# Why do things accelerate? [closed]

Why do things accelerate? Let's say you are pushing an object in space for example, why would the object accelerate? I know that when a net force acts on an object, it accelerates but that is my question, why does this happen? Why wouldn't the velocity stay the same with a net force acting on an object?Because isn't it the same force? I'm trying to figure this out and it's making me crazy! EDIT: I changed the scenario because what i'm asking has nothing to do with gravity

• Do you understand that if the velocity stayed the same, nothing would ever start moving?
– Wood
Nov 11 '16 at 4:03
• everything has a mass, and thus have gravitational pull, and if the distance between the two objects decrease, the pull gets stronger, so they accelerate. Nov 11 '16 at 4:24
• Physics does not answer the "why" for questions like these, it describes the "how." If you want Physics.SE to answer "why," you may need to explain what models you find intuitively acceptable, and we can try to explain acceleration in terms of those models. However, truly answering "why" is a philosophy problem. Nov 11 '16 at 4:36
• Possible duplicate of Why do objects accelerate as they fall? Nov 11 '16 at 4:55
• Please don't vandalise your question. Nov 19 '16 at 22:48

## 4 Answers

Many sciences are a posteriori, and physics, the study of nature, attempts to form theories and models through induction. We observe stuff, we take data, and we try to come up with generalizations. The fact is, we aren't really sure if such generalizations exist or not. We simply belief so, because it makes life easier, and so far this belief has not been shown wrong.

Back to your question: why do things accelerate? This is a question that might never be answered. What physics is doing is trying to answer how things accelerate, or become hot or cold, or increase or decrease in mass. The reason why "truths" from the past are no longer true today, and "truths" today may very likely be no longer true some day, is that we are going bottom up, instead of top down: we can only see the manifestations of those generalizations, if they do exist, and we are doing nothing more than guessing what they are.

Imagine an alien seeing an Earth person pressing gas pedal, then the car moved. The alien never saw the interior of a car, and when she went back, she theorized that some mechanism makes the action of pressing gas pedal move the car. After 1 million observations of this correlation and none of them showing anything that contradicts her theory, she was so sure that pressing gas pedal will lead to the movement of the car.

But, she does not really know what kind of mechanism it is, that is hidden inside the car, that makes this theory valid. Although her theory is sound, as pressing gas pedal will 100% lead to the car moving (given the right conditions), she has no idea why, and she can never find out why if she is unable to disassemble a car and see for herself what is within.

We are aliens, and nature is the car. The sad thing is, we might never have a chance to see what's within the car. All we can do is observe, theorize, and use these theories to make our lives easier.

• That is an excellent analogy! Nov 11 '16 at 5:17
• @CortAmmon Thanks! I was inspired by your comment. Nov 11 '16 at 5:25

If you're asking about why objects with mass are attracted to each other, that comes from theory on relativity. So it's a difficult question to answer. If you're wondering why objects accelerate, I'd like you to think about what happens when you press the gas pedal in your car. You speed up, or accelerate, correct? Once you hold down that pedal and start accelerating, your acceleration and net force does not change. Yet you change velocity. That's because every second, every millisecond, every infinitesimally small increment of time, the car is being pushed a bit to make it go faster.

The same idea goes for gravity. You drop an object, so the net force is gravity. It starts moving, but gravity keeps pulling on it, so it starts moving faster and faster. When you bench press or do weight lifting, you're accelerating the object, right? The object starts from rest, but you give it some velocity by pushing up and overcoming gravity. If you are thinking that all objects must move at some constant velocity, then no objects would ever move at all because their velocities would stay at zero!

• Sorry, I mean it'll go a little faster. Like if you are running and someone pushes you forward. You're going to move forward faster in that second because of the added energy you have. Keep getting pushed forward and you will keep speeding up. If you didn't accelerate, how do you start moving? Nov 11 '16 at 5:59
• I think I have a good example for you. When you are driving and you let go of the gas and don't touch the brake, what happens? The car slows down! Why? Friction is the net force on the car. Each second, these tiny particles are coming into contact and sticking, causing the rotation of the wheel to slow. This continues until it stops. Nov 11 '16 at 6:04

Don't go crazy just yet. Many of laws of nature are inverse distance laws, if you put in numbers to $1/r^2$, as you go further away from the source, say gravity, you can ses that the force on you reduces by the square of the distance you have moved away.

So move 2 metres in distance and the force drops, not to half what it was, but to 1/4 of what it was. Now reverse that idea, imagine you are moving towards the source, you will need to accelerate to comply with this law. That's why acceleration graph lines are curved.

If you use the law, force equals mass by acceleration, in 2 metres, the force on you has doubled, so to balance the equation, assuming your mass is constant, the acceleration has to increase as well, not just stay constant.

• @moe You seem to accept that there will be an initial change in velocity. Okay, what happens if you keep tapping on this object in space? It'll have an initial change in velocity as you stated. But as you tap faster, it becomes so fast that you just push on the object. Then it'll be a constant change in velocity. Nov 11 '16 at 6:13

Force and acceleration

Not applying a force on a mass the mass will move on a straight path. This was not intuitive before Newton because the resistance of air and friction led to slowing down any movement on earth.

Best example in today's life is the bicycle, not applying energy you will without any doubt come to a stop. Excluding this two reasons for energy loss you would be able to accelerate your bicycle more and more, applying all the time the same amount of energy. BTW you would accelerate the mass to infinity time but by this you would nether reach the velocity of light, but this is an other question.

Gravitation and geodesic path

Somehow different from the acceleration by a force is the freefall in a gravitational field. Accelerating your bicycle or more sensitive accelerating a car you feel the acceleration. Falling down you will not feel any acceleration. That is why Einstein does not call it an acceleration and the gravitational interaction not a force. Freefalling body follows a geodesic path and gravitation bends the space and time.

The movement of a satellite around the earth is not an acceleration. The astronauts in the space lab do not feel any acceleration and no force is needed to stay on the orbital (200 km above the earth there is a little atmosphere and this will brake a little bit the satellites and Lab and that is why they have to accelerate time to time to stay on the same height).