Motion Definitions

Question 1

Define displacement

Answer 1

Displacement is the distance travelled in a particular direction from a reference point (vector quantity)

Question 2

Define speed

Answer 2

Speed is the rate of change of distance or the distance per unit time. ms^-1 (scalar quantity)

Question 3

Define average speed

Answer 3

Average speed is the total distance or displacement travelled divided by the total time taken for a journey.

Question 4

Define instantaneous speed

Answer 4

Instantaneous speed is the speed of an object at a particular point in time during a journey

Question 5

Define velocity

Answer 5

Velocity is the rate of change of displacement OR the speed in a given direction (vector quantity)

Question 6

Define acceleration

Answer 6

Acceleration is the rate of change of velocity or the change in velocity per unit time (vector quantity)

Question 7

What is the acceleration g of free fall?

Answer 7

The acceleration of free fall ‘g’ is the acceleration of an object due to the gravitational attraction between the Earth and the object, with air resistance and buoyancy being ignored.

Objects of all masses fall with the same acceleration under the influence of gravity.

Question 8

Define a projectile

Answer 8

A projectile is an object thrown at an angle to the horizontal and therefore moves in both the horizontal and vertical planes.

The horizontal/vertical components are independent to one another, but the time is interchangeable.

Question 9

Define reaction time

Answer 9

Reaction time is the time taken by the driver to respond to the hazard and apply the brakes

Question 10

Define thinking distance

Question 11

Define braking distance

Answer 11

Braking distance is the distance travelled by the vehicle between applying the brakes and coming to a rest

Question 12

Define thinking distance

Answer 12

The distance travelled by the vehicle between the driver seeing the hazard and applying the brakes. Or distance travelled during the reaction time of the driver.

Question 13

Define stopping distance

Answer 13

Stopping distance is the thinking distance + braking distance

Question 14

List 5 factors which influence thinking distance

Answer 14

Tiredness
High speed
Influence of substances (alcohol/drugs)
Distractions
Age of driver

Question 15

List 5 factors which influence the braking distance

Answer 15

High speed
Poor road conditions (icy or wet)
Poor brake conditions
Poor tyre conditions
Mass of car (people, luggage etc.)

Question 16

What is the equation to calculate thinking distance?

Answer 16

Thinking distance = Reaction time x Speed of car

Question 17

Is the relationship between braking distance and speed of the car linear? Explain why?

Answer 17

No, the relationship is not linear. When the car brakes, kinetic energy is transferred into thermal energy in the brakes and the work done by the frictional force must be equal to the transfer of energy to bring the car to a standstill.

Work done = Force x Displacement (W=Fs) and KE = 1/2(mv^2) so W = KE so Fs = 1/2(mv^2).

From this, we can see Displacement (s) is directly proportional to the square of the car’s initial speed (v)