Help with this problem

[ramin86]

A golf ball is hit off a tee at the edge of a cliff. Its x and y coordinates as functions of time are given by the following expressions.

x = (15.0 m/s)t

y = (2.00 m/s)t - (4.90 m/s2)t2

(a) Write a vector expression for the ball's position as a function of time, using the unit vectors i and j. (Use t, i and j as necessary.)

[vsage]

You have all the information you need. v = a*i+b*j where i is the x direction and j is the y direction. a and b specify the magnitude of the i and j directions, respectively. I am not sure how to answer this question without just flat out telling you.

Say you have the cartesian point (3,4)

in vector form this is 3i+4j

[ramin86]

yeah lol, got that one and the next couple, but having trouble with the next few:
(a) Write a vector expression for the ball's position as a function of time, using the unit vectors i and j. (Use t, i and j as necessary.) Got this one
r = correct check mark m
(b) By taking derivatives, obtain an expression for the velocity vector v as a function of time. Got this one
v = correct check mark m/s
(c) By taking derivatives, obtain an expression for the acceleration vector a as a function of time. Got this one
a = correct check mark m/s2

Don't get these :
(d) Next use unit vector notation to write an expression for the position of the golf ball at t = 1.20 s.
r(1.20 s) = ( m ) i + ( m ) j
(e) Write an expression for the velocity at this time.
v(1.20 s) = ( m/s ) i + ( m/s ) j
(f) Write an expression for the acceleration at this time.
a(1.20 s) = ( m/s2 ) j

[ramin86]

lol nevermind, got them all

[vsage]

yeah lol, got that one and the next couple, but having trouble with the next few:
(a) Write a vector expression for the ball's position as a function of time, using the unit vectors i and j. (Use t, i and j as necessary.) Got this one
r = correct check mark m
(b) By taking derivatives, obtain an expression for the velocity vector v as a function of time. Got this one
v = correct check mark m/s
(c) By taking derivatives, obtain an expression for the acceleration vector a as a function of time. Got this one
a = correct check mark m/s2

Don't get these :
(d) Next use unit vector notation to write an expression for the position of the golf ball at t = 1.20 s.
r(1.20 s) = ( m ) i + ( m ) j
(e) Write an expression for the velocity at this time.
v(1.20 s) = ( m/s ) i + ( m/s ) j
(f) Write an expression for the acceleration at this time.
a(1.20 s) = ( m/s2 ) j

I think semantics are messing you up. All they're asking is using the formulae you derived, plug t = 1.20s into each of them: position for d, velocity for e and acceleration for f. Your previous equations should have told you what the i and j components will be at any time t.