Free body diagram question (1 Viewer)

[.ben]

11. A car is coasting to the right and slowing down. The free-body diagram for this situation is shown below:

I just have a question. If a car is moving rightward, shouldn't there also be a rightward force in the diagram to balance the frictional force?

 

[pLuvia]

But, it states the car is slowing down, so if there was a rightward force then it wouldn't be slowing down

 

[Mill]

If you consider a normal car, you could probably consider two main forces acting on it.

The friction force against the direction of motion.

And the force due to the 'engine' that is obviously being applied in the direction of motion.

In most resisted motion questions, you ignore the effects of a 'engine' force (or rather, you assume there isn't one taking place).

So for example, in your question, only the friction force is acting on the car, and so it is slowing down.




There are other possible scenarios (that aren't really explored in resisted motion).

Maybe the car will have friction force F and engine force E.

If E > F, then the car is speeding up.

If E < F, then the car is slowing down.




But like I said, in general, there is no propulsion force in 4u resisted motion.

 

[.ben]

But, it states the car is slowing down, so if there was a rightward force then it wouldn't be slowing down

But can't the scenario be say 400newtons to the left and 100newtons to the rigth so therefore net force is 300 newtons to teh left so it is accelerating toward teh left and thus slowing down?

If you consider a normal car, you could probably consider two main forces acting on it.

The friction force against the direction of motion.

And the force due to the 'engine' that is obviously being applied in the direction of motion.

In most resisted motion questions, you ignore the effects of a 'engine' force (or rather, you assume there isn't one taking place).

So for example, in your question, only the friction force is acting on the car, and so it is slowing down.




There are other possible scenarios (that aren't really explored in resisted motion).

Maybe the car will have friction force F and engine force E.

If E > F, then the car is speeding up.

If E < F, then the car is slowing down.




But like I said, in general, there is no propulsion force in 4u resisted motion.

so basically in 4u there won't be any of this type of question? why would they ignore applied force? doesn't that make the situation unrealistic?

 

[Riviet]

so basically in 4u there won't be any of this type of question? why would they ignore applied force? doesn't that make the situation unrealistic?

Yes, it is very unrealistic but this is mainly to keep things simple for the 4u course.

 

[.ben]

well if it's to keep it simple, then i'm all for it

 

[gamecw]

11. A car is coasting to the right and slowing down. The free-body diagram for this situation is shown below:

I just have a question. If a car is moving rightward, shouldn't there also be a rightward force in the diagram to balance the frictional force?

Definitions of coast on the Web:
slide: the act of moving smoothly along a surface while remaining in contact with it; "his slide didn't stop until the bottom of the hill"; "the children lined up for a coast down the snowy slope"
move effortlessly; by force of gravity

lol i guess there's no rightward force then.. the engine of car is stoped

 

[gamecw]

But, it states the car is slowing down, so if there was a rightward force then it wouldn't be slowing down

not really, if the leftward force is greater than tbe rightward force, then the car will still be slowing down... its possible for the car to experience forces in both opposed direction..

but in this case, the car's a 'free body', its 'coasting' alone the road, hence theres no rightward force..

Edit:

But can't the scenario be say 400newtons to the left and 100newtons to the rigth so therefore net force is 300 newtons to teh left so it is accelerating toward teh left and thus slowing down?

Exactly, this Can happen, but not in this particular scenario. explained above.

when u see questions like this, dont treat the object as a car, treat it as a rolling ball or sumthing..