of doing work)
What is power measured in?
Watts (W)
What is one watt (W) equal to?
One watt (W) = one joule of energy transferred per second (J/s)
What is a force?
A push or a pull on an object caused by it interacting with something
What is an interaction pair?
Two objects that when they interact, exert an equal but opposite force onto each other
What are 2 types of forces?
- Contact forces
- Non contact forces
What is a contact force?
A force created only when two objects touch
What is a non contact force?
A force created when objects don’t need to be touching
What are 3 examples of contact forces?
- Friction
- Air resistance & water resistance
- Normal contact force
What are 3 examples of non contact forces?
- Electrostatic force
- Gravitational force
- Magnetic force
What is a scalar quantity?
A quantity that only has a magnitude but no direction
What is a vector quantity?
A quantity that has a magnitude and a direction
Examples of scalar quantities?
Speed
Mass
Distance
Energy
Examples of vector quantities?
Force
Displacement
Momentum
Velocity
Acceleration
Weight
What are 3 examples of scalar-vector pairs?
- Speed and velocity
- Distance and displacement
- Mass and weight
What can forces and other vectors be represented as?
Arrows
What does the direction of an arrow show in a vector diagram?
The direction of the quantity
What does the length of an arrow show in a vector diagram?
The magnitude
What does a free body diagram show?
All forces acting on an isolated body
What do arrows show in a free body diagram?
Relative magnitudes and directions of the forces acting on the object/body
What is equilibrium?
When the forces acting on an object are balanced and the resultant force is zero
When forces are drawn tip to tail in a scale drawing, what does it create?
A closed loop e.g. a triangle
What is a resultant (net) force?
A single force that can replace all the forces acting on an object to give the same effect as all the original forces acting together
What are the 2 ways to calculate resultant (net) force?
- Using free body diagrams
- Drawing forces to scale and tip-to-tail
How can free body diagram be used to calculate resultant (net force)?
- Add all of the forces pointing in the same direction
- Then use the total forces pointing in one direction and subtract total forces pointing in the opposite direction to calculate resultant force
USE F1 - F2 = resultant force
How can forces being drawn to scale and tip-to-tail be used to calculate resultant (net force)?
- Draw all of the forces to scale and tip to tail (the end of one force connecting to the beginning of another force)
- Measure the length of the resultant force (the line connecting the beginning of the first force and the end of the other force/s) to find its magnitude (size)
- Measure the angle between the resultant force line and one of the other lines, to find its direction
What is a moment?
The turning effect of a force
If a force is applied at any angle but the normal (right angle), what happens to the distance and thus the moment?
The normal distance will be smaller so there’ll be a smaller moment
What is the principle of moments?
If an object is in equilibrium:
Sum of clockwise moments = sum of anticlockwise moments about a pivot
Why does a smaller mass need to be at a longer distance from the pivot than the bigger mass?
Smaller masses have smaller forces SO the distance from pivot needs to increase to balance the bigger mass
BECAUSE…
moment of a force = force x distance normal to the direction of the force
What is the unit for a moment?
Nm
What are gears used for?
To transmit the rotational effect of a force from one place to another
What are levers used for?
To make it easier to do work e.g. lift a load
What is the effort/applied force in a lever?
The force applied by a person
What is the load in a lever?
What the person is trying to move
What is the fulcrum in a lever?
The pivot
Do larger or smaller gears rotate faster?
Smaller gears
If one gear rotates clockwise, what direction will the gear next to it rotate in?
Anti clockwise
If a large gear with 20 teeth rotates twice a second, then how many times in a second will a smaller gear with 10 teeth rotate? Why?
4 times a second
BECAUSE…
The small gear is 2x smaller than the large gear SO it’s thus 2x faster than the large gear, and the large gear rotates twice a second so 2x2=4
If the ratio of teeth of gearA:gearB is 3:1, what the ratio of speed? Why?
1:3
BECAUSE…
Number teeth and speed are inversely proportional so their ratios are too
What happens if you increase the distance between pivot and and the applied force in a lever?
Less force will be required to get the same moment - making it easier to lift the load
