P2 Forces (dynamics)

Question 1

define distance and displacement

Answer 1

distance: scalar - how far an object has travelled (eg. 5km)
displacement: vector - how far an object has travelled in a particular direction (eg. 5km east)

Question 2

typical values of speed for human walking/running/cycling

Answer 2

walk: 1.5 m/s
run: 3 m/s
cycle: 6 m/s

Question 3

factors affecting a person’s speed (5)

Answer 3

age, health, mode of transport, terrain, length of journey

Question 4

describe a distance-time graph showing constant speed

Answer 4

straight line
* horizontal = stationary
* diagonal = away/towards the start

Question 5

describe a distance-time graph showing acceleration

Answer 5

curved line
steeper as speed increases

Question 6

state equation for average acceleration

Answer 6

acceleration = change in velocity / time
a = ∆v / t
(m/s²) (m/s) (s)
-> change in velocity = final - initial

Question 7

describe the speed+velocity of an object moving in a circular path

Answer 7

• moves at a constant speed
• there is a change in velocity due to the change in direction
  (as velocity is the speed of something in a given direction)

Question 8

describe a velocity-time graph showing uniform acceleration

Answer 8

straight, diagonal line

Question 9

describe a velocity-time graph showing constant velocity (steady speed)

Answer 9

straight, horizontal line

Question 10

describe a velocity-time graph showing increasing acceleration

Answer 10

curved line
moves upwards
gets increasingly steeper

Question 11

describe a velocity-time graph showing decreasing acceleration

Answer 11

curved line
moves upwards
gets increasingly shallower

Question 12

describe what the area under a velocity-time graph shows

Answer 12

distance travelled

Question 13

state the equation for uniform acceleration (used when time is unknown)

Answer 13

final velocity² - initial velocity² = 2 x acceleration x displacement
v² - u² = 2as
(m/s) (m/s) (m/s²) (m)

Question 14

describe Newton’s 1st law of motion

Answer 14

• an object will remain stationary / at constant velocity when the resultant force is 0N
• acceleration is 0 and velocity is constant

Question 15

describe Newton’s 2nd law of motion +equation

Answer 15

• the accleration of an object depends on its mass and the resultant force
• resultant force = [inertial] mass x acceleration
  F = m x a
  (N) (kg) (m/s²)
• resultant force is directly proportional to acceleration (when mass is constant)

Question 16

define inertia

Answer 16

an object’s tendency to remain stationary/uniform motion - to resist changes in its state of motion

Question 17

define inertial mass

Answer 17

• the mass calculated from m = F / a (Newton’s 2nd law)
• the ratio of force to acceleration
• how difficult it is to change the velocity of an object (by applying resultant force)

Question 18

method for investigating relationship between resultant force + acceleration for an object of constant mass (Newton’s 2nd law)

Answer 18

• set up apparatus: dynamics trolley connected to hanging masses by string+ over a bench pulley, metre ruler along bench
• measure a distance (78cm) that the trolley will accelerate over
• apply a force of 1N to the string (hang a mass of 100g)
• release trolley and time over the distance
• repeat twice and calculate a mean time
• repeat for forces of 2 - 5 N

Question 19

describe Newton’s 3rd law of motion

Answer 19

• if object A exerts a force on object B, B will exert the same amount of force on A to maintain equilibrium
• force exerted by B will be equal in size / opposite in direction / of the same type

Question 20

define reaction time

Answer 20

time taken between stimulus (seeing the hazard) and the reaction (muscle movement)

Question 21

range of human reaction time

Answer 21

0.2 - 0.9 secs

Question 22

define thinking distance

Answer 22

distance travelled by vehicle from stimulus (when a hazard is seen) to when brakes are applied
(reaction time)

Question 23

factors increasing thinking distance/ reaction time (5)

Answer 23

• alcohol
• drugs (prescribed or legal)
• tiredness
• distractions - phone, passengers, radio, eating
• increased speed of vehicle - thinking distance is directly proportional to speed

Question 24

define braking distance

Answer 24

distance travelled by vehicle from when brakes are applied to a stop

Question 25

factors increasing braking distance (4)

Answer 25

* worn-down tyres * worn-down/faulty breaks * road conditions: ice, wet, spillages * **increased speed** of vehicle

Question 26

equation for braking distance

Answer 26

**force x distance = 1/2 x mass x velocity²** [work done = kinetic energy] F x d = 1/2 x m v² (N) (m) - (kg) (m/s)

Question 27

what happens to braking distance when speed doubles

Answer 27

distance increases 4x (speed increases x2 so distance increases x2²) F x **d** = 1/2 x m x **v²**

Question 28

name the main factor affecting stopping distance

Answer 28

speed of the vehicle

Question 29

equation for calculating stopping distance

Answer 29

thinking distance + braking distance

Question 30

describe why a vehicle with a greater speed is more dangerous for drivers use: braking force, deceleration

Answer 30

* the greater the speed of a vehicle, the greater the braking force needed to stop it * the greater the braking force, the greater the deceleration of the vehicle * large decelerations may cause brakes to overheat / loss of control (skidding)

Question 31

describe how brakes heat up when a driver applies force to the brakes

Answer 31

* work done by friction between brakes+wheels reduces the wheel's kinetic energy * energy is transferred from wheel's kinetic energy store to the brake's thermal energy store * so brake's temperature increases

Question 32

method of calculating human reaction time RP

Answer 32

* partner holds a ruler above you * position your hand infront of you so the ruler will sit between your index finger + thumb - top of your finger should be level with 0cm on ruler * partner drops ruler unexpectedly, catch asap * measure the point at which you caught the ruler above your thumb * repeat three times and find a mean reaction time

Question 33

typical results for reaction time for ruler drop test RP

Answer 33

<7.5cm - excellent <16cm - good <20cm - average <28cm - fair +28cm - poor

Question 34

state equation for momentum

Answer 34

momentum = mass x velocity p = m x v (kg m/s) (kg) (m/s)

Question 35

describe conservation of momentum +equation

Answer 35

total momentum before an event (collision/explosion) is equal to total momentum after an event - in a closed system (m1 x v1) + (m2 x v2) = (m3 x v3) + (m4 x v4)