Work and kinetic energy

The magnitude of a certain one-dimensional force varies according to:

F = 6.68x^2 + 1.56

where x is the displacement from the origin in meters, and F is the force in Newtons. At t = 0, a 677 g mass is at the origin moving in the positive x-direction at speed 8.46 m/s when this force begins to act on it.
ASSUME: there are no other forces acting on the mass.

a) How much work is done by the force on the mass when it reaches x = 2.78 m?
b) What is the speed of the mass when it reaches 2.78 m?

W=FS

i put x=2.78 into the equation. w=(6.68*2.78^2)*2.78
and get the wrong answer.

The Attempt at a Solution

Shooting Star
Homework Helper
i put x=2.78 into the equation. w=(6.68*2.78^2)*2.78
and get the wrong answer.

If you put the value of x, you get the force, not the work. Think about what is work done in terms of force and distance.

not, i put the value of x. and get the force, then use the force * distance.w=(6.68*2.78^2+1.56)*2.78

Shooting Star
Homework Helper
When the force is not constant, then the work done is Integral(from x1 to x2)[Fdx].