Physics for Kids
Force
What is force?
In physics, force is a push or pull on an object. A force can cause an object to
accelerate, slow down, remain in place, or change shape.
How to Measure Force
The unit of measure for force is the newton which is abbreviated as "N". One
newton is the force needed to accelerate one gram of mass by one centimeter
per second squared. Other units of force include the dyne and the poundforce.
Force, Mass, and Acceleration
Force can be figured out if you know the mass and acceleration of an object.
This equation comes from Newton's Second Law of Motion:
f = m * a
Where f = force, m = mass, and a = acceleration.
Forces and Vectors
Force not only has a magnitude (which is what we get in newtons when we use the
equation above), but it also has a direction. This makes force a vector. Vectors
are shown by an arrow that indicates the direction of the force and a number
that indicates the magnitude. See the pictures to the right to see how the
arrow is used to show the direction of the force.


Forces in Equilibrium
Sometimes there can be many forces acting on an object, but the object remains
still. In this case the forces are in equilibrium. The sum of the forces, or
the net force, is zero.
The picture below shows an object sitting on a table. The object isn't moving.
This is because the force of gravity pulling the object down is equal and opposite
to the force of the table pushing up. The net force is zero and the forces are in
equilibrium.
Combined Forces
When multiple forces are acting on an object, the resultant force is the sum
of the vectors of the individual forces. We won't get into complex vector math
here, but take for example a tug of war. The two sides are each pulling. If one
side is pulling with a force of 2 N in left direction and the other side is
pulling with a force of 3 N in the right direction, then the resultant force
is 1 N in the right direction.
Types of Forces
 Friction  Friction is a force caused when one object rubs against
another. It works in the opposite direction of the main force.
 Gravity  Gravity is a force caused by a large body, such as the
Earth. Gravity pulls objects toward the Earth with an acceleration of "g"
which equals 9.8 m/s^{2}.
 Electromagnetic  Electromagnetic force is a force associated with
electric and magnetic fields.
 Nuclear  Nuclear forces are the forces that hold atoms and their
particles together.
 Tension  A pulling force that is exerted by a string, cable, or
chain on another object.
 Elastic  An elastic force is a force exerted by an object trying
to return to its natural length. This is modeled by a spring that has
been pulled by an external force, but is pulling back while trying to
return to is original length.
Interesting Facts about Force
 An object that is accelerating in a circular motion experiences "centripetal"
force.
 The four fundamental forces are gravity, electromagnetic force, the strong
nuclear force, and the weak nuclear force.
 Torque is a type of force that measures changes in the rotational speed of
an object. Torque is an important feature of automobiles, especially trucks.
 Drag is a force that decreases the velocity of an object. Thrust is a force
that increases the velocity of an object.
More Physics Subjects on Motion, Work, and Energy

