5.6 - forces and motion

Question 1

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?

Answer 1

● The object has zero displacement
● Displacement is a vector quantity so it also
involves direction
● The object starts and ends at the same point

Question 2

State a typical value for the speed of
sound.

Answer 2

330 m/s

Question 3

What is a typical value for human
walking speed?

Answer 3

1.5 m/s

Question 4

What is a typical value for human
running speed?

Answer 4

3 m/s

Question 5

What is a typical value for human cycling speed?

Answer 5

6 m/s

Question 6

Why can an object travelling at a
constant speed in a circle not have a
constant velocity?

Answer 6

● 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

Question 7

How can speed be calculated from a
distance-time graph?

Answer 7

The speed is equal to the gradient of the
graph.

Question 8

How can the distance travelled by an
object be calculated from a velocity-time
graph?

Answer 8

It is equal to the area under the graph.

Question 9

Give an approximate value for the
acceleration of an object in free fall
under gravity near the Earth’s surface

Answer 9

9.8 m/s²

Question 10

What can be said about the resultant
force acting on an object when it is falling at terminal velocity?

Answer 10

● The resultant force is zero
● When at terminal velocity, the object is
moving at a constant speed and so
isn’t accelerating

Question 11

State Newton’s first law for a stationary
object

Answer 11

If the resultant force on a stationary
object is zero, the object will remain at
rest.

Question 12

State Newton’s first law for a moving object.

Answer 12

If the resultant force on a moving object
is zero, the object will remain at constant
velocity

Question 13

What can be said about the braking forces and
driving forces when a car is travelling at constant
velocity?

Answer 13

The braking forces are equal to the
driving forces.

Question 14

If an object changes direction but
remains at a constant speed, is there a
resultant force?

Answer 14

Since there is a change in direction,
there is a change in velocity and so there
must be a resultant force

Question 15

What is inertia?

Answer 15

The tendency of an object to continue in
its state of rest or uniform motion.

Question 16

State Newton’s Second Law in words.

Answer 16

An object’s acceleration is directly
proportional to the resultant force acting
on it and inversely proportional to its
mass.

Question 17

What is inertial mass?

Answer 17

● A measure of how difficult it is to
change a given object’s velocity
● The ratio of force over acceleration

Question 18

State Newton’s Third Law

Answer 18

Whenever two objects interact, the
forces that they exert on each other are
always equal and opposite.

Question 19

What is the stopping distance of a
vehicle equal to?

Answer 19

The sum of thinking distance and braking
distance.

Question 20

For a given braking distance, if the
vehicle’s speed is increased, what can
be said about its stopping distance?

Answer 20

The stopping distance is increased with
an increase in speed.

Question 21

Give a typical range of values for human
reaction time.

Answer 21

0.2 seconds - 0.9 seconds

Question 22

Give three factors which can affect a
driver’s reaction time.

Answer 22

1. Tiredness
2. Drugs
3. Alcohol

Question 23

Give two factors which may affect braking distance

Answer 23

1. Adverse (wet/icy) road conditions
2. Poor tyre/brake conditions

Question 24

Describe the energy transfers that take
place when a car applies its brakes

Answer 24

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

Question 25

To stop a car in a given distance, if its
velocity is increased, what must happen
to the braking force applied?

Answer 25

The braking force must also be
increased.

Question 26

State two consequences of a vehicle undergoing
very large decelerations.

Answer 26

1. Kinetic energy converted to heat is
   very high causing brakes to overheat
2. Loss of control of the vehicle