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Physics > Topic 4: Forces and Energy > Flashcards

Topic 4: Forces and Energy Flashcards

1
Q

State the law of energy conservation.

A

Energy cannot be created or destroyed

it can only be transferred into different forms

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2
Q

Describe the energy changes involved
when a ball is thrown upwards and then
returns to its starting position. Ignore air resistance.

A

● Upwards: KE is converted to GPE
● Peak: Maximum GPE, zero KE
● Downwards: GPE is converted to KE

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3
Q

State any changes in the total energy of

a ball that is kicked, assuming that no external forces act.

A

The total energy of the system remains
constant due to the conservation of
energy.

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4
Q

Describe the energy changes that occur in a filament light-bulb.

A

● Electrical energy is transferred into
light and heat energy
● Light is a useful energy form, heat is
waste energy

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5
Q

Describe the energy transfers for a bungee jumper.

A

● When falling, the GPE is converted to KE of
jumper
● As the cord tightens, KE is converted and
stored as Elastic Potential Energy (EPE)
● At lowest point, the jumper’s initial GPE
equals the EPE stored in the cord

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6
Q

Explain why a bungee jumper slows

down once the cord begins to stretch.

A

● Kinetic energy decreases since it is
converted to elastic potential energy
● Since KE is proportional to (velocity)2,
as KE decreases, so does velocity.

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7
Q

Describe the energy changes in a power

station.

A

● Heat energy released in the reaction heats water to
make steam
● Steam moves the turbine, converting kinetic energy
● Kinetic energy turns the generator
● Electrical energy as the generator generates
electricity

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8
Q

In what ways can the energy of a system

be changed?

A

● Through work done by forces
● Through electrical input
● Through heating

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9
Q

How can energy be changed through work done by forces?

A

A force which moves an object (kinetic
energy) causes work to be done over a
distance.

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10
Q

What equation is used to measure the

work done on an object?

A 
Work done (J) = force (N) x distance (m)
W = Fd
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11
Q

Why do wasteful mechanical processes
cause a rise in the temperature of the
surroundings?

A

They cause energy to be transferred by

heating the surroundings.

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12
Q

What is the equation for kinetic energy?

Give SI units for all quantities involved.

A

E = 1⁄2 m v2

Energy = 1⁄2 x Mass x (Velocity)2

Energy (J), Mass(kg), Velocity(m/s)

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13
Q

What is the equation for gravitational
potential energy? Give SI units for all
quantities involved.

A

E = mgh

Energy = Mass x Gravitational Field Strength x Height

Energy (J), Mass(kg), Gravitational Field Strength (N/kg),

Height (m)

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14
Q

What is the definition of ‘Power’?

A

The rate at which energy is transferred

or rate at which work is done

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15
Q

State two equations for power. Give SI

units for all quantities involved.

A

power = energy transferred / time

power = work done / time

Energy (J), Work Done (J), Time (s)

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16
Q

What is the unit of power?

A

Watt (W)

17
Q

What is one Watt equivalent to?

A

1 Joule per second (J/s).

18
Q

What does a power rating of 10W mean

for an electrical device?

A

It converts 10 Joules of energy every

second.

19
Q

Two motors lift the same mass through
the same height. Motor A does this in
half the time of Motor B. Which
dissipates the most power?

A

The energy transferred is the same but

the time taken is less (P=E/t).

20
Q

What is efficiency?

A

The proportion of energy that is
converted usefully, rather than
wasted/dissipated.

21
Q

How do you calculate efficiency? What are the units of efficiency?

A

Efficiency = useful energy out / total energy in
Efficiency is a ratio so has no units, but it
can be expressed as a percentage.

22
Q

What is a force?

A

A push or pull acting on an object due to

an interaction with another object.

23
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)

24
Q

Give three examples of contact forces.

A
  1. Friction
  2. Air resistance
  3. Tension
25
Q

Give three examples of non-contact

forces.

A
  1. Gravitational forces
  2. Electrostatic forces
  3. Magnetic forces
26
Q

Is force a vector or a scalar quantity?

Why?

A

● Vector

● It has both magnitude and direction

27
Q

What is a scalar quantity?

A

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

28
Q

What is a vector quantity?

A

A quantity that has both magnitude and

direction.

29
Q

Give three examples of vector quantities.

A
  1. Velocity
  2. Displacement
  3. Force
30
Q

Give examples of scalar quantities.

A 
● Temperature
● Time
● Mass
● Speed
● Distance
● Energy
31
Q

How are vectors represented?

A

● Vector arrows
● Length indicates magnitude
● Arrow indicates direction

32
Q

How can you calculate the resultant
magnitude and direction of forces using
a scale drawing?

A

● Draw the component forces as scaled arrows, joined tip-to-tail
● Draw a line connecting the two open ends.
● Measure the length of this line and convert into the magnitude.
● Measure the angle between the resultant line and the vertical/horizontal to find the direction.

33
Q

What is an alternative name for the

turning effect of a force?

A

A moment.

34
Q

State the equation used to calculate the
moment of a force. Give appropriate
units.

A

Moment of force = Force x Distance

Moment (Nm), Force (N), Distance (m)

35
Q

What distance measurement is used

when calculating a moment?

A

The perpendicular distance from the

pivot to the line of action of the force.

36
Q

If an object is in equilibrium, what can be
said about the moments acting on the
object?

A

The clockwise moments are equal to the

anticlockwise moments.

37
Q

How do levers make use of moments?

A

They increase the perpendicular
distance of the force from the pivot, so
decrease the force needed to produce
the same moment.

38
Q

How are levers used to decrease the force

required to lift something?

A 
Levers use a pivot point and
the concept of moments to
reduce the force. The further
you are away from the pivot
point, the less force you
need to lift the object up.
39
Q

How does lubricating levers/gears

improve efficiency?

A

There is less resistance; less energy is
wasted/dissipated (as heat) overcoming
frictional forces.

Physics (7 decks)

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