P5- Forces Flashcards
Do vectors have a magnitude and direction
Yes
Do scalars have a magnitude and direction
No, just a magnitude
What are examples of vector quantities
+-Force
- Velocity
- Displacement
- Acceleration
- entum
What are examples of scalar quantities
- Speed
- Distance
- Mass
- Temperature
- Time
What is a contact force
When two objects have to be touching for a force to act
What is a non-contact force
When the objects do no need to be touching for the force to act
What is the difference between weight and mass
Mass is the amount of stuff in an object which will be the same anywhere in the universe but weight is the amount of force acting on the object due to gravity
What is weight measured in
newtons
Is mass a force
No
What is mass measured
Kg
Mass and weight are_ proportional
Directly
Weight =
Mass X Gravitational Field Strength
Work done (J)=
Force (N) X Distance (M)
Draw all the forces acting on an object -to-
Tip
Tail
What is an object called that can go back to it’s original shape after being stretched,bent or compressed
Elastic
Extension is _ proportional to force
Directly
Spring Constant (N/M)=
Force ÷ Extension
Elastic potential energy=
1/2 Spring constant X Extension^2
Pressure(Pa) =
Force normal to surface(N) ÷Area of that surface(m^2)
Or
Height of the column of liquid (the depth) in m X Gravitational field strength (N/Kg) X Density of the liquid (Kg/m^3)
Density is a measure of __ of a substance
Compactness
What has to be equal for an object to float
Upthrust and weight
Define atmosphere
A layer of air that surrounds the Earth
Is distance a scalar or vector
Scalar
Is displacement a scalar or vector
Vector
Is velocity a vector or scalar
Vector
Distance travelled (m)
Speed (m/s) X Time (s)
Define acceleration
How quickly you’re speeding up
Acceleration=
Change in velocity (m/s) ÷ Time (s)
What is deceleration
How quickly something slows down
What is the typical speed of someone walking
1.5m/s
What is the typical speed of a person running
3m/s
What is the typical speed of a cyclist
6 m/s
What is the typical speed of a car
25 m/s
What is the typical speed of a train
55 m/s
What is the typical speed of a plane
250 m/s
Velocity^2 (m/s) - initial velocity (m/s)
2 X Acceleration(m/s^2) X Distance (m)
True/ False
Drag increases as speed decreases
False speed increases
What is drag
The resistance you get in a fluid. Air resistance is a type of drag
What is the most important factor by reducing drag
Make the object streamline
What is terminal velocity
The constant speed that a freely falling object eventually reaches when the resistance of the medium through which it is falling prevents further acceleration
What 2 things does terminal velocity depend on
Shape and area
What does Newton’s first law state
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
What was Newton’s second law
Acceleration is proportional to the resultant force
Resultant force (N) =
Mass (kg) X Acceleration (m/s^2)
What is inertia
The tendency for motion to remain unchanged
What is newton’s third law
When two objects interact, the forces they exert on each other are equal and opposite
Stopping distance=
Thinking distance + breaking distance
What two things effect your thinking distance
- Your speed
- Your reaction time
What 4 things effect your breaking
- Your speed
- The weather or road surface
- The condition of your tyres
- How good your brakes are
Breaking realise on ___ between the brakes and wheels
Friction
What can test reaction
Ruler drop test
What is conservation of momentum
Is a closed system, the total momentum before an event is the same as after the event
Momentum=
Mass X Velocity
Change in momentum=
Force X change in time
Is momentum always concerved
Yes
Is momentum a scalar
No- a vector
Momentum is _ within a system unless an external _ acts on the system
Conserved
Force
Acceleration is the _ of change of velocity
Rate
Acceleration=
Change in velocity / time
Do all objects have momentum if they have mass and velocity
Yes
What is the average speed of somebody cycling
6m/s
What is the average speed of somebody walking
1.5m/s
What is the average speed of somebody running
3m/s
What is the rough equation for m/s to mph
m/s= mph ÷ 2
What does collision mean
Stick together
What is a scalar quantity?
● A quantity that only has a magnitude
● A quantity that isn’t direction
dependent
What is a vector quantity?
A quantity that has both a magnitude and
an associated direction
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 force?
A push or pull acting on an object due to
an interaction with another object.
What are the two categories that all
forces can be split into?
- Contact forces (objects touching)
2. Non-contact forces (objects separated)
Give three examples of contact forces.
- Friction
- Air resistance
- Tension
Give three examples of non-contact
forces.
- Gravitational forces
- Electrostatic forces
- Magnetic forces
Is force a vector or a scalar quantity?
● Vector
● It has both a magnitude and an
associated direction
Give three examples of vector quantities.
- Velocity
- Displacement
- Force
Give three examples of scalar quantities.
● Temperature ● Time ● Mass ● Speed ● Distance ● Energy
What is weight?
The force that acts on an object due to
gravity and the object’s mass.
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 the unit used for weight?
The Newton (N).
What is the unit used for gravitational
field strength?
N/kg
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 piece of equipment can be used to
measure an object’s weight?
A calibrated spring-balance or newtonmeter.
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 does it mean if a force is said to do
‘work’?
The force causes an object to be
displaced through a distance.
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 distance must be used when
calculating work done?
It must be the distance that is moved
along the line of action of the force
Under what circumstance is 1 joule of
work done?
When a force of 1 Newton causes a
displacement of 1 metre.
How many Newton-metres are equal to 1
joule of energy?
1 Nm = 1 J
How much work is done by the force acting on the
below object over a distance of 5m?
10 x 5 = 50 Nm
= 50 J
What occurs when work is done against
frictional forces?
● A rise in temperature of the object
occurs
● Kinetic energy is converted to heat
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
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
What is meant by an inelastic (plastic)
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
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 type of energy is stored in a spring
when it is stretched?
Elastic potential energy.
What can extension be replaced with in
the equation for spring force?
Compression.
What is an alternative name for the
turning effect of a force?
A moment.
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 distance measurement is used
when calculating a moment?
The perpendicular distance from the
pivot to the line of action of the force.
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 three parts make up a lever
system?
- Load
- Effort
- Pivot
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.
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.
What determines the moment of a gear
wheel?
The size of the wheel.
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 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?
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 a typical value for the speed of
sound.
330 m/s
What is a typical value for human
walking speed?
1.5 m/s
What is a typical value for human running speed?
3 m/s
What is a typical value for human cycling
speed?
6 m/s
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?
● 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?
● 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?
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?
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?
● 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.
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.
- Tiredness
- Drugs
- Alcohol
Give two factors which may affect braking distance.
- Adverse (wet/icy) road conditions
2. 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
State the equation used to calculate an object’s
momentum.
Momentum = Mass x Velocity
What is the unit used for momentum?
kg m/s
kilogram metres per seconds
In a closed system, what can be said about the
momentum before and after a collision?
The total momentum before is equal to
the total momentum afterwards
State an equation linking change in momentum,
force and time.
Force x Time = Change in Momentum
F Δt = mΔv
What quantity is equal to the force
experienced in a collision?
The rate of change of momentum.
If an object’s change of momentum is fixed, what is
the only way to reduce the force that the object
experiences?
Increase the length of time over which
the change of momentum occurs.
Explain how a seatbelt improves a
passenger’s safety during a collision.
● 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
