# Body trajectory and force direction

1. Mar 6, 2014

### foggy37

Hello Forum,

Let's consider a particle that is giving an initial speed in a certain direction. The particle is now moving in that initial direction at a constant speed.
If a later moment a force starts acting on the particle and the force direction is not along the original particle direction of motion, will that applied force eventually pull the particle in such a way that the particle travels along the force direction?

If I think of projectile motion with the object launched at a certain angle to the horizontal direction, the particle will trace an inverted parabolic path. The force of gravity is always pointing straight down and the particle direction (velocity vector direction) never ends up pointing exactly along the vertical direction....Would it if enough time was allowed to pass and the vertical distance was large enough?

Another situation: centripetal force. No matter what, if the force is perpendicular to the particle direction, the particle will change direction but will never end up pointing along the force direction....

what is the general concept?

thanks,
foggy37

2. Mar 6, 2014

### dauto

No, a vertical force will never slow a horizontal component of the motion. Why should it?

3. Mar 6, 2014

### foggy37

Thanks dauto.

As far as the projectile motion, the force of gravity is straight down vertical while the initial motion has both a vertical and a horizontal component. As you mention, the force will only modify the y-component of the velocity vector and not the x-component.

Surely the y-component v_y of the velocity vector grows and grows until it becomes so much larger than the horizontal component v_x as the projectile is moving downward. That means that the velocity vector (direction of motion) will never point exactly along the direction of the vertical gravity force but will get very close to it if enough time goes by, i..e the velocity vector will become almost vertical too.

In general, we can say that if applied force is in a constant direction and it affects only one component of motion, the motion will eventually move very close to the force direction.

If the force affects both components of motion (both v_x and v_y) the object will never end up pointing along the force direction...

thanks
foggy37