# Force and acceleration

1. Mar 8, 2009

### jamesabc

i was just wondering about the relationship between force and acceleration.

from what i have learned in school and at university is that when a force is applied to an object, the object is accelerated. But what i am a bit confused about is why objects accelerate.

this is what i think but am not sure if it is right. it takes a certain amount of force to start an object moving, but as soon as the object starts moving the force required to keep it moving is less than the force to get to start moving in the first place. so if the initial force is keep constant then there will be acceleration? but eventually the friction from air resistance or whatever will eventually work against the acceleration and acceleration will be zero.

is this right?

2. Mar 8, 2009

### Staff: Mentor

As long as there is a net force on an object, it will accelerate.

3. Mar 8, 2009

### HallsofIvy

But if you maintain a constant force and friction increases with velocity, eventually the friction force will be equal to the applied force so the net[\b] force will be 0 and there will no longer be any acceleration.

4. Mar 8, 2009

### jamesabc

by net force do you mean force greater than the force to get the object moving? or something else?

5. Mar 8, 2009

### Staff: Mentor

The net force on an object is the vector sum of all individual forces acting on the object. As long as that net force is non-zero, the object will accelerate. When the net force is zero, the acceleration is zero. (Newton's 2nd law.)

To start something moving (from rest) some kind of force is needed. Once that force is removed, or other forces (like friction) are added, such that the net force is zero, the object will continue moving in a straight line at constant speed. (Newton's 1st law.)

In other words: You need a net force to change an object's motion (get it moving, for example); but once it's moving, it does not require a net force to keep moving. Of course, if friction acts on the moving object, you need to keep pushing to maintain the motion. But your push merely acts to overcome friction, making the net force zero.