P8-P11 Flashcards
What is meant by the term ‘fluid’?
A liquid or a gas.
In any fluid, at what angle do the forces
due to pressure act on a given surface?
At right angles (normal to) the surface.
State the equation used to calculate
pressure. Give appropriate units.
Pressure = Force/Area
Pressure (Pascals), Force (Newtons),
Area (metre²)
What happens to the density of the
atmosphere with increasing altitude?
The atmosphere becomes less dense as
altitude increases.
Explain why atmospheric pressure
decreases with an increase in height.
● Pressure is created by collisions of air
molecules
● The quantity of molecules (and so weight)
decreases as the height increases
● This means atmospheric pressure decreases
with an increase of height
What is the Earth’s atmosphere?
A thin (relative to the magnitude of the
Earth) layer of gas surrounding the
Earth.
How does pressure in fluids increase
with depth?
As the depth increases, the mass of liquid
above that depth also increases. This means
that the force due to the mass increases.
Since the force has increased whilst the area
has remained constant, the pressure will
increase.
Why does pressure in fluids increase
with density?
As the density of a fluid increases, the number of
particles in a given volume increases.
Consequently the weight of the fluid is greater.
This means that the force from the fluid above a
certain point is larger. Since the force has
increased, the pressure also increases.
What equation shows the magnitude of
pressure in liquids at different depths?
Pressure due to a column of liquid(Pa) =
Column height(m) x density(kg/m³) x
gravitational field strength(N/kg)
P = hρg
Why are objects in a fluid subject to an
upthrust?
Why are objects in a fluid subject to an upthrust?
When an object is submerged in a fluid, it has a
higher pressure below it than it does above, as there is more weight above it at the bottom than there is at the top. This leads to an upwards force called upthrust. You float or sink depending on whether the
upthrust is more or less than your weight.
What is upthrust always equal to?
The weight of the fluid that the object
displaces.
What factors influence whether an object
will sink or float?
● Upthrust
● Weight
● Density of fluid
Explain why an object with a density
greater than that of water can never
float.
Upthrust is equal to the weight of the displaced
fluid. If the density of the object is high, there
would not be enough volume displaced to
produce an upthrust larger than the object’s
weight. This means that it will sink.
Does a distance quantity require a
specific direction? i.e. Is it a scalar or
vector quantity?
No specific direction is required so it is a
scalar quantity.
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?
● The object has zero displacement
● Displacement is a vector quantity so it also
involves direction
● The object starts and ends at the same point
State the equation linking distance,
speed and time. Give appropriate units.
Distance = Speed x Time
Distance (m), Speed (m/s), Time (s)
Why can an object travelling at a
constant speed in a circle not have a
constant velocity? (Higher)
● 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
How can speed be calculated from a
distance-time graph?
The speed is equal to the gradient of the
graph.
What must be done to calculate speed at
a given time from a distance-time graph
for an accelerating object? (Higher)
● Drawing a tangent to the curve at the
required time
● Calculating the gradient of the tangent
State the equation for the average
acceleration of an object. Give
appropriate units.
Acceleration = (Change in Velocity)/(Time
Taken)
Acceleration (m/s²), Velocity (m/s), Time (s)
How can the distance travelled by an
object be calculated from a velocity-time
graph? (Higher)
It is equal to the area under the graph.
Give an approximate value for the
acceleration of an object in free fall
under gravity near the Earth’s surface.
9.8 m/s²
What can be said about the resultant
force acting on an object when it is falling
at terminal velocity?
● The resultant force is zero
● When at terminal velocity, the object is
moving at a constant speed and so
isn’t accelerating
State Newton’s first law for a stationary
object.
If the resultant force on a stationary
object is zero, the object will remain at
rest.
State Newton’s first law for a moving
object.
If the resultant force on a moving object
is zero, the object will remain at constant
velocity (same speed in same direction).
What can be said about the braking
forces and driving forces when a car is
travelling at constant velocity?
The braking forces are equal to the
driving forces.
If an object changes direction but
remains at a constant speed, is there a
resultant force?
Since there is a change in direction,
there is a change in velocity and so there
must be a resultant force.
What is inertia? (Higher)
The tendency of an object to continue in
its state of rest or uniform motion.
State the defining equation for Newton’s
Second Law.
Resultant force = Mass x Acceleration
F = ma
State Newton’s Second Law in words.
An object’s acceleration is directly
proportional to the resultant force acting
on it and inversely proportional to its
mass.
What is inertial mass? (Higher)
● A measure of how difficult it is to
change a given object’s velocity
● The ratio of force over acceleration
State Newton’s Third Law.
Whenever two objects interact, the
forces that they exert on each other are
always equal and opposite.
What is the stopping distance of a
vehicle equal to?
The sum of thinking distance and braking
distance.
For a given braking distance, if the
vehicle’s speed is increased, what can
be said about its stopping distance?
The stopping distance is increased with
an increase in speed.
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Give a typical range of values for human
reaction time.
0.2 seconds - 0.9 seconds
Give three factors which can affect a
driver’s reaction time.
Give three factors which can affect a
driver’s reaction time.
- Tiredness
- Drugs
- Alcohol
Give two factors which may affect
braking distance.
- Adverse (wet/icy) road conditions
- Poor tyre/brake conditions
Describe the energy transfers that take
place when a car applies its brakes.
● 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
To stop a car in a given distance, if its
velocity is increased, what must happen
to the braking force applied?
The braking force must also be
increased.
State two consequences of a vehicle
undergoing very large decelerations.
- Kinetic energy converted to heat is
very high causing brakes to overheat - Loss of control of the vehicle
What is the name given to the single force that is
equivalent to all the other forces acting on a given
object?
The resultant force.
What piece of equipment can be used to measure an
object’s weight?
A calibrated spring-balance or newton-
meter.
What is meant by an object’s centre of mass?
The single point where an object’s
weight can be considered to act through.
What is the unit used for gravitational field strength?
N/kg
What is the unit used for weight?
The Newton (N).
What quantities does weight depend on?
Weight = mass x gravitational field strength
● The object’s mass
● The gravitational field strength at the
given position in the field
What is weight?
The force that acts on an object due to
gravity and the object’s mass.
Give three examples of scalar quantities.
Temperature
● Time
● Mass
● Speed
● Distance
● Energy
Give three examples of vector quantities.
- Velocity
- Displacement
- Force
Is force a vector or a scalar quantity?
● Vector
● It has both a magnitude and an
associated direction
Give three examples of non-contact forces.
- Gravitational forces
- Electrostatic forces
- Magnetic forces1.
Give three examples of contact forces.
- Friction
- Air resistance
- Tension
What are the two categories that all forces can be
split into?
- Contact forces (objects touching)
- Non-contact forces (objects separated)
What is a force?
A push or pull acting on an object due to
an interaction with another object.
How can a vector quantity be drawn and what does it
show?
● As an arrow
● The length of the arrow represents the
magnitude
● The arrow points in the associated
direction
What is a vector quantity?
A quantity that has both a magnitude and
an associated direction.
What is a scalar quantity?
● A quantity that only has a magnitude
● A quantity that isn’t direction
dependent
Why does air resistance slow down a projectile?
● The object does work against the air
resistance
● Kinetic energy is converted in to heat,
slowing down the object
What occurs when work is done against frictional
forces?
● A rise in temperature of the object
occurs
● Kinetic energy is converted to heat
How many Newton-metres are equal to 1 joule of
energy?
1 Nm = 1 J
Under what circumstance is 1 joule of work done?
When a force of 1 Newton causes a
displacement of 1 metre.
What distance must be used when calculating work
done?
It must be the distance that is moved
along the line of action of the force.
What is the equation used to calculate work done?
Give appropriate units.
Work done = Force x Distance
Work done (Joules), Force (Newtons),
Distance (metres)
What does it mean if a force is said to do ‘work’?
The force causes an object to be
displaced through a distance.
What can extension be replaced with in the equation
for spring force?
Compression
What type of energy is stored in a spring when it is
stretched?
Elastic potential energy.
State the equation relating force, spring constant and
extension. Give appropriate units.
Force = Spring Constant x Extension
Force (N), Spring Constant (N/m)
Extension (m)
What is meant by an inelastic deformation?
● A deformation which results in the
object being permanently stretched
● The object doesn’t return to its original
shape when the force is removed
Explain the relationship between the force applied
and the extension of an elastic object.
The extension is directly proportional to
the force applied, provided that the limit
of proportionality is not exceeded.
Explain how a seatbelt improves a passenger’s
safety during a collision. (Higher)
● 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
If an object’s change of momentum is fixed, what is
the only way to reduce the force that the object
experiences? (Higher)
Increase the length of time over which
the change of momentum occurs.
What quantity is equal to the force experienced in a
collision? (Higher)
The rate of change of momentum.
State an equation linking change in momentum,
force and time. (Higher)
Force x Time = Change in Momentum
F Δt = mΔv
In a closed system, what can be said about the
momentum before and after a collision? (Higher)
The total momentum before is equal to
the total momentum afterwards.
What is the unit used for momentum? (Higher)
kg m/s
kilogram metres per seconds
State the equation used to calculate an object’s
momentum. (Higher)
Momentum = Mass x Velocity
Describe the moment and speed of a larger gear
being driven by a smaller gear.
The larger wheel will rotate more slowly but
will also produce a larger moment.
What determines the moment of a gear wheel?
The size of the wheel.
Give an example of when a lever may be used to
multiply a force.
A wrench, which has a long handle so that the
force applied by the user is multiplied.
How can a lever be used as a force multiplier?
If the distance between the effort and pivot is
greater than the distance between the pivot
and load, the force applied on the load is
greater than the effort force. This is since the
moment on both the effort and load must be
the same.
What three parts make up a lever system?
- Load
- Effort
- Pivot
If an object is in equilibrium, what can be said about
the moments acting on the object?
The clockwise moments are equal to the
anticlockwise moments.
What distance measurement is used when
calculating a moment?
The perpendicular distance from the
pivot to the line of action of the force.
State the equation used to calculate the moment of a
force. Give appropriate units.
Moment of force = Force x Distance
Moment (Nm), Force (N), Distance (m)
What is an alternative name for the turning effect of
a force?
A moment.
