5.6 - forces and motion Flashcards
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
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.
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
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 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
- 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
