# PhysicsMechanics- General motion in a straight line.

#### Shah 72

##### Member
A particle starts at the origin and moves along the X- axis. The acceleration of the particle in the direction of the positive x axis is a= 6t-c for some constant c. The particle is initially stationary and it is stationary again when it is at the point with x coordinate = -4. Find the value of c.
I don't understand how to calculate.

#### topsquark

##### Well-known member
MHB Math Helper
What have you been able to do so far?

-Dan

#### Shah 72

##### Member
What have you been able to do so far?

-Dan
V= integration (6t-c) with limits 0 and -4. I got the ans c= -12. But it's wrong and the textbook ans is 6

#### topsquark

##### Well-known member
MHB Math Helper
I got c=12 not -12
$$\displaystyle a = \dfrac{dv}{dt} = 6t - c$$

so $$\displaystyle v = \int_0^t (6t - c) ~ dt = 3t^2 - ct$$

$$\displaystyle v = \dfrac{dx}{dt} = 3t^2 - ct$$

Try to finish it.

-Dan

#### Shah 72

##### Member
$$\displaystyle a = \dfrac{dv}{dt} = 6t - c$$

so $$\displaystyle v = \int_0^t (6t - c) ~ dt = 3t^2 - ct$$

$$\displaystyle v = \dfrac{dx}{dt} = 3t^2 - ct$$

Try to finish it.

-Dan
You mean I do s=t^3-ct^2/2

#### Shah 72

##### Member
$$\displaystyle a = \dfrac{dv}{dt} = 6t - c$$

so $$\displaystyle v = \int_0^t (6t - c) ~ dt = 3t^2 - ct$$

$$\displaystyle v = \dfrac{dx}{dt} = 3t^2 - ct$$

Try to finish it.

-Dan
You take both the equations = 0 as stationary .
Then solve the quadratic equation by using b^2-4ac=0. I still don't get the ans

#### topsquark

##### Well-known member
MHB Math Helper
You take both the equations = 0 as stationary .
Then solve the quadratic equation by using b^2-4ac=0. I still don't get the ans
Your formula for s (I've been calling it x) is correct. Now you know when the motion of the particle stops, v(t) = 0 and x(t) = -4. So
$$\displaystyle 0 = 3t^2 - ct$$
and
$$\displaystyle -4 = t^3 - \dfrac{1}{2} c t^2$$

My advice is to solve the top equation for ct, multiply it by t (to get $$\displaystyle ct^2$$) and then plug that into the bottom equation.

-Dan

#### Shah 72

##### Member
Your formula for s (I've been calling it x) is correct. Now you know when the motion of the particle stops, v(t) = 0 and x(t) = -4. So
$$\displaystyle 0 = 3t^2 - ct$$
and
$$\displaystyle -4 = t^3 - \dfrac{1}{2} c t^2$$

My advice is to solve the top equation for ct, multiply it by t (to get $$\displaystyle ct^2$$) and then plug that into the bottom equation.

-Dan
Thank you so much!!