Thread: Where am I wrong?

Hey guys I'm new here but I need help with this exercise to which I was said I was wrong but not fully understood why.

It's from a test and it is in Portuguese, I'll try my best to use the most "equal" words so that everything is understand by you like I did.

Ex:

1-In Figure 3, there is shown a side view section of a bridge, which is believed to form an arc of a circle in a vertical plane. The positions P and Q are situated in the same horizontal line (same height).

Over this bridge, a car moves at constant velocity module. (meaning the velocity values changes except the intensity, so the vector's direction and way are not the same during the movent, but if we were in the car it would always be at the same Km/h or miles/h)


Consider that the car can be represented by its center of mass.


In which of the following figures is correctly represented the resultant of the forces acting on the car.
--


And then there is 4 multiple answers to choose from, I tried uploading the scan of the exercise but somehow it did not work, so I'll try my best explaining each of them:
Note: The same car is represented in 2 diferent positions. First being at the begging of the bridge leaned backwards and the second at the middle of the bridge parallel to our X axis.

A- Both positions have the arrow of the resultant of the forces vector pointing down parallel to the Y axis

B- Both positions have the arrow of the resultant of the forces vector pointing to the center of the bridge (so if it were a complete circle it would be pointing towards the center of it)

C- Both positions have the arrow of the resultant of the forces vector pointing tangential to the line of the bridge.

D- Both resultant of the forces are equal to 0 vector

---


The correct one, that the teacher said, is B. And I think it is is in fact D.

I know there is a centripetal force, but my point is, if there were any resultant in forces, how would the car maintain the same velocity intensity?
Isn't the centripetal force being compensated by our car motor? So we can maintain the same speed? Aren't we accelerating the car so it keeps the same speed (velocity intensity) contradicting all the other forces?

I tried to understand the teachers explanation and explain my point, but I think she didn't understand my point of view or if I was wrong where, she just kept insisting to not confuse that the velocity is not the same as the intesity of it.

If anyone can explain me this and prove me where I am thinking wrong I would be grateful.

Thank you

Originally Posted by NDest

Hey guys I'm new here but I need help with this exercise to which I was said I was wrong but not fully understood why.

It's from a test and it is in Portuguese, I'll try my best to use the most "equal" words so that everything is understand by you like I did.

Ex:

1-In Figure 3, there is shown a side view section of a bridge, which is believed to form an arc of a circle in a vertical plane. The positions P and Q are situated in the same horizontal line (same height).

Over this bridge, a car moves at constant velocity module. (meaning the velocity values changes except the intensity, so the vector's direction and way are not the same during the movent, but if we were in the car it would always be at the same Km/h or miles/h)


Consider that the car can be represented by its center of mass.


In which of the following figures is correctly represented the resultant of the forces acting on the car.
--


And then there is 4 multiple answers to choose from, I tried uploading the scan of the exercise but somehow it did not work, so I'll try my best explaining each of them:
Note: The same car is represented in 2 diferent positions. First being at the begging of the bridge leaned backwards and the second at the middle of the bridge parallel to our X axis.

A- Both positions have the arrow of the resultant of the forces vector pointing down parallel to the Y axis

B- Both positions have the arrow of the resultant of the forces vector pointing to the center of the bridge (so if it were a complete circle it would be pointing towards the center of it)

C- Both positions have the arrow of the resultant of the forces vector pointing tangential to the line of the bridge.

D- Both resultant of the forces are equal to 0 vector

---


The correct one, that the teacher said, is B. And I think it is is in fact D.

I know there is a centripetal force, but my point is, if there were any resultant in forces, how would the car maintain the same velocity intensity?
Isn't the centripetal force being compensated by our car motor? So we can maintain the same speed? Aren't we accelerating the car so it keeps the same speed (velocity intensity) contradicting all the other forces?

I tried to understand the teachers explanation and explain my point, but I think she didn't understand my point of view or if I was wrong where, she just kept insisting to not confuse that the velocity is not the same as the intesity of it.

If anyone can explain me this and prove me where I am thinking wrong I would be grateful.

Thank you

If I understand the problem correctly, the car is traveling on a level, circular track at a constant speed. The weight is balanced by the reaction force from the track, such that there is no vertical acceleration. Therefore the net vertical force is zero. The car is accelerating toward the center of the circle, so there is a net centripetal force caused by the force of the tires against the track. Since the force is at right angle to the direction of motion, the car does not slow down or speed up, but only changes direction. The car motor is not needed except to compensate for losses, such as wind resistance or friction.