Why does the force of gravity works on a car that rolls down hill, but doesn't work on a car on a level road?

Question

Think: is the force of gravity always at work? is Newton's third law correct (For every action there is an equal and opposite reaction)? on a level road is the car reacting?

Answer 1

Gravitational force is acting on the car whether it's on a slope or on the level. What you need to understand first is the application of Newton's second law: An object will accelerate in the direction of an applied force. If the car is on a downhill slope gravity pulls it forward and since there is no opposing force (unless the brakes are set) that's the direction in which it moves. On level ground, gravity pulls the car downward, not forward. In this case there is an opposing force. That of the solid ground. It's simply a matter of unbalanced forces resulting in movement in the first case and balanced forces resulting in no movement in the second. Once you understand that concept you can go on to Newton's third law. When the air is expelled rapidly from a balloon, the balloon moves in the opposite direction. The momentum (FXM) of the balloon will be equal to the momentum (FXM) of the expelled air and their directions of travel will be opposite. However, this is also another case of unbalanced forces. As long as the balloon was sealed the air exerted force equally in all directions - thus, no movement. But when the balloon was opened the forces suddenly became unbalanced resulting in equal but opposite motions. Same as the car.

Answer 2

Well, the first thing you should realize is that gravity is not a force. That being said, the weight (which is a force), is indeed acting on the ground, and according to Newton's Third Law of Motion, there has to be an equal and opposite reaction, so the ground is acting on the car itself.

However, the car doesn't move, because in order for the car to move, the force applied to the car must be on the same component as the direction the car is to move. That's why force is a vector. If we pushed on the car from behind, applying force to the back, the car will move (slowly) forward. That's because the force we apply is on the horizontal component, and so the motion of the car will also go along the horizontal component.
The acting of gravity and weight, on the other hand, is a vertical component. The car will ONLY move on a vertical component if the only forces acting on the car is weight. With the ground underneath and pushing up, however, the car does not move because the net force is now 0.

The thing about a hill is that it has both vertical and horizontal components; therefore, weight can actually make the car move due to no level ground, and not a true net force of 0.

Answer 3

Gravity works the same in both these examples. The difference is the road pushing back up against the wheels. If the wheels weren't there, the car would have to be on a pretty steep hill to start sliding down it because of the car's inertia. Since it does have those nice round wheels, any difference from 90 degrees and the wheels help translate downward motion into lateral motion because of the angle (other than 90 degrees or perpendicular) with the pavement and gravity.

Think of it this way, just because the ground directly below the hill and perpendicular is 'down' doesn't mean it's towards the center of the earth's mass...the center is where gravity pulls from (for all intents and purposes of answering this question anyway.)

And this is a seriously hard question to answer without a diagram. :)

Oh, and of course gravity works on a level road...what else keeps the car on the road?

Answer 4

Do you say that gravity is not working because the car doesn't move? Gravity is at work and it is proven by the fact that the car stays firmly planted on the road. Gravity is keeping it there. Gravity will pull any object on wheels to the lowest point. The car on the hill will stop when it reaches the bottom.

Answer 5

Gravity is working otherwise the car would float into space if there was no gravity. There are 2 forces when the car is on level ground and not moving. Gravity pushing the car to the ground and the normal force, that is the ground pushing back on the car. If the car is not working then they are equal and there is no net force.

Answer 6

It is always at work on earth. There is a normal force opposing it on a horizontal road.

on a slanted surface, the normal force is perpendicular to the surface while the gravity is still down. Thus, Force of gravity ...has a bit of an advantage over the normal force.

and wow, the first two posts are wrong, 3rd and 4th are partly right and 5 has no relation to your question.

First post: the force of gravity is always the same, it is the increased velocity you are experiencing.

second post; no gravity doesnt equate to flying off the road. No graviational force also means no normal force.

third post: same as second post although the second half is correct.

forth post: same with 2nd and third...no gravity doesnt mean it wont stay but her second half was correct.

fith post; this question is about force, not work.

Answer 7

Gravity is ALWAYS acting on any given object. With the car on a flat road, gravity is pushing the car down twoards the earth. When a car is facing downhill, gravity is still pushing the car down, but its down a slope now and car has someplace to move.

Answer 8

Your car doesn't fly off the road, right? So gravity is working, ALL the time.

You need to review your physics. Gravity is about mass and distance; it knows nothing about whether you're on a hill or on level ground.

Answer 9

If gravity wasn't working the car would float off like a balloon.

Answer 10

Work is defined as displacement times component of force along the displacement.
Since gravity has no component alongside the displacement on a level road, there is no work done.