Topic 5 - Forces Flashcards
(120 cards)
1
Q
What type of quantity is
force?
(scalar or vector)
A
vector
2
Q
Define
vector quantity.
A
a quantity which has a magnitude and a direction
3
Q
What are some examples of
vector quantities?
(5)
A
- force
- velocity
- displacement
- acceleration
- momentum
4
Q
Define
scalar quantity.
A
a quantity which only has magnitude and no direction
5
Q
What are some examples of
scalar quantities?
(5)
A
- speed
- distance
- mass
- temperature
- time
6
Q
Define
force.
A
a push or a pull on an object that is caused by it interacting with something
7
Q
Define
contact force.
A
when two objects have to be touching for a force to act
8
Q
What are some examples of
contact forces?
(3)
A
- friction
- air resistance
- tension in ropes
9
Q
Define
non-contact force.
A
when the objects do not need to be touching for the force to act
10
Q
What are some examples of
non-contact forces?
(3)
A
- magnetic force
- gravitational force
- electrostatic force
11
Q
Define
interaction pair.
A
a pair of forces that are equal and opposite and act on two interacting objects
12
Q
What are the
two important effects of gravity
around a planet?
A
- on the surface of a planet, it makes all things
- it gives everything a weight
13
Q
What is
mass?
A
the amount of ‘stuff’ in an object
for any given object this will have the same value anywhere in the universe
14
Q
Define
gravity.
A
the pull of the gravitational forces on the object
15
Q
Define
weight.
A
the force acting on an object due to gravity
close to earth, this force is caused by the gravitational field around the earth
16
Q
How does
gravitational field strength vary?
A
it varies with location
it’s stronger the closer you are to the mass causing the field
it’s stronger for larger masses
17
Q
What is
weight
dependent on?
A
the strength of the gravitational field at the location of the object
this means that the weight of an object changes with its location
18
Q
What is an object’s
centre of mass?
A
a point at which you assume the whole mass is concentrated
19
Q
How is
weight
measured?
A
a calibrated spring balance
(or a newtonmeter)
20
Q
How is
mass
measured?
A
a mass balance (kg)
mass is not a force
21
Q
What equation links
mass, weight and gravitational field strength?
A
weight = mass x gravitational field strength
W = mg
W: N
m: kg
g: N/kg
22
Q
What is the relationship between
weight and mass?
A
they are directly proportional
23
Q
What equation links
distance, force and work done?
A
work done = force x distance
W = Fd
W: joules, J
F: newtons, N
d: metres, m
24
Q
How do you convert between
joules and newton metres?
A
1J = 1Nm
25
# How would you use a
scale drawing to find the resultant force?
1. Choose a **sensible scale**.
2. Draw all of the **forces** acting on an object, to scale, '**tip to tail**'.
3. Draw a **straight line** from the start of the **first force** to the end of the **last force** (this is the **resultant force**).
4. Measure the **length** of the **resultant force** on the diagram to find the **magnitude**.
5. Measure the **angle** of the **resultant force** on the diagram to find the **direction** of the force.
26
# Define
equilibrium.
when **all** of the forces on an object **combine** to give a resultant force of zero
27
# When has an object been
elastically deformed?
when the object can **go back** to its **original shape** and **length** after the force has been removed
28
# When has an object been
inelastically deformed?
when the object **doesn't** return to its **original shape** and **length** after the force has been removed
29
# What equation links
force, spring constant and extension?
force = spring constant x extension
| F = ke
## Footnote
F: newtons, N
k: newton metres, N/m
e: metres, m
30
# Define
limit of proportionality.
the **maximum** force above which a **force against extension** graph **curves**, showing that extension is **no longer** proportional to force
31
# Describe an experiment that investigates the
link between force and extension.
| (5 steps)
1. Measure the **natural length** of the spring (when **no load** is applied) with a **millimetre ruler** clamped to the stand.
2. Add a mass to the spring and allow it to come to **rest**.
3. Record the mass and measure the new **length** of the spring. The **extension** is the change in length.
4. **Repeat** this process until you have enough measurements (at least 6).
5. **Plot** a **force-extension graph** of your results.
## Footnote
it will only start to **curve** if you **exceed** the **limit of proportionality**
32
# How would you interpret a
force-extension graph?
- when the line of best fit is a **straight line** it means that there is a **linear** relationship between force and extension (they're **directly proportional**), so the **gradient** of the straight line is equal to the **spring constant**
- when the line begins to **bend**, the relationship is now **non-linear** between force and extension
33
# What is the equation for the
work done in stretching a spring?
## Footnote
(as long as the spring is not stretched past its limit of proportionality)
EPE = (ke^2) /2
| (this is equal to the area under a force-extension graph)
## Footnote
EPE: joules, J
k: newton metres, N/m
e: metres, m
34
# Define
moment.
the **turning effect** of a force
35
# What equation gives the
size of the moment of the force?
moment = force x perpendicular distance from pivot to the line of action of the force
| M = Fd
## Footnote
M: newton metres, Nm
F: newtons, N
d: metres, m
36
# How do
levers make it easier for us to do work?
they **increase** the **distance** from the pivot at which the **force** is applied
| this means that **less force** is needed to get the **same moment**
37
# What is a
gear?
a circular disc with **teeth** around its edge
38
# What do
gears do?
| and how?
they **transmit** the **rotational effect** of a **force** from one place to another
## Footnote
this is because their teeth **interlock** so that **turning** one causes **another** to turn, in the **opposite** direction
39
# Define
fluid.
a substance that can '**flow**' because its particles are able to **move around**
| (liquids or gases)
40
# Define
pressure.
force per unit area
41
# What equation allows you to calculate the
pressure at the surface of a fluid?
pressure = force normal to surface / area of that surface
| p = F/A
## Footnote
p: pascals, Pa
F: newtons, N
A: metres squared, m^2
42
# How does
density affect pressure?
| (in a liquid)
the **more dense** a given liquid is, the **more particles** it has in a certain space, this means that there are more particles that are able to **collide** so the **pressure is higher**
43
# How does
density vary in a liquid?
it doesn't
for a given **liquid**, the **density** is **uniform** and it **doesn't vary** with **shape** or **size**
44
# How does
depth affect pressure
| in a liquid?
as the **depth** of the liquids increases, the number of particles **above** that point increases, the weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth
45
# What equation allows you to calculate the
pressure at a certain depth due to the column of liquid above?
pressure = density of the liquid x gravitational field strength xheight of column of liquid (the depth)
| p = ρgh
## Footnote
p: pascals, Pa
ρ: kg/m^3
g: N/kg
h: metres, m
46
# What is
upthrust?
the **resultant force** of the force exerted on the **bottom** of a submerged object and the force acting on the **top** of the object
| (the force on the bottom is larger)
## Footnote
this is **equal** to the **weight** of fluid that has been **displaced** by the object
47
# When does an object
float?
when the **upthrust** on an object is **equal to** the object's **weight**
| (the forces **balance**)
48
# When does an object
sink?
when the object's **weight** is **more than** the **upthrust**
49
# What does
an object's ability to float depend on?
its **density**
## Footnote
An object that is **less dense** than the fluid it is placed in **weighs less** than the **equivalent volume** of the fluid. This means that it **displaces** a **volume** of fluid that is **equal to its weight** before it is **completely submerged**.
At this point, the object's weight is **equal** to the upthrust, so the object **floats**.
An object that is **denser** than the fluid it is placed in is **unable** to displace enough fluid to equal its weight. This means that its weight is always **larger** than the upthrust, so it **sinks**.
50
# How do
submarines make use of upthrust to sink and rise?
Submarines hold large tanks.
To **sink**, these tanks are **filled with water** to increase the **weight** of the submarine (but not the volume) so that it is **more than** the upthrust.
To rise to the surface, the tanks are filled with **compressed air** to reduce the weight so that it's **less than** the upthrust.
51
# What is the relationship between
atmospheric pressure and altitude?
| and why?
atmospheric pressure decreases with altitude
## Footnote
As the altitude increases, the atmosphere gets **less dense**, so there are **fewer air molecules** that are able to collide with the surface.
There are also **fewer** air molecules **above** a surface as the height increases. This meanse that the **weight** of the air **above** it, which contributes to atmospheric pressure, **decreases** with altitude.
52
# What is the
atmosphere?
a **layer** of **air** that surrounds the Earth
| this is **thin compared** to the size of the Earth
53
# What is
atmospheric pressure?
the pressure that is created on a surface by **air molecules** colliding with the surface
54
# Define
distance.
| what type of quantity is this?
**how far** an object has moved
| this is a **scalar** quantity (so it doesn't involve **direction**)
55
# What is
displacement?
| and what type of quantity is this?
the distance and direction in a **straight line** from an object's **starting point** to its **finishing point**
| this is a **vector** quantity
## Footnote
the direction could be **relative to a point**, or a **bearing**
56
# What is a
bearing?
a **three-digit angle from north**
| e.g. 035°
57
# What is
speed?
| and what type of quantity is this?
**how fast** you're going with no regard to the direction
| this is a **scalar** quantity
58
# What is
velocity?
| and what type of quantity is this?
a speed in a given **direction**
| this is a **vector** quantity
59
# What equation links
distance, time and speed?
distance = speed x time
60
# What is the typical speed for
a person **walking**?
| (in m/s)
1.5 m/s
61
# What is the typical speed for
a person **running**?
| (in m/s)
3 m/s
62
# What is the typical speed for
a person **cycling**?
| (in m/s)
6 m/s
63
# What is the typical speed for
a **car**?
| (in m/s)
25 m/s
64
# What is the typical speed for
a **train**?
| (in m/s)
30 m/s
65
# What is the typical speed for
a **plane**?
| (in m/s)
250 m/s
66
# What does
the speed at which a person can travel depend on?
| (4 things)
- their **fitness**
- their **age**
- the **distance travelled**
- the **terrain**
67
# What are some examples of things that affect
wind speed?
| (3 things)
- **temperature**
- atmospheric **pressure**
- if there are any large **buildings** or structures nearby
68
# What is
uniform acceleration?
**speeding up** (or **slowing down**) at a **constant rate**
69
# Define
acceleration.
the **change in velocity** in a certain amount of **time**
70
# What equation allows you to calculate
the average acceleration of an object?
acceleration = change in velocity/time
| a = Δv / t
## Footnote
a: m/s^2
v: m/s
t: s
71
# What is
deceleration?
**negative** acceleration
| (if something **slows down**, the change in velocity is **negative**)
72
# What is
acceleration **due to gravity** near the Earth's surface?
roughly 9.8 m/s^2
| (the same value as gravitational field strength)
## Footnote
this is **uniform** for objects in free fall
73
# What equation links
final velocity, initial velocity, distance and acceleration?
| (only for **uniform** acceleration)
(final velocity)^2 - (inital velocity)^2 = 2 x acceleration x distance
| v^2 - u^2 = 2ad
74
# What does the
**gradient** of the line represent on a distance-time graph?
gradient = speed
| (the **steeper** the graph, the **faster** it's going)
75
# What does a
**flat** section represent on a distance-time graph?
the object is **stationary**
76
# What does a
**straight** line represent on a distance-time graph?
the object is travelling at a **steady speed**
77
# What does a
**curve** represent on a distance-time graph?
an **acceleration** or a **deceleration**
## Footnote
a **steepening** curve means it's **speeding up**
a **levelling off** curve means it's **slowing down**
78
# What does the
**gradient** of the line represent on a velocity-time graph?
gradient = acceleration
| the steeper the graph, the **greater** the acceleration or deceleration
## Footnote
**uphill** sections are **acceleration**
**downhill** sections are **deceleration**
79
# What does a
**flat section** represent on a velocity-time graph?
the object is travelling at a **steady speed**
80
# What does a
**curve** represent on a velocity-time graph?
**changing acceleration**
81
# How could you calculate
**distance travelled** using a velocity-time graph?
by calculating the **area** under any section of the graph
82
# In what direction does
friction work?
the **opposite** direction to the movement
83
# When do you get
friction?
| (2 cases)
between **two surfaces** or when an object passes **through a fluid**
84
# Define
drag.
the **resistance** you get in a **fluid**
| (**air resistance** is a type of **drag**)
85
# What is the most important factor in
reducing drag?
keeping the shape of the object **streamlined**
86
# What makes an object
streamlined?
the object is designed to allow fluid to **flow easily** across it
| (this reduces drag)
## Footnote
parachutes work in the **opposite** way
87
# What is the relationship between
friction and speed?
| (from fluids)
**frictional forces** from fluids always increase with speed
88
# How does a
falling object reach its terminal velocity?
| (3 steps)
1. When the falling object first **sets off**, the force of gravity is **much more** than the **frictional force** slowing it down, so it accelerates.
2. As the **speed increases** the friction **builds up**.
3. This gradually **reduces** the **acceleration** until eventually the **frictional force** is **equal** to the **accelerating force** (so the **resultant force is zero**). It will have reached its maximum speed or **terminal velocity** and will fall at a steady speed.
89
# What is
Newton's First Law?
If the resultant force on a **stationary** object is **zero**, the object will **remain stationary**. If the **resultant force** on a moving object is **zero**, it'll just carry on moving at the **same velocity**.
## Footnote
(a resultant force is needed to make something **start moving**, **speed up**, or **slow down**)
90
# What is the relationship between
resultant force and acceleration?
they are **directly proportional**
91
# What is the relationship between
acceleration and mass of the object?
| (for a **fixed resultant force**)
acceleration is **inversely proportional** to the **mass** of the object
## Footnote
so an object with a **larger** mass will accelerate **less** than one with a smaller mass (for a **fixed resultant force**)
92
# What equation describes
Newton's Second Law?
| (linking acceleration, resultant force and mass)
resultant force = mass x acceleration
| F = ma
## Footnote
F: N
m: kg
a: m/s^2
93
# Define
inertia.
the tendency to continue in the **same state of motion**
94
# What does
inertial mass
| measure?
how **difficult** it is to change the **velocity** of an object
95
# How can you calculate
inertial mass?
by rearranging **Newton's Second Law**
| m = F/a
## Footnote
(inertial mass is just the **ratio** of **force** over **acceleration**)
96
# What is
Newton's Third Law?
when **two objects interact**, the forces they exert on each other are **equal and opposite**
97
# Describe how you would carry out the
trolley experiment (which tests Newton's second law).
| (7 steps)
1. Set up two light gates and connect them to a computer.
2. Set up the **trolley** so it holds a **piece of card** in the middle that will **interrupt** the signal on the light gates.
3. Connect the trolley to a piece of string that goes over a pulley (at the end of the table) and is connected on the other side to a hook.
4. Mark a **starting line** on the table so that the trolley always travels the **same distance** to the light gate.
5. Place the trolley on the **starting line**, holding it so the string is **taut**, and **release** it.
6. Record the acceleration measured by the **light gate** as the trolley passes through it.
7. Repeat this twice more to get an **average acceleration**.
98
# What are the
two different variations of the trolley experiment?
1. Investigating the **effect of mass**.
**Add masses** to the **trolley** one at a time to increase the mass of the system. Record the average **acceleration** for each mass.
2. Investigating the **effect of force**.
Keep the **total mass** of the system the **same**, but **change** the mass on the hook. To do this, start with **all** the masses loaded onto the **trolley**, and **transfer** the masses to the hook one at a time, to increase the **accelerating force**. Record the **average acceleration** for each **force**.
99
# What is an
emergency stop?
| (for a car)
where **maximum force** is applied by the **brakes** in order to stop the car in the **shortest possible distance**
| the **longer** it takes to do this, the **higher the risk** of crash
100
# What is the general equation to find
stopping distance?
| (for a car)
stopping distance = thinking distance + braking distance
101
# What is the
thinking distance?
| (in an emergency stop)
how far the car travels during the driver's **reaction time**
## Footnote
(the time **between** the driver **seeing** a hazard and **applying the brakes**)
102
# What is the
braking distance?
| (in an emergency stop)
the distance taken to stop under the **braking force**
103
# What is a
typical **car** braking distance at
30mph?
60mph?
70mph?
30 - 14m
60 - 55m
70 - 75m
104
# What is
thinking distance
| affected by? (2 things)
1. Your **SPEED** - the **faster** you're going the **further** you'll travel during the **time** you take to **react**.
2. Your **REACTION TIME** - the **longer** your **reaction time**, the **longer** your **thinking distance**.
105
# What is
braking distance
| affected by? (4 things)
1. Your **SPEED** - for a **given** braking force, the **faster** a vehicle travels, the **longer** it takes to stop.
2. The **WEATHER** or **ROAD SURFACE** - if it is **wet** or **icy**, or there are **leaves** or **oil** on the road, there is **less grip** (and so less **friction**) between a vehicle's tyres and the road, which can cause tyres to **skid**.
3. The **CONDITION** of your **TYRES** - if the tyres of a vehicle are **bald** (they don't have **any tread left**) then they cannot **get rid of water** in wet conditions. This leads them to **skidding** on top of the water.
4. How good your **BRAKES** are - if brakes are **worn** or **faulty**, they won't be able to apply as much **force** as well-maintained brakes, which could be dangerous when you need to brake hard.
106
# What happens when the
brake pedal is pushed?
| (in a car)
The brake pads are **pressed** onto the wheels.
## Footnote
This contact causes **friction**, which **causes work to be done**. The work done between the brakes and the wheels transfers **energy** from the **kinetic energy stores** of the **wheels** to the **thermal energy stores** of the **brakes**. The brakes **increase** in **temperature**.
107
# Why are
very large decelerations (due to a very large braking force) dangerous?
they may cause brakes to **overheat** (so they don't work as well)
or could cause the vehicle to **skid**
108
# What is a
typical human reaction time?
between 0.2 and 0.9s
109
# What affects
reaction time?
| (4 examples)
- tiredness
- drugs
- alcohol
- distractions
110
# Describe the
ruler drop test.
| (for reaction time) (steps)
1. Sit with your arm resting on the edge of a table.
2. Get someone else to hold a ruler so it **hangs between** your thumb and forefinger, lined up with **zero**.
3. Without giving any warning, the person holding the ruler should **drop it**. Close your thumn and finger to try to **catch the ruler as quickly as possible**.
4. The measurement on the ruler at the point where it is caught is **how far** the ruler dropped in the time it takes you to react.
5. You can calculate **how long** the ruler falls for (the **reaction** time) because **acceleration due to gravity is constant**.
| (do a lot of **repeats** and calculate an **average**)
## Footnote
To calculate this you would use these two equations:
v^2 - u^2 = 2ad
a = Δv/t
111
# What are the
typical stopping distances at
30mph?
50mph?
70mph?
30mph - 21m (9 + 14)
50mph - 53m (15 + 38)
70mph - 96m (21 + 75)
112
# What is the relationship between
braking distance and speed?
as speed doubles, the braking distance increase **4-fold**
## Footnote
The **work done** to stop the car is **equal** to the energy in the car's **kinetic energy store** (1/2 mv^2). So, as speed doubles, the kinetic energy increases **4-fold** , and so **work done** to stop the car also increases 4-fold. Since W = Fd and the **braking force** is **constant**, the **braking distance increases 4-fold**.
113
# What type of quantity is
momentum?
a vector
114
# What is the equation for
momentum?
momentum = mass x velocity
| p = mv
## Footnote
p: kgm/s
m: kg
v: m/s
115
# What is the
conservation of momentum?
in a **closed system**, the total momentum **before** an event, is the same as **after** the event
116
# What equation links
time, mass, force and velocity?
| (and sort of momentum)
force = (mass x change in velocity) / change in time
| F = mΔv / t
## Footnote
momentum = mass x velocity
p = mΔv
so this can be rewritten as F = p / t
117
# Why are
cars designed to slow people down over a longer time then they have a crash?
The longer it takes for a change in **momentum**, the **smaller** the **rate of change of momentum**, and so the smaller the **force**. Smaller forces mean the **injuries** are likely to be **less severe**.
118
# What are some safety features of
cars?
| and how do they work? (3)
- **crumple zones** - crumple on impact, increase the time taken for the car to stop
- **seat belts** - stretch slightly, increasing the time taken for the wearer to stop
- **air bags** - inflate before you hit the dashboard, slows you down more gradually
119
# How does a
**bike helmet** reduce the severity of injury after a crash?
it contains a **crushable layer** of foam which helps to lengthen the time taken for your **head** to stop in a crash
| this reduces the impact on your brain
120
# How do
**crash mats** reduce the severity of injury after a crash?
they increase the time taken for you to stop if you **fall** on them
| this is because they are made from **soft**, **compressible** materials
