Physics Question Help???(movement)??

Question

Please state which formula you are using as it will be a great help. Thanks in advance.

The road is clear and a car traveling at 100km/h wants to overtake a truck which is traveling at 80km/h without changing speed. The truck is a length of 15m and the car is 4 car lengths behind the truck and wants to pull in 4 car lengths in front of the truck (car length is 4.5m).

a) Calculate how long this would take and how far the overtaking car will have to travel in the other lane to overtake the truck.

b) Calculate also the absolute minimum distance of an oncoming car moving at the same speed could be from the overtaking car as it overtakes the truck.

Answer 1

a.

consider the truck to be standing still then the car will be approaching it at 20 km/h.

relative car speed = 20 km/hr = 2x10^4 m/hr

the car is 4(4.5) m behind the truck. d1 = 18

length of truck: d2 = 15

front of car must be 4(4.5) m in front of truck. d3 = 18

the distance the car must travel is the sum of these:

D = d1 + d2 + d3 = 18 + 15 + 18 = 51 m

time = D/speed = 51/2x10^4 hours = 2.55x10^(-3) hours

time = 2.55x10^(-3) hours x (3600 secs/hour)

time = 9.18 seconds

the car will travel 51 meters and take 9.18 seconds to pass the truck

b. I am not quite sure I understand this. I think it means at what distance from the car will an oncoming car be to just meet the front of the car as it finishes passing the truck. Since the oncoming car is travelling at the same speed as the car it will cover the same distance. So it must be twice this distance from the initial position of the passing car.

distance = 2(51) m = 102 meters

Answer 2

a) Car and truck are moving in the same direction.

Therefore, speed of car relative to truck = car's speed - truck's speed

= 100 - 80 = 20 km/h

Car is 4 car lengths behind truck. It needs to pull in 4 car lengths in front of truck.

Required distance relative to truck = 4 * car's length + truck's length + 4*car's length

= 4 * 4.5 + 15 + 4 * 4.5 = 18 + 15 + 18 = 51 m

Time taken t = relative distance/relative speed = 51/20 h = 2.55 h.

Total distance travelled by car in this time = car's speed * t

= 100 * 2.55 m = 255 m.

Ans: It will take 2.55 h and car will have to travel 255 m

b) Not clear. Is this complete question?