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If a balanced combination of forces acts on an object, then, if it is at rest, it will remain at rest, and if it is in motion, it will continue to move in a straight line path at a constant speed. (A situation in which no forces acting on an object is just the simplest case of balanced forces.)

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If an unbalanced combination of forces acts on an object, its speed and/or its direction will change. How quickly the speed (and/or direction) changes is directly related to the strength of the net force and inversely related to the object’s mass. (A situation in which only a single force acts on an object is just the simplest case of unbalanced forces.)

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During a contact push/pull interaction, the two objects involved exert a force on each other. These two forces are equal in strength and opposite in direction. Only one of these two forces acts on each of the objects involved.

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Interactions described in terms of forces

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Interactions described in terms of forces

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The interaction between two objects may be described in terms of the force (a push or pull) that one object exerts on the other. A force diagram uses arrows to represent the forces acting on an object at a particular moment. The length of the arrows represents the relative strengths of the forces. When an object is in motion, its force diagram should also include a speed arrow to show in which direction it is moving.

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When two objects interact, they exert forces on each other that are equal in strength and

opposite in direction. Only one force acts on each object involved. When more than one force acts on a single object (which means it is involved in more than one interaction at the same time), the effect they have on its motion is due to the combination of these forces and not just the strongest one. If the strengths and directions of the individual forces are such that their combination results in an exact balancing of forces in opposite directions, the forces are said to be balanced. f the strengths and directions of the individual forces are such that their combination does not result in an exact balancing of forces in opposite directions, the forces are said to be unbalanced. (Note that a situation in which only a single force acts on an object is just the simplest case of an unbalanced combination of forces.) When the forces acting on an object are added and/or subtracted to give an equivalent single force, scientists call the result the net force.

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If a non-zero net force acts on an object at rest, the object will begin to move in the direction of the net force.

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If a non-zero net force acts on an object in the same direction as its motion, the object’s speed will increase. (When the speed of an object changes scientists say it is accelerating.)

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Net force in direction opposite to motion

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Net force in direction opposite to motion

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If a non-zero net force acts on an object in the direction opposite to its motion, the object’s speed will decrease. (Scientists call this accelerating too, since a decrease in speed is also a change.)

If the net force continues to act, the object may eventually stop and even reverse direction.

Friction (example of a backward force)
An example of a force that always opposes motion is friction. It is caused by the contact

push/pull interaction between tiny bumps and irregularities in the surfaces of objects as they rub together.

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If a balanced combination of forces acts on an object at rest, then the object will remain at rest.

If a balanced combination of forces acts on an object in motion, then the object will remain in motion at a constant speed.

If a balanced combination of forces acts on an object in motion, then the path of the object will be a straight line

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When a non-zero net force acts on an object, the greater the strength of the net force, the

higher the rate of change of the object’s speed.
When a non-zero net force acts on an object, the more mass the object has, the lower the

rate of change of the object’s speed. (Scientists call this property of objects their inertia.) Rateofchange∈speed=Strengthofnetforce/Mass of object Scientists define the rate of change in speed of an object (that is, how much it’s speed changes in one second) to be its acceleration.

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If a force acts on a moving object in a direction that is neither along the line of the object’s motion nor directly opposed to it, then the object’s path will change direction.

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If a continuous force acts on a moving object in a direction that always points toward a common center, the object’s path may curve around the center.

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As an object moves through the air, either horizontally or vertically, the air exerts a force on it in the direction opposite to its motion. This force is called air resistance or drag. The strength of the drag force depends on two factors. First, it depends on the speed of the object: the faster the object is moving, the greater the strength of the drag force. Second, it depends on the surface area of the object in the direction of motion: the greater the surface area, the greater the strength of the drag force.