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Motion and Newton's Laws

# GED Science Practice Test: Force And Newton’s Laws Of Motion

A force is a push or pull upon an object resulting from an interaction between objects. Forces only exist as a result of an interaction; when the interaction ceases, the two objects no longer experience the force.  Forces between objects can be placed into two broad categories: contact and non-contact forces.  The following table shows the two categories of forces:

Contact forces are those types of forces that require physical contact between objects. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces.  While some of these may be intuitive to you, you may be unclear what applied and normal forces are.  An applied force is an “on purpose” push or pull.  If you push a box across the floor, the force between you and the box is an applied force.  A normal force is a sort of passive force between two objects, once of which is resting on the other. When a box rests on the floor, the floor pushes up on the box with a normal force.  The normal force is always perpendicular to the surface; in fact, the word “normal” means perpendicular. When a box rests on a ramp, the ramp pushes perpendicularly to the box with a normal force.

The idea of the normal force may seem odd to you.  If you imagine yourself standing on the ground (instead of a box resting on the floor), the idea of the floor pushing up on you seems unusual.  However, unless there was a force pushing up on you, your force pushing down on the ground would cause you to fall into the earth.

Non-contact forces are those types of forces that do not require contact between the objects.. Examples of action-at-a-distance forces include gravitational, electrical, and magnetic forces. For example, the sun and planets exert a gravitational pull on each other despite their large spatial separation. When you jump, and break your physical contact with the earth, you still feel the force from the earth.

Force is a quantity that is measured using the standard metric unit known as the Newton. A Newton is abbreviated by an “N,” and represents the amount of force required to push a 1-kg mass with an acceleration of 1 m/s/s. To say “10 N” means 10 Newton of force.

A force is a vector quantity, meaning that it has both magnitude and direction. To fully describe the force acting upon an object, you must describe both the magnitude (size or numerical value) and the direction. Thus, 10 N is not a full description of the force acting upon an object. In contrast, 10 Newton, downward is a complete description of the force acting upon an object; both the magnitude (10 Newton) and the direction (downward) are given.

Because a force is a vector that has a direction, it is common to represent forces acting on an object using arrows in a diagram. The size of the arrow represents the magnitude of the force and the direction of the arrow reveals the direction that the force is acting. The following shows a diagram of the forces acting on a box resting on the floor:

In this example, gravity pulls down on the box with a force of 8N and the normal force of the ground pushes up on the box with a force of 8N.  This diagram is simple, but also corrects one common misconception that people have about forces; that an unmoving object does not experience any forces.  Indeed, this box is experiencing two forces.  One force is the result of the interaction between the box and the floor.  The other is the result of the interaction between the earth and the box.

As you can see from the box example above, forces can sometimes effectively cancel each other out. The 8N force of the earth pulling down on the box and the 8N normal force of the ground pushing up on the box cancel each other out.   In such instances, it is said that the two individual forces are balanced.  There are no unbalanced forces acting on the box.  In this case, we can describe the net force (or overall sum of forces) as zero.

However, sometimes when adding up forces, the forces do not cancel out.  Remember how we added vector quantities above?   The following example shows an object experiencing unbalanced forces, with a net force of 2N to the left:

This example shows unbalanced forced, with a net force of 14 N to the right.

When an object experiences unbalanced forces, it will accelerate in the direction of the net force.  When an object experiences balanced forces, it will not accelerate (stay still or stay at the same velocity).  Newton’s Laws of Motions further describe how motion can be predicted for objects or groups of objects.

Motion and forces can be explained by Newton’s Laws of Motion. Isaac Newton was a 17th century scientist who put forth a variety of laws that explain why objects move (or don’t move) as they do. These three laws have become known as Newton’s three laws of motion. These three laws state the following:

Newton’s First Law of Motion (Law of Inertia): This law states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. The behavior of all objects can be described by saying that objects tend to “keep on doing what they’re doing” (unless acted upon by an unbalanced force). If at rest, they will continue in this same state of rest. If objects are in motion with an eastward velocity of 5 m/s, they will continue in this same state of motion (5 m/s, East). If in motion with a leftward velocity of 2 m/s, they will continue in this same state of motion (2 m/s, left). All objects resist changes in their state of motion – they tend to “keep on doing what they’re doing.” This tendency of an object to “keep on doing what it is already doing” is called inertia. The inertia of an object is maintained as long as the object is not acted upon by an unbalanced force.

A great example of Newton’s First Law of Motion in everyday life is driving a car.  As you travel with the car, both you and the car have inertia….you’re moving forward, and the tendency is to keep moving forward.  However, if you have to press the brakes suddenly, the car has experienced an unbalanced force that causes it to decelerate.  You, however, are still moving forward, due to inertia so your body moves forward into the seat belt.  If you are not wearing a seat belt, you could potentially get thrown from the car as your body continues forward through the windshield.

Newton’s Second Law of Motion: Newton’s Second Law of Motion pertains to the behavior of objects for which all existing forces are not balanced, or in other words, when the net force is not zero. Newton’s Second Law states that the acceleration of an object is directly proportional to the magnitude of the net force, and inversely proportional to the mass of the object.  The second law is best expressed through an equation:

Sometimes, the equation is solved for acceleration, in order to more easily describe the relationship:

Thus, as the force acting upon an object is increased, the acceleration of the object is increased. As the mass of an object is increased, the acceleration of the object is decreased. For example, if you exert 20 newtons of force on an object, it will accelerate twice as fast as an object of the same mass that you have exerted 10 newtons of force on. Likewise, if you exert 10 newtons of force on a 2 kg object, you would have to exert 20 newtons of force on a 4kg object to get it to accelerate at the same rate.

An interesting thing about Newton’s law is that it suggests that when there is no net force on an object (i.e., the forces on an object are balanced), there is no acceleration.   Remember that no acceleration can mean no motion or constant motion (i.e., constant velocity).  So Newton’s Second Law helps to explain the Newton’s First Law.  Remember that Newton’s First Law stated that an object in motion will stay in motion, and an object at rest will stay at rest, unless acting on by an unbalanced force.  The following diagram shows how Newton’s First and Second Law interact:

Newton’s Third Law of Motion: Newton’s Third Law states that for every action, there is an equal and opposite reaction.   This statement is a little misleading, and could best be restated as “for every force, there is an equal and opposite force.” So for every interaction between two objects, there is a pair of forces.  When we described the normal force above, we said that when a box pressed down on the floor due to gravity, the floor presses up on the box with a normal force.  These two forces are the action and reaction forces.  Notice that we are not talking about force diagrams, here; we are not describing the forces acting on a single object.  Rather, force pairs exist between two objects:  box pushes on floor, floor pushes on box.

The size of each of the forces in a force pair are equal in size and opposite in direction. This relationship is intuitive to understand for a stationary box on a floor.  It is not difficult to believe that the downward force of the box pushing on the floor is equal to the upward force of the floor on the box.  However, Newton’s Third Law of Motion is true for all forces.  Imagine for a moment that you throw a ball at the wall.  The force pair diagrams are shown in the diagram below:

Newton’s Third Law of Motion states that the force that the ball exerts on the wall is opposite in direction and equal in size to the force with which the wall pushed back on the ball.  Intuitively, this makes you wonder:  if the forces are equal, why does the ball move but the wall stay still?  The answer is that while the forces are equal in size, they are acting on two objects with very different masses;  a 10N force acting on a ball is likely to have a bigger effect than a 10N force acting on a wall.

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