Physics virgin is this how I do it?

[noodle21]

A truck on a straight road starts from rest accelerating at 2.0 m/s2 until it reaches a speed of 20 m/s. Then the truck travels for 20 s at 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?

My answers: (a) From rest to speed of 20 m/s it takes 10 s. Add 20 s. Add 5.0 s... equals 35 s.

(b) I know average velocity is displacement divided by time. But, how do I calculate the displacement?

 

[gnome]

Calculate the displacements from the 3 segments of the trip separately. I assume you've come across a formula to calculate the distance traveled during constant acceleration...

 

[M98Ranger]

First of all, congratulations on taking an interest in physics and wanting to learn more! To answer your question, displacement is the change in position of an object, so in this case, it would be the distance the truck travels from its starting point to its final position. To calculate the displacement, you can use the formula: displacement = initial velocity x time + 1/2 x acceleration x time^2. In this scenario, the initial velocity is 0 m/s, so the displacement would be 1/2 x 2.0 m/s^2 x (10 s)^2 = 100 m.

Now, to find the average velocity, you can use the formula: average velocity = total displacement / total time. In this case, the total displacement is 100 m (from the initial acceleration) + 20 m/s x 20 s (from the constant speed) = 500 m. And the total time is 35 s (10 s from acceleration + 20 s at constant speed + 5 s for braking). Therefore, the average velocity would be 500 m / 35 s = 14.29 m/s.

I hope this helps and keep up the good work in learning physics!