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Physics > Unit 5 - Forces > Flashcards

Unit 5 - Forces Flashcards

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1
Q

What is a scalar quantity?

A

● A quantity that only has a magnitude
● A quantity that isn’t direction
dependent

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2
Q

What is a vector quantity?

A

A quantity that has both a magnitude and
an associated direction.

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3
Q

How can a vector quantity be drawn and
what does it show?

A

● As an arrow
● The length of the arrow represents the
magnitude
● The arrow points in the associated
direction

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4
Q

What is a force?

A

A push or pull acting on an object due to
an interaction with another object.

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5
Q

What are the two categories that all
forces can be split into?

A
  1. Contact forces (objects touching)
  2. Non-contact forces (objects separated)
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6
Q

Give three examples of contact forces.

A
  1. Friction
  2. Air resistance
  3. Tension
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7
Q

Give three examples of non-contact
forces

A
  1. Gravitational forces
  2. Electrostatic forces
  3. Magnetic forces
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8
Q

Is force a vector or a scalar quantity?

A

● Vector
● It has both a magnitude and an
associated direction

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9
Q

Give three examples of vector quantities.

A
  1. Velocity
  2. Displacement
  3. Force
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10
Q

Give three examples of scalar quantities.

A

● Temperature
● Time
● Mass
● Speed
● Distance
● Energy

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11
Q

What is weight?

A

The force that acts on an object due to
gravity and the object’s mass.

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12
Q

What quantities does weight depend on?

A

Weight = mass x gravitational field strength
● The object’s mass
● The gravitational field strength at the
given position in the field

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13
Q

What is the unit used for weight?

A

The Newton (N).

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14
Q

What is the unit used for gravitational
field strength?

A

N/kg

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15
Q

What is meant by an object’s centre of
mass?

A

The single point where an object’s
weight can be considered to act through.

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16
Q

What piece of equipment can be used to
measure an object’s weight?

A

A calibrated spring-balance or newton-meter.

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17
Q

What is the name given to the single
force that is equivalent to all the other
forces acting on a given object?

A

The resultant force.

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18
Q

What does it mean if a force is said to do
‘work’?

A

The force causes an object to be
displaced through a distance.

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19
Q

What is the equation used to calculate
work done? Give appropriate units

A

Work done = Force x Distance
Work done (Joules), Force (Newtons),
Distance (metres)

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20
Q

What distance must be used when
calculating work done?

A

It must be the distance that is moved
along the line of action of the force.

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21
Q

Under what circumstance is 1 joule of
work done?

A

When a force of 1 Newton causes a
displacement of 1 metre.

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22
Q

How many Newton-metres are equal to 1
joule of energy?

A

1 Nm = 1 J

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23
Q

What occurs when work is done against
frictional forces?

A

● A rise in temperature of the object
occurs
● Kinetic energy is converted to heat

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24
Q

Why does air resistance slow down a
projectile?

A

● The object does work against the air
resistance
● Kinetic energy is converted in to heat,
slowing down the object

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25
Q

Explain the relationship between the
force applied and the extension of an
elastic object

A

The extension is directly proportional to
the force applied, provided that the limit
of proportionality is not exceeded.

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26
Q

What is meant by an inelastic (plastic)
deformation?

A

● A deformation which results in the
object being permanently stretched
● The object doesn’t return to its original
shape when the force is removed

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27
Q

What type of energy is stored in a spring
when it is stretched?

A

Elastic potential energy.

28
Q

What can extension be replaced with in
the equation for spring force?

A

Compression.

29
Q

Does a distance quantity require a
specific direction? i.e. Is it a scalar or
vector quantity?

A

No specific direction is required so it is a
scalar quantity.

30
Q

If an object moves 3 metres to the left
and then 3 metres back to its initial
position, what is the object’s total
displacement?

A

● The object has zero displacement
● Displacement is a vector quantity so it also
involves direction
● The object starts and ends at the same point

31
Q

State a typical value for the speed of
sound.

A

330 m/s

32
Q

What is a typical value for human
walking speed?

A

1.5 m/s

33
Q

What is a typical value for human
running speed?

A

3 m/s

34
Q

What is a typical value for human cycling
speed?

A

6 m/s

35
Q

State the equation linking distance,
speed and time. Give appropriate units.

A

Distance = Speed x Time
Distance (m), Speed (m/s), Time (s)

36
Q

Why can an object travelling at a
constant speed in a circle not have a
constant velocity?

A

● Speed is a scalar quantity
● Velocity is a vector quantity which means it can
only be constant if the direction is constant
● In circular motion, the direction is continuously
changing

37
Q

How can speed be calculated from a
distance-time graph?

A

The speed is equal to the gradient of the
graph.

38
Q

What must be done to calculate speed at
a given time from a distance-time graph
for an accelerating object?

A

● Drawing a tangent to the curve at the
required time
● Calculating the gradient of the tangent

39
Q

State the equation for the average
acceleration of an object. Give
appropriate units

A

Acceleration = (Change in Velocity)/(Time
Taken)
Acceleration (m/s²), Velocity (m/s), Time (s)

40
Q

How can the distance travelled by an
object be calculated from a velocity-time
graph?

A

It is equal to the area under the graph.

41
Q

Give an approximate value for the
acceleration of an object in free fall
under gravity near the Earth’s surface

A

9.8 m/s²

42
Q

What can be said about the resultant
force acting on an object when it is falling
at terminal velocity?

A

● The resultant force is zero
● When at terminal velocity, the object is
moving at a constant speed and so
isn’t accelerating

43
Q

State Newton’s first law for a stationary
object.

A

If the resultant force on a stationary
object is zero, the object will remain at
rest.

44
Q

State Newton’s first law for a moving
object.

A

If the resultant force on a moving object
is zero, the object will remain at constant
velocity (same speed in same direction).

45
Q

What can be said about the braking
forces and driving forces when a car is
travelling at constant velocity?

A

The braking forces are equal to the
driving forces.

46
Q

If an object changes direction but
remains at a constant speed, is there a
resultant force?

A

Since there is a change in direction,
there is a change in velocity and so there
must be a resultant force.

47
Q

What is inertia?

A

The tendency of an object to continue in
its state of rest or uniform motion.

48
Q

State the defining equation for Newton’s
Second Law

A

Resultant force = Mass x Acceleration
F = ma

49
Q

State Newton’s Second Law in words

A

An object’s acceleration is directly
proportional to the resultant force acting
on it and inversely proportional to its
mass.

50
Q

What is inertial mass?

A

● A measure of how difficult it is to
change a given object’s velocity
● The ratio of force over acceleration

51
Q

What is the symbol used to represent an
approximate value?

A

≈

52
Q

State Newton’s Third Law.

A

Whenever two objects interact, the
forces that they exert on each other are
always equal and opposite.

53
Q

What is the stopping distance of a
vehicle equal to?

A

The sum of thinking distance and braking
distance.

54
Q

For a given braking distance, if the
vehicle’s speed is increased, what can
be said about its stopping distance?

A

The stopping distance is increased with
an increase in speed.

55
Q

Give a typical range of values for human
reaction time.

A

0.2 seconds - 0.9 seconds

56
Q

Give three factors which can affect a
driver’s reaction time.

A
  1. Tiredness
  2. Drugs
  3. Alcohol
57
Q

Give two factors which may affect
braking distance.

A
  1. Adverse (wet/icy) road conditions
  2. Poor tyre/brake conditions
58
Q

Describe the energy transfers that take
place when a car applies its brakes.

A

● Work is done by the friction force between
the brakes and wheel
● Kinetic energy of the wheel is converted to
heat and is dissipated to the surroundings
through the brake discs

59
Q

To stop a car in a given distance, if its
velocity is increased, what must happen
to the braking force applied?

A

The braking force must also be
increased.

60
Q

State two consequences of a vehicle
undergoing very large decelerations.

A
  1. Kinetic energy converted to heat is
    very high causing brakes to overheat
  2. Loss of control of the vehicle
61
Q

State the equation used to calculate an
object’s momentum.

A

Momentum = Mass x Velocity

62
Q

What is the unit used for momentum?

A

kg m/s
kilogram metres per seconds

63
Q

In a closed system, what can be said
about the momentum before and after a
collision?

A

The total momentum before is equal to
the total momentum afterwards

64
Q

State an equation linking change in
momentum, force and time.

A

Force x Time = Change in Momentum
F Δt = mΔv

65
Q

What quantity is equal to the force
experienced in a collision?

A

The rate of change of momentum.

66
Q

If an object’s change of momentum is
fixed, what is the only way to reduce the
force that the object experiences?

A

Increase the length of time over which
the change of momentum occurs.

67
Q

Explain how a seatbelt improves a
passenger’s safety during a collision.

A

● Passenger must decelerate from the vehicle’s velocity at impact
to zero, meaning they undergo a fixed change of momentum
● The force they experience is equal to the rate of change of
momentum
● Seatbelts increase the time over which the force is applied,
reducing the rate of change of momentum and therefore
reducing the force experienced

Physics (7 decks)

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