Section 4: Forces & Energy

Question 1

What are the 8 types of energy stores?

Answer 1

1. Kinetic
2. Gravitational potential
3. Elastic potential
4. Electrostatic
5. Thermal
6. Chemical
7. Magnetic
8. Nuclear

Question 2

What are the 4 types of energy transfer?

Answer 2

1. Mechanical - a force doing work
2. Electrical - work done by moving charges
3. Heating
4. Radiation - e.g. light or sound

Question 3

What is a system?

Answer 3

A single object or a group of objects

Question 4

What is conservation of energy?

Answer 4

Energy can be transferred usefully, stored or dissipated but not created or destroyed

Question 5

What is a closed system?

Answer 5

When no energy or matter is transferred in or out of the system - there is no net change in total energy

Question 6

What are the energy transfers for an arm throwing up a ball?

Answer 6

Chemical energy store of the arm —> kinetic energy store of ball and arm

BECAUSE WORK DONE MECHANICALLY BY FORCE EXERTED BY ARM

Question 7

What are the energy transfers for a ball falling?

Answer 7

Gravitational potential energy store of ball —> kinetic energy store of the ball

BECAUSE WORK DONE MECHANICALLY BY GRAVITATIONAL FORCE

Question 8

What are the energy transfers for a kettle boiling water?

Answer 8

Mains —> thermal energy store of kettles heating element —> thermal energy store of water

BECAUSE ENERGY WAS TRANSFERRED ELECTRICALLY FIRST THEN TRANSFERRED BY HEAT

Question 9

What is the equation for efficiency?

Answer 9

Efficiency = useful energy transferred/total energy supplied

Question 10

Why is no device 100% efficient?

Answer 10

In all systems, energy is dissipated (wasted) to a store that’s not useful

Question 11

What does the width of an arrow show in a sankey diagram?

Answer 11

Energy supplied to that transfer (to the useful transfer and to the wasted transfer)

Question 12

Steps to drawing a sankey diagram?

Answer 12

1. Create a key
2. First relate the TOTAL energy supplied to the key and draw that width
3. Coming horizontally from the total energy line, draw an arrow the size of the USEFUL energy that’s related to the key
4. 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

Question 13

What are 2 ways to increase the efficiency useful energy transfers?

Answer 13

1. Applying lubricant
2. Thermal insulation

Question 14

What is the process of lubrication?

Answer 14

1. The frictional force acts between moving gears, so energy is wasted and dissipated in an unwanted energy transfer
2. Apply lubricant
3. Frictional force is reduced so less energy is wasted

Question 15

What does thermal insulation do?

Answer 15

Reduce unwanted energy transfers by heating

Question 16

What are the 2 ways to decrease how quickly a building cools?

Answer 16

1. Increase thickness of it’s walls
2. Make the walls out of material with a lower thermal conductivity

Question 17

As a materials thermal conductivity increases, what happens to the rate at which it transfers energy by conduction?

Answer 17

The rate increases

Question 18

What is conduction?

Answer 18

When vibrating particles transfer energy to neighbouring particles

Question 19

What is work done the same as?

Answer 19

Energy transferred

Question 20

When is work done on an object?

Answer 20

When a force moves an object from one to another

Question 21

What is work done measured in?

Answer 21

Joules (J)

Question 22

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?

Answer 22

The box does work against the frictional forces - causes temperature of the box to increase

Question 23

What is power?

Answer 23

Rate of energy transfer (or rate of doing work)

Question 24

What is power measured in?

Answer 24

Watts (W)

Question 25

What is one watt (W) equal to?

Answer 25

One watt (W) = one joule of energy transferred per second (J/s)

Question 26

What is a force?

Answer 26

A push or a pull on an object caused by it interacting with something

Question 27

What is an interaction pair?

Answer 27

Two objects that when they interact, exert an equal but opposite force onto each other

Question 28

What are 2 types of forces?

Answer 28

1. Contact forces 2. Non contact forces

Question 29

What is a contact force?

Answer 29

A force created only when two objects touch

Question 30

What is a non contact force?

Answer 30

A force created when objects don’t need to be touching

Question 31

What are 3 examples of contact forces?

Answer 31

1. Friction 2. Air resistance & water resistance 3. Normal contact force

Question 32

What are 3 examples of non contact forces?

Answer 32

1. Electrostatic force 2. Gravitational force 3. Magnetic force

Question 33

What is a scalar quantity?

Answer 33

A quantity that only has a magnitude but no direction

Question 34

What is a vector quantity?

Answer 34

A quantity that has a magnitude and a direction

Question 35

Examples of scalar quantities?

Answer 35

Speed Mass Distance Energy

Question 36

Examples of vector quantities?

Answer 36

Force Displacement Momentum Velocity Acceleration Weight

Question 37

What are 3 examples of scalar-vector pairs?

Answer 37

1. Speed and velocity 2. Distance and displacement 3. Mass and weight

Question 38

What can forces and other vectors be represented as?

Answer 38

Arrows

Question 39

What does the direction of an arrow show in a vector diagram?

Answer 39

The direction of the quantity

Question 40

What does the length of an arrow show in a vector diagram?

Answer 40

The magnitude

Question 41

What does a free body diagram show?

Answer 41

All forces acting on an isolated body

Question 42

What do arrows show in a free body diagram?

Answer 42

Relative magnitudes and directions of the forces acting on the object/body

Question 43

What is equilibrium?

Answer 43

When the forces acting on an object are balanced and the resultant force is zero

Question 44

When forces are drawn tip to tail in a scale drawing, what does it create?

Answer 44

A closed loop e.g. a triangle

Question 45

What is a resultant (net) force?

Answer 45

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

Question 46

What are the 2 ways to calculate resultant (net) force?

Answer 46

1. Using free body diagrams 2. Drawing forces to scale and tip-to-tail

Question 47

How can free body diagram be used to calculate resultant (net force)?

Answer 47

1. Add all of the forces pointing in the same direction 2. 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

Question 48

How can forces being drawn to scale and tip-to-tail be used to calculate resultant (net force)?

Answer 48

1. Draw all of the forces to scale and tip to tail (the end of one force connecting to the beginning of another force) 2. 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) 3. Measure the angle between the resultant force line and one of the other lines, to find its direction

Question 49

What is a moment?

Answer 49

The turning effect of a force

Question 50

If a force is applied at any angle but the normal (right angle), what happens to the distance and thus the moment?

Answer 50

The normal distance will be smaller so there’ll be a smaller moment

Question 51

What is the principle of moments?

Answer 51

If an object is in equilibrium: Sum of clockwise moments = sum of anticlockwise moments about a pivot

Question 52

Why does a smaller mass need to be at a longer distance from the pivot than the bigger mass?

Answer 52

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

Question 53

What is the unit for a moment?

Answer 53

Nm

Question 54

What are gears used for?

Answer 54

To transmit the rotational effect of a force from one place to another

Question 55

What are levers used for?

Answer 55

To make it easier to do work e.g. lift a load

Question 56

What is the effort/applied force in a lever?

Answer 56

The force applied by a person

Question 57

What is the load in a lever?

Answer 57

What the person is trying to move

Question 58

What is the fulcrum in a lever?

Answer 58

The pivot

Question 59

Do larger or smaller gears rotate faster?

Answer 59

Smaller gears

Question 60

If one gear rotates clockwise, what direction will the gear next to it rotate in?

Answer 60

Anti clockwise

Question 61

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?

Answer 61

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

Question 62

If the ratio of teeth of gearA:gearB is 3:1, what the ratio of speed? Why?

Answer 62

1:3 BECAUSE… Number teeth and speed are inversely proportional so their ratios are too

Question 63

What happens if you increase the distance between pivot and and the applied force in a lever?

Answer 63

Less force will be required to get the same moment - making it easier to lift the load