Forces

A force is defined as a push or pull that causes an object to move or speed up (accelerate), to slow down (decelerate) or even stop, or to change the direction of the object's motion. Since any change in velocity is considered acceleration, it can be said that a force on an object results in the acceleration of an object. The acceleration cause by a force depends on the mass of the object. You can directly measure force with a scale or indirectly through calculations.

Questions you may have about this include:

• What is a force?

• What factor does mass have with forces?

• How do you measure force?

This lesson will answer those questions.

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).

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.

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?

Vectors help measure forces

Just as vectors are used in measuring velocities in different directions, vectors are also used to help calculate the sum of forces in a given direction.

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.

If you wanted to measure the factors, you could use the equation or relationship that the force (F) equals the mass of the object (m) times the amount of resulting acceleration (a):

F = m*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 = F*t / m,

which means that the velocity (v) equals the force applied (F) times the time interval (t) divided by the mass of the object (m).

We got the equation from taking knowing that acceleration (a) equals velocity (v) divided by time (t), a = v/t, to get F = m*a = m*v/t. Thus v = F*t/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.

Measuring a force

You can measure the amount of force with a device such as a scale. The unit of force in the English system is a pound and in the metric system is a Newton.

Force of gravity

Gravity is a force that accelerates falling objects. You can measure the force of gravity or weight of an object with a scale.

Likewise, You can easily use a scale to measure how much force is being exerted with you push or pull an object.

Sometime calculations will determine the amount of force. If you knew the mass of an object and could measure its change in velocity or acceleration, you could easily calculate it from the F = m*a equation.

In conclusion

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. You can often use a scale to measure force.