Unit 5 - Forces Flashcards
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)
- 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 newton-meter.
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
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
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.
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
What is the symbol used to represent an
approximate value?
≈
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
- 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
