From the documentation of raw_input:
The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that.
So what happens is that you try to multiply a string with a float, something like y="3" * x - 0.5 * "3" *x**2, which is not defined.
The easiest way to circumvent this is to cast the input string to float first.
x = np.linspace(0., 9., 10)
a = float(raw_input('Acceleration ='))
v = float(raw_input('Velocity = '))
y = v * x - 0.5 * a * x**2
Mind that if you’re using Python 3, you’d need to use input instead of raw_input,
a = float(input('Acceleration ='))