When you have varying accelerations like these it’s only possible to deter
mine the acceleration at a specific time. This is achieved by calculating the
gradient of the tangent to the curve at that point.
According to Isaac Newton:
An impressed force is an action exerted upon a body in order to change
its state, either of rest, or of uniform motion in a right line.
Therefore, force is that quality that can alter an object’s speed or line of
motion. Force has nothing to do with motion itself though. For example, a
flying arrow does not need a constant force applied to it to keep it flying
(as was thought by Aristotle). Force is only present where changes in
motion occur, such as when the arrow is stopped by an object or when a
drag racer accelerates along the strip. The unit of force is the Newton,
abbreviated to N, and is defined as:
The force required to make a one-kilogram mass move from rest to a
speed of one meter per second in one second.
There are two different types of force: contact and non-contact forces. Con-
tact forces occur between objects that are touching each other, such as the
frictional force present between the snow and skis of a downhill skier.
Non-contact forces are those that occur between objects not touching each
other, such as the gravitational force of the Earth upon your body or the
magnetic force of the Earth upon a compass needle.
It’s important to note that many forces can act upon a single object
simultaneously. If the sum of those forces equals zero, the object remains
in motion with the same velocity in the same direction. In other words, if
an object is stationary or moving in a straight line with a constant velocity,
the sum of all the forces acting upon it must be zero. If, however, the sum
of the forces is not equal to zero, the object will accelerate in the direction
of the resultant force. This can be confusing, especially in relation to static
objects. For instance, how can there be any forces acting upon an apple sit
ting on a table? After all, it’s not moving! The answer is that there are two
forces acting upon the apple: the force of gravity trying to pull the apple
toward the Earth and an equal and opposite force from the table pushing it
away from the Earth. This is why the apple remains motionless. Figure
1.33 shows examples of varying amounts of forces acting upon everyday
38 | Chapter 1