# Why does something on an inclined plane move forward at all?

We just started studying about the inclined plane and vectors in motion, but i don't understand one thing: Why on earth does the object on the inclined plane move forward (I.e in the direction where the inclined plane is 'falling') at all?

I understand that the force due to gravity is (almost) perfectly downward. I can also understand that since the object sits on the inclined plane, there should be an equal and opposite force on the object (Owing to the fact that when the object pushes down on the inclined plane, it gets slightly closer to the inclined plane, which increases electrostatic repulsion and makes the atoms that are part of the upper layer of the inclined plane in turn push the object 'up'), but that should be (almost) perfectly upwards [The almost part owing to the curvature of the earth would be too small to make a difference].

So in this case, how on earth does the object on the inclined plane gain any 'forward' velocity at all? My question is, when you put an object on an inclined plane, how the heck does it get horizontal velocity?

• The force that the plane exerts has two components, normal to the plane and parallel to the plane. The two have to add up to the force of gravity or the object will move, right? In the normal direction you can push as hard on the plane as you want, nothing will move, correct? Now, then, what happens in you push parallel to the plane? Mar 16, 2016 at 6:25
• @CuriousOne I'm sorry but i still don't understand. The force that is parallel to the plane is the frictional force (?), which doesn't come into effect until the object actually starts to move, since it's only at that point when weak covalent bonds start to be broken and hence apply a retardation to the motion. Also, gravity isn't perpendicular to the plane. It's straight 'down', or straight towards the y component alone. There is no part of gravity that is along the horizontal direction, and hence, the plane shouldn't apply anything in the horizontal direction either. Mar 16, 2016 at 6:55
• There are two types of friction, remember? Static friction keeps you in one place, even against a finite amount of force that is trying to accelerate you. Once that finite static friction force is exceeded, the sliding begins. Mar 16, 2016 at 6:56
• @CuriousOne Yes, i think i understand the general area of where the gap in my understanding is, but i still can't put a finger on it. When there is only a force in the vertical direction (And the object can't move in the vertical direction because of the plane putting an equal and opposite force on the vertical direction), there should be no motion. And without motion, there should be no friction. I can understand when there is some force and the static friction is cancelling it, but in this case, that's what i don't understand. Where the heck is the horizontal force coming from? Mar 16, 2016 at 7:01

Have you ever walked down a mountain slope? Did you ever feel that you climb down faster than you normally walk?

How did you get any horizontal velocity at all?

This image is a classic textbook illustration of an object on an inclined plane. mg is the weight of the object. It is acting vertically downwards.

But you can imagine that this vertical force is made up of two forces - One acting perpendicular to the wedge and other parallel to the slant of the inclined plane.

A little trigonometry reveals that these forces are mg cos$\theta$ and mg sin$\theta$ respectively.

Now you need to forget about the original vertical force mg. Consider you have these two mentioned forces only.

mgcos$\theta$ is acting perpendicular to the incline of the wedge. This will not provide the object with any horizontal velocity. It is responsible for the reaction force N by the wedge. The frictional force f is proportional to this component.

mgsin$\theta$ is parallel to the incline. This force will try push the object down the incline. But behold! f is opposing this force!

If mgsin$\theta$ is greater than maximum possible value of friction, the object will experience a force mgsin$\theta$ - f, and this force will be responsible for making the object move down the slope.

That's how you get a horizontal force.

• Platypus Perry, I think OP understands resolution of vectors. He wants to know why they resolute. May 3, 2016 at 6:02

I understand that the force due to gravity is (almost) perfectly downward. I can also understand that since the object sits on the inclined plane, there should be an equal and opposite force on the object

No. If we are considering the pull of the earth on the object as the force in question, then the "equal and opposite" force is the one that the object exerts on the earth, pulling it upward.

(Owing to the fact that when the object pushes down on the inclined plane, it gets slightly closer to the inclined plane, which increases electrostatic repulsion and makes the atoms that are part of the upper layer of the inclined plane in turn push the object 'up'), but that should be (almost) perfectly upwards

The electrostatic repulsion does push the object, but the net direction is perpendicular to the plane, not upward. This is the normal force.

When this force is not vertical (the plane is not horizontal), and there is no friction, then the forces are unbalanced and the object accelerates with some horizontal component.