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Characteristics of Force

by Ron Kurtus (revised 13 October 2007)

A force is defined as a push or pull that causes an object to change its velocity. A force can be applied by a direct push on another object. But also there are forces that can push or pull on other objects from a distance. The mass of an object affects how much force is required to accelerate it.

Questions you may have include:

• How does a force change velocity?
• What what are the two classes of forces?
• How is force affected by mass?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

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Time = 6 min. 40 sec.

Force changes velocity

A force acting on an object will cause the object to change its velocity. That means it will start moving, increase its speed, decrease its speed, stop moving, or change direction. A change in velocity is defined as acceleration (or deceleration). While velocity is measured in meters per second (m/s), acceleration is measured in meters per second per second (m/s/s) or meters per second-squared (m/s²).

Common factor in motion

In general, most things are standing still. But as you look around, you may see things that were still suddenly start to move for one reason or another. For example:

• An apple falls from the tree to the ground.
• Some men push a stalled car down the street.
• You blow up balloon, and it flies away as the air rushes out.
• You hold a stick under water, and after you let it go, it shoots up to the surface.

There is a common factor in the motion of the above examples. In every case, a force was acting on the object. The forces in the examples are respectively gravity, pushing, air pressure, and buoyancy.

Two classes of forces

Forces can be divided into two classes:

1. Those forces that act by direct contact, such as when you push on a door to open it
2. Forces that act at a distance, with no physical contact between the objects; the forces of gravity or magnetism are examples of forces at a distance

Direct forces make sense

Direct contact forces seem to make sense. It only seems natural that if a moving object smashes into another object, it will cause the second object to move in the same direction, as explained by Newton's Laws.

Force at a distance is strange

On the other hand, how can a force act on another object at a distance? What is "pushing" on the second object to make it move? This can be seen in the case of two magnets of like poles. Not only that, most forces at a distance seem to "pull" more than they push.

What is this mystical force of gravity that pulls an object to the earth from a distance? There really isn't a good explanation, except that they are "forces" that act at a distance.

(See The Mysterious Force at a Distance for more information.)

Direction of force affects motion

If an object is moving and you push on it in a direction at an angle to what the object is traveling, the force of the push will cause the object to change its direction.

Forces in a line add or subtract, according to their direction. For example, if one person pulls on a rope with a force of 10 pounds and another person pulls in the opposite direction with a force of 8 pounds, the total force will be 2 pounds in the direction of the 10 pound force.

If several forces or velocities are applied at angles, it is not so easy to calculate the total force in a given direction. For example, suppose an jet plane was moving through a cross-wind. What would be the total force in the direction of motion? It would depend on the force and the angle.

Force affected by mass

The heavier an object, the more force is needed to speed it up (accelerate it) or slow it down (decelerate it). Another way of saying this is that the amount of force needed to accelerate an object is proportional to the mass of the object.

Relationship

If you wanted to measure the factors, you could use the equation or relationship that the force equals the mass of the object times the amount of resulting acceleration or

F = ma

where:

• F is the force applied to the mass
• m is the mass
• a is the acceleration caused by the force
• ma is m times a

In other words, it would take twice the force to accelerate a 2-kilogram object to a given velocity than it would for a 1-kilogram object.

Accelerates until force stops

As long as the force is applied to a given object, it will to accelerate. Once the force is withdrawn, the object will continue to move at a constant velocity.

Measure velocity

If you wanted to measure that velocity, you could use the equation

v = Ft / m

where

• v is the velocity to be determined
• F is the force applied on the object
• t is the time interval the force is applied
• m is the mass of the object
• Ft/m is F times t divided by m

We obtained the equation by knowing that acceleration (a) equals velocity (v) divided by time (t), a = v/t. , to get F = ma = mv/t. Thus v = Ft/m.

Impulse

If the amount of time the force is applied is very short, that force is called an impulse. An example of an impulse is when you hit a golf ball. The force is applied for a very short time and accelerates the ball up to a high speed. The ball then continues to travel at the same velocity, being slowed down some by the force of air resistance and being pulled to the earth by the force of gravity.

Summary

A force is a push or pull that causes an object to accelerate. There are direct contact forces and those that act at a distance. Newton's Laws of Inertia state that force is required to change the motion of matter. The force require to accelerate an object is proportionate to the mass of the object.

Answers to Readers' Questions

Become a positive force in your community.

Resources

The following resources provide information on this subject:

Websites

Physical Science Resources

Books

Forces In Nature by Liz Sonneborn Rosen; Publishing Group (2004) $25.25 - Understanding gravitational, electrical and magnetic force

The Science of Forces by Steve Parker; Heinemann (2005) $29.29 - Projects with experiments with forces and machines

Glencoe Science: Motion, Forces, and Energy, by McGraw-Hill; Glencoe/McGraw-Hill (2001) $19.32 - Student edition (Hardcover)

Miscellaneous

Mini-quiz to check your understanding

1. What causes things to accelerate?

2. Why is gravity a force?

3. What is an impulse?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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Characteristics of Force