When an object is in motion, there is sometimes a force acting on it. This force can be the result of another object or force field, such as a floor pushing on your foot when you try to walk away with something on the floor, or gravity pulling an object down.
When this force is in action for a certain amount of time, then we can conclude that the object is moving at a certain speed and in a certain direction.
So how do we know if the net work done on an object is positive or negative? Well, let’s start with what “work” actually means.
Work is the amount of energy transferred to an object due to a force exerted over a certain distance. The term “net work” refers to the total work done by all forces acting on an object over some time period.
If an object moves away from a source of net work, then its motion is either sideways or forward
If an object moves either sideways or forward as a result of net work done on it, then you can safely say that it is moving.
If the net work done on an object is positive, then you can also say that its motion is directed away from the source of net work. This is a very important concept!
Let’s look at an example to make this clear. Suppose that there is a source of net work whose force does positive work on an object. Then we can say that the object’s motion is either sideways or forward, and that its motion is directed away from the source of net work.
What if there is no source of net work? In that case, we cannot say that its motion is directed in any particular direction.
The direction of the net force can tell you the direction of the object’s acceleration
If the net force on an object is directed in the same direction as the object’s current motion, then you can conclude that there is a positive acceleration of the object.
Acceleration, , is defined as the rate of change of velocity, so if the velocity of an object increases, then its acceleration is considered to be a positive value.
You can think about acceleration as how fast something changes its speed. If your car accelerates from 0 mph to 60 mph, its acceleration is said to be 60 mph per hour, or 60 m/s².
Acceleration can be negative, meaning that an object’s velocity changes in the opposite direction than what it is currently moving in.
Net forces can be represented with vectors, which have a length and a direction. By determining the length of the vector and whether it is pointing in the same direction as the object’s current motion, you can determine whether there is an acceleration or not.
The magnitude of the net force can tell you how fast the object is moving
If the net force on an object is positive, then the object will be accelerated in the direction of the force.
Because of this acceleration, you can conclude that the object’s speed or velocity will increase. The speed will increase in magnitude, i.e., it will get faster.
You can also conclude that the object will keep moving in that direction until some other force acts on it. It will not stop unless something else stops it.
If the net force is negative, then the object’s speed or velocity will decrease and it may ultimately stop moving in that direction. It may still continue to move in some other direction, however.
Net forces always act upon objects according to Newton’s second law of motion. This law states that a force is proportional to the rate of change of velocity (velocity = changing direction + speed).
Only use these rules if there are no external forces besides gravity affecting the object
In a world without gravity, if you know the initial and final positions of an object, you can determine whether or not it’s moving, and in which direction it’s moving.
If the net work done on the object is positive, then you can conclude that the object is moving to the right. If the net work done on the object is negative, then you can conclude that the object is moving to the left.
This is because only work done in the positive direction results in a change in velocity (i.e., it takes some amount of time). No work was done in the negative direction, so there was no change in velocity.
When there are external forces besides gravity affecting an object, things get a little more complicated. There are several equations that help determine whether an object’s motion changes or not due to these external forces.
