# 2 acceleration components

1. Aug 7, 2009

### firavia

if a body that is going in space in a straight line and suddenly 2 forces acts on it the first with the motion direction and the other is perpendicular to the motion direction , would the object start to go in circles ??? or it will change its direction of motion and will keep going sideways , please check my picture for better explanation , thanku ...

when an airplain banks should not make a turn and start go in cirlces , it should go sideways and change the direction of flight without changing the direction of its nose with the new flight trajectory , but thats not what reall happens why

p.s" : the case I am talking about do not include the uyse of rudder , only banking...

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2. Aug 7, 2009

### RoyalCat

The trajectory will be curved, since the object has an initial velocity solely in the x direction, but is then accelerated in the y direction as well.

At every point along the motion, the object has an acceleration parallel to its velocity, and one perpendicular to its velocity, so it'll spin, and accelerate as well. Note, however, that it will not go in perfect circles.

I'm not quite sure what you mean by the rest of your post, though, could you please clarify? I'm having a hard time following what you're talking about.

3. Aug 7, 2009

### Staff: Mentor

Several issues:
(1) Don't confuse velocity and trajectory with acceleration.
(2) The acceleration component perpendicular to its velocity will change its direction; the component parallel will change its speed.
(3) Whether the object rotates about its center of mass depends on whether there's a torque on it.

4. Aug 7, 2009

### firavia

so the 2nd option that I drew in my added picture is wrong , the object would be changing its direction continuesoly ??

if both force continue to act on this object 1 with the motion and the other perpendicualr to the motion , would the object be making turns continuesly ???

how can I calcualte or draw the change of velocity vector at each second whenever I have an acceleration vector ??

5. Aug 7, 2009

### firavia

so DOC Al , in this situation the object will change direction and will continue traveling at the same new direction without making a turn ?? , or a curve trajectory ??? ,

beeter saying , the object will continue as in option 2 in my picture added ??? please check the picture and answer me .

thank u .

6. Aug 7, 2009

### rcgldr

The path will be curved only if there is a component of force always pependicular to the current path. Regardless of the forwards force, in order to get a circle, the perpendicular force has to be equal to the current mass x speed 2 / radius. Otherwise, the path may be curved, but it won't be a circle.

With only aileron and elevator inputs, the plane will start moving sideways in the direction of the bank. Even without rudder inputs, the sideways motion puts a force on the vertical stabilizer in the tail and cause the plane to yaw in the direction of the turn. This is called weather vane effect. To prevent this, opposite rudder must be applied. If a plane has enough power and enough fuselage area, it can fly with it's wings oriented vertically, called knife edge flight, using the rudder as an elevator, and the fuselage as a wing.

At high speeds, such as cruise speed for most aircraft, the rudder doesn't have to be used as the turn is only slightly "uncoordinated". For gliders, flying at slower speeds, it's important to use the rudder (and a bit of outside aileron force since the inside wing flies slower and generates a bit less lift unless a bit of outside aileron input is used), to keep the turn "coordinated" (so the glider isn't sliding inwards or outwards).

To make an airplane roll, most of the input is aileron, with a touch of elevator (mostly down elevator when inverted) and rudder, to keep the fuselage going straight.

7. Aug 7, 2009

### firavia

This was very usefull jeff Reid , thank u .

but still I havent been answered on my question that concern the picture I added , which option in the picture added is correct ??

would the body change direction and continue its path according to the new direction , or the object will keep curving its trajectory ???

8. Aug 7, 2009

### rcgldr

Assuming that your picture is of a rocket in outer space (no air to affect it), then since both engines apply a force through the center of mass, the rocket will not rotate. It's still possible to create a curved path, but the rocket orientation will remain the same (it won't rotate). If you had 4 engines, one on each side, then you could make the rocket move in a circle without the rocked rotating. With just the two engines, there's no way to decelerate the rocket, so although you can make it curve, you wouldn't be able to make that path circular (or even part of a circle). For real rockets, small engines are used to control rotation about all 3 axis, pitch, yaw, roll, and small lateral movments (for docking), and in this case just a single main engine is needed.

9. Aug 7, 2009

### firavia

thank u jeff

10. Aug 7, 2009

### Staff: Mentor

One problem I have with your option 2 diagram is that it shows a straight line path. Since the object was already moving in the x-direction, adding a force in the y-direction will give it a curved path.

(Having a curved path is distinct from having the object rotate.)

11. Aug 7, 2009

### firavia

DOC Al , in order to have a curve path the perpendicular force that is acing on the body must bee seeking a center , "centripital force" , but in this case this force will change the body's path , the trajectory will stay linear but with a diferent direction , in order to change the path into curvilinear the force must changing angle in order to be kept tracking a center ,
the second option must not be a curve ??? am i not right ??? the addition of the perpendicular force should make the body go sideway .

Last edited: Aug 7, 2009
12. Aug 7, 2009

### rcgldr

Doc's point was in your diagram, the left image shows a rocket with only an horizontal component of speed, but the right images shows one with an angled path. Somewhere between these two paths a curved path occured.

13. Aug 7, 2009

### firavia

the first image the rocket was going straight , after 2 minutes lets say another perpendicular force acted on the rocket , now which 1 of the 2 picture describe the real path ?? the option1 or the option2 ...

even if a body was going in certain velocity at x direction and after a while a force Fy acted on that body that dosent mean that the body will travel in a curvelinear trajectory , but what must happen , is that the body will change its direction of motion and keep going linearly in another direction like in option 2 .

Doc Al is trying to say that whenever an Fy force acted on the body that was traveling in x direction the body will start to travel curvelinearly , and I am not convinced with that .. please help

14. Aug 7, 2009

### Staff: Mentor

Neither. The center of mass of the object will execute a curved path, but the orientation of the body will not change unless there's a torque acting on it.

Why do you think that? Throw a ball horizontally. What happens?

Don't just say that you're not convinced--give the reasons why you think the way you do.

15. Aug 7, 2009

### firavia

taking the example of a satellite orbiting earth, the satellite will try to go in a linear direction whenever the force of gravity of earth act on the satellite it will change its direction of motion , but while changing its direction of motion the gravity force of earth will face the satelite again "PERPENDICULARY " so the angle of the centripital force is changing with the cange of direction of the satellite thats what keep changing the satellite direction infinetly and it will revolve around the earth , but in my picture the perpendicular force when it acted
the force direction was never changed anyomore, only the body changed its direction due to 2nd law of newton > a body will change direction or velocity whenever it is exited by a force < . so the body changed its direction due to the new perpendicular force , now this new force created an new path for the body , this path will not change unless a new force act on the body , the path is curvilinear because curvilinearity occur due to infinit change of direction . the body will continue going linearly but not in the x direction.
thats why I am not convinced

16. Aug 7, 2009

### Staff: Mentor

Again I ask you to consider the case of a ball thrown horizontally. What happens? Does it travel in a straight line (at an angle) towards the ground? Or does it curve towards the ground?

As long as a force acts on the body, the velocity (magnitude and/or direction) will continue to change. The object moves in the direction of its velocity, not in the direction of the force. If its velocity keeps changing, so will its path. Only if the velocity has a constant direction will the path be a straight line.

17. Aug 7, 2009

### firavia

allright thanks DOC Al

18. Aug 7, 2009

### firavia

if the body was at rest and suddenly the 2 forces act on it would the body travel curvilenarily as well ???

19. Aug 7, 2009

### Cyrus

You really need to get a good book on dynamics and sit down and study it.

20. Aug 7, 2009

### Staff: Mentor

No. It will begin moving in the direction of the net force. It doesn't matter (as far as the motion of the center of mass goes) whether 2 or 20 forces act--they add up to a single net force, which determines the direction of the acceleration.