Answers wrong in Dot Point 1.11.1 'Moving About' ? (1 Viewer)

[dkazzle]

Just wondering if the answer for 1.11.1 in 'Moving About' is wrong in the Dot Point Physics book. It's a simple question and I've used four different types of equations, and keep getting the answer 550m. However, the answer says it's 390 and I'm not so sure they're right... Could someone please confirm this for me?

For those of you who don't actually have this book, the question is just "How far did the car travel in 20 seconds." You're given a velocity vs. time graph of motion of a car, with a car accelerating from 0ms at 0seconds to 55ms at 20 seconds.
Using formulas r = 0.5t(u+v) or r = ut + 0.5at^2 etc. all gave me the same answer of 550..

 

[Rathaen]

If you look at the graph, you'll notice the velocity on time graph is not a straight line. This means that the car is undergoing non uniform linear motion, or in other words it doesn't have a constant acceleration, and this is the reason your answer is wrong. (because your kinematics equations assume a constant acceleration).

The distance travelled will be equal to the area under the velocity time curve.

 

[dkazzle]

Oh okay, that makes sense then. But how do I work out the area under if the line is curved?

 

[Rathaen]

Well normally you'd integrate the function of velocity with respect to time, but since this is HSC I assume you'll just sort of fudge it and visually examine how many squares and little triangle bits are underneath and then estimate the area. Hence the answer being given as "about 390".