Hey everyone! i'm new to the forums(adsbygoogle = window.adsbygoogle || []).push({});

I just started AP Physics a few days ago and i'm having some trouble on the following problems:

1) A truck on a straight road starts from rest accelerating at 2.0 m/s^2 until it reaches a speed of 20 m/s. Then the truck travels for 20 s at a constant speed until the brakes are applied, stopping the truck in a uniform manner in an additional 5.0 s. (A)How long is the truck in motion? (B)What is the average velocity of the truck for the motion described?

______________________________________________________________

For (A) I found the correct time to be 35 s.

For (B), I used the equation:

Average velocity = (Vo + V)/2

I set it up like:

V = [(0 m/s) + (20 m/s)] / 2

i came up with 10 m/s as my velocity which was wrong. I know this is kind of a multifaceted problem and i'm having trouble adjusting the equation accordingly. Can someone please point out my error and how i can solve this?

______________________________________________________________

2) A cesna aircraft has a lift-off speed of 120 km/h. (A) What minimum constant acceleration does this require if the aircraft is to be airborne after a takeoff run of 240 m? (B) How long does it take the aircraft to become airborne?

_____________

For this problem i used the equation:

V^2 = Vo^2 + 2ax

I set it up like:

(120 km/h)^2 = (0 km/h)^2 + 2(a)(.240 km)

i came up with 30,000 km/h^2 which doesn't sound right at all

Can someone please point out my error and how i am able to better approach this problem?

Help would be very much appreciated thanks!

edit: sorry for the vague title, i'll be sure to make it more specific next time.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Having some trouble on these physics questions!

**Physics Forums | Science Articles, Homework Help, Discussion**