10.3 Forces and braking Q1. a. road surface affects the braking distance b. Tiredness affects the thinking distance c Poorly maintained brakes affects the braking distance

10.3 Forces and braking Q1. a. road surface affects the braking distance b. Tiredness affects the thinking distance c Poorly maintained brakes affects the braking distance Q2. a. At 30mph thinking distance = 30 feet At 50 mph thinking distance = 50 feet That is an increase of 20 feet (or 67% increase) b. At 30mph braking distance = 45 feet At 50 mph braking distance = 125 feet That is an increase of 80 feet (or 178% increase) c. At 30mph stopping distance = 75 feet At 50 mph stopping distance = 175 feet That is an increase of 100 feet (or 133% increase)

Q3. a. When the speed of a car is doubled i) the thinking distance is also doubled because thinking distance = speed x reaction time ii) the braking distance is more than doubled because braking distance (s) is proportional to the speed (u) squared

Q3. a. When the speed of a car is doubled i) the thinking distance is also doubled because thinking distance = speed x reaction time ii) the braking distance is more than doubled because braking distance (s) is proportional to the speed (u) squared b. To check the above relationship with data: if s ∝ u2 then s = k u2 and s/ u2 = k data set for 30 mph: 45/ (30) 2 = 0.05 data set for 50 mph: 125/ (50) 2 = 0.05 Relationship is confirmed as k= 0.05 for both sets of data

Q4. m= 1500kg u= 31m/s s= 75 m v=0m/s a) Deceleration a ? v2 = u2 + 2as 0 = 312 + 2a75 -961 = 150a -961/150 = a = 6.4 m/s2

Q4. m= 1500kg u= 31m/s s= 75 m v=0m/s a) Deceleration a ? v2 = u2 + 2as 0 = 312 + 2a75 -961 = 150a -961/150 = a = 6.4 m/s2 m= 1500kg u= 31m/s s= 75 m v=0m/s a = - 6.4 m/s2 b) braking force: F = ma F = 1500 x -6.4 F = - 9600 N or - 9.6 kN