Determining Acceleration of Object on x-Axis

[patelneel1994]

Homework Statement

An object moving on the x-axis with a constant acceleration increases its x coordinate by 80 m in a time of 5.0 s and has a velocity of +20 m/s at the end of this time. Determine the acceleration of the object during this motion.
x = 80m t = 5.0 second v = + 20m/s a = while in motion

Homework Equations

80 m = 100 m + 1/2 a 25 s2
-1.6 m/s2

The Attempt at a Solution

Supppos to be positive I don't know why I get negative acceleration?

 

[patelneel1994]

Can we apply this law?
Velocity is positive direction and if the velocity increases with time, then acceleration is said to be positive. In this case, if the velocity is reducing with time, then the acceleration said to be negative

 

[BvU]

Equation is not good. Where does the 100 come from?
This time, constant acceleration during 5 seconds gives an increase in velocity. There is an equation for that.
The other equation is the full expression for x(t). Two equations, initial velocity and acceleration unknown. Should be solvable.

 

[patelneel1994]

is it Xt = X0 + V0 . t + 1/2 . a .t^2?

 

[patelneel1994]

100 m came from 20 m/s . 5 s

 

[BvU]

#4 is correct.

In #5 you get meters allright, but that expression is for uniform (constant speed) motion. In this case the thing accelerates, so the average speed in these 5 sec was less.In #4 you don't know X(5) or X(0), but you know their difference, which is all you need.
You also don't know V(0) so you can't plug in a numerical value.

You still need an equation for V(t), where you plug in the knowledge that V(5 sec) = 20 m/s.