When a force is exerted on an object, that object will experience a change in velocity (speed and direction) or state of motion. For example, if you pull on a box with a constant force, the box will move forward.
If the force pulling on the box is variable (like changing how hard you pull on the box), then the box will have a variable speed and direction.
You can describe this concept with physics equations, but it is easiest to understand through examples. There are many examples of this concept in everyday life, from doing chores to playing sports.
Chores can involve forces applied to objects depending on what chore you are doing. If you are washing the dishes, then water and soap act as forces against the dish. If you are sweeping floors, then the floor dust acts as a force against the broom. If you are drying dishes, then air pressure acts as a force against them while being dried.
If an object experiences a net negative work, then its motion will be either downward or backward
If the net work done on an object is negative, then the object’s motion will be either downward or backward.
Imagine that you are pushing a heavy box across a floor. You exert a certain amount of force over a certain distance, and the box moves in the other direction.
But because you are exerting force in the wrong direction, someone else could come along and pull on the box with equal force in the opposite direction, and they would end up moving the box forward.
That is, they would do enough net work to move the box forward, which would make their total work positive. Your contribution to the total work was negative, so their contribution had to be positive.
In other words, if the net work done on an object is negative, what can you conclude about the object’s motion? It must be either downward or backward.
Objects that experience a net positive work tend to move forward
If the net work done on an object is positive, then you can conclude that the object’s motion is forward. The reason for this is because the force acting on the object was stronger than the resistance it encountered, so it must have moved forward as a result.
If the net work done on an object is negative, then you can conclude that the object’s motion is backward. The reason for this is because the force acting on the object was weaker than the resistance it encountered, so it must have moved backward as a result.
You can use this principle to determine whether an object moving at constant speed is going forward or backward. If you find that there is a net positive work done on an object, then it is moving forward. If you find that there is a net negative work done on an object, then it is moving backward.
This principle can be used to determine if an unknown motion is forward or backward, but only if there is some measurable work done during that motion.
Objects that experience a net negative work tend to move backward
If the net work done on an object is negative, what can you conclude about the object’s motion? If the net work done on an object is negative, then you can conclude that the object’s motion is backward.
Imagine a box that weighs one kilogram sitting on a frictionless surface, where you push it to the right with a force of one newton. Then you apply a force of -1 newton to the left, so that it moves in that direction. You calculate the work you do, and find that it is -1 joule.
You have decreased its kinetic energy by 1 joule, which means its speed has decreased by 1 newton per second.
The relationship between the net work and the direction of the motion depends on the sign of the velocity before collision
If the final velocity of the object is in the same direction as its initial velocity, then the work done on the object is negative, and the object loses speed.
This happens when the colliding objects stick to each other, like two magnets. The force of attraction between them increases their total kinetic energy, so it takes work to pull them apart.
If the net work is negative, then you can conclude that the final speed of an object is lower than its initial speed.
If the net work is positive, then you can conclude that the final speed of an object is higher than its initial speed. This happens when the colliding objects do not stick to each other, like billiard balls. The force of collision sends one ball away at a higher speed than it had before collision.
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