The Student Room Group

M1 Question

Hi guys.... I'm a bit stuck on this question... I've being trying to do this for quite a while now....The question is....

A car starts from rest at traffic lights and moves in a straight line. The car moves with constant acceleration until it is moving with speed 9m/s. It then moves for 5s with an acceleration of 2m/s² before moving at constant speed of 19 m/s.
Given that the car is still moving at 19m/s 12s afer leaving the traffic lights, calculate the distance travelled in 12s.

This is what I think the speed-time graph looks like....


What I'm having trouble with is the time "t" on the graph. It doesn't tell me how long it has travelled 9 m/s and 19 m/s....

Thanks...
Reply 1
super_baros


Can you post the FULL question... it looks like you have omitted some info
A car starts from rest at traffic lights and moves in a straight line. The car moves with constant acceleration until it is moving with speed 9m/s. It then moves for 5s with an acceleration of 2m/s² before moving at constant speed of 19 m/s.
Given that the car is still moving at 19m/s 12s afer leaving the traffic lights, calculate the distance travelled in 12s.


They must have mentioned the acceleraion for that bit is 3 m/s2...if not you cannot assume the time is 3 seconds.
Reply 3
Here is the fulll question, I omitted part a) because I did that already...

A car starts from rest at traffic lights and moves in a stragiht line. The car moves with constant acceleration 3m/s² until is it is moving with speed 9m/s. It then moves for 5s with an acceleration of 2m/s² before moving at a constant speed.
a) Calculate the constant speed reached by the car.
Given that the car is still moving at this constant speed 12s after leaving the traffic lights,
d)calculate the distance travelled in 12s.
The graph is wrong. The question does not say that the car was travelling at a constant speed after 3 seconds. It only decelerates to 2ms-2. So there is not T seconds and that straight line after 3 seconds doesn’t exist. Instead there is a less steep line showing acceleration of 2 ms-2 for 5 seconds (so the steep line ends at 3+5= 8s). The car travels for 4 more seconds at a constant speed …so the total journey adds up to 12s.

Just find the area under the graph….triangle+ trapezium+ rectangle.
The graph should look like this...
Reply 6
although speed is ms^-1
oops my bad...edited.
Reply 8
wow... thanks guys.... it worked out right.... I misunderstood the question.. I thought it actually travelled at 9ms^-1 for a short time...

Thanks again!....
Reply 9
:biggrin: Yay