Answer:

Given:
- Particle is moving along a straight line x=a
- Constant velocity voj
- Particle is at (a,0) at t=0

To find:
- Vr at t

Solution:

Understanding the problem:
- As the particle is moving along a straight line, its motion is one-dimensional.
- So, we can represent the motion of the particle by its position x as a function of time t, i.e., x(t).
- As the velocity of the particle is constant, it means that it covers equal distances in equal intervals of time.
- So, the velocity of the particle can be represented as the rate of change of its position with respect to time, i.e., V = dx/dt.
- In this case, as the velocity is constant, it means that the rate of change of position is constant, i.e., dx/dt = voj.
- So, the position of the particle as a function of time can be represented as x(t) = x(0) + V*t = a + voj*t.

Finding Vr at t:
- As the particle is moving along a straight line, its velocity V is also a vector along that line.
- However, Vr is the radial velocity component of V, i.e., the component of V perpendicular to the line connecting the particle to the origin.
- In this case, as the particle is at (a,0), the line connecting the particle to the origin is along the x-axis.
- So, Vr is the y-component of the velocity vector V, i.e., Vr = dy/dt = 0.
- Hence, the radial velocity component Vr of the particle is zero at all times.

Conclusion:
- The radial velocity component Vr of the particle is zero at all times as it is moving along a straight line x=a with constant velocity voj and is at (a,0) at t=0.