Contact Forces

Have you ever been slapped in the face? If so, you have experienced contact forces first-hand. These are forces that only exist between objects when the objects physically touch each other. The force that was exerted on your face was the result of the contact of someone's hand with your face. However, there is more to these forces than just getting slapped across the face. Keep reading to learn more about contact forces!

Definition of a contact force

A force can be defined as a push or pull. A push or pull can only happen when two or more objects interact with each other. This interaction can take place while the objects involved are touching, but it can also take place while the objects are not touching. This is where we distinguish a force as a contact or non-contact force.

Contact forces are responsible for most of the interactions we see in our daily lives. Examples include pushing a car, kicking a ball, and holding a cigar. Whenever there is a physical interaction between two objects, equal and opposite forces are exerted on each of the objects by one another. This is explained by Newton's third law which states that every action has an equal and opposite reaction. This is clearly visible in contact forces. For example, if we push against a wall, the wall pushes back at us, and if we punch a wall, our hand will hurt because the wall exerts a force on us that is equal in size to the force we exert on the wall! Now let's look at the most common type of contact force that is visible everywhere on Earth.

Normal force: a contact force

The normal force is present everywhere around us, from a book lying on a table to a steam locomotive on rails. To see why this force exists, remember that Newton's third law of motion states that every action has an equal and opposite reaction.

The normal force on an object will always be normal to the surface it is placed on, hence the name. On horizontal surfaces, the normal force is equal to the weight of the body in magnitude but acts in the opposite direction, namely up. It is represented by the symbol$N$(not to be confused with the upright symbol$\mathrm{N}$for the newton) and given by the following equation:

$\mathrm{normal}\mathrm{force}=\mathrm{mass}\times \mathrm{gravitational}\mathrm{acceleration}$.

If we measure the normal force in, the mass$m$in$\mathrm{kg}$and the gravitational acceleration$g$in$\frac{\mathrm{m}}{{\mathrm{s}}^2}$, then the equation for the normal force on a horizontal surface in symbolic form is

$N=mg$

or in words,

$\mathrm{normal}\mathrm{force}=\mathrm{mass}\times \mathrm{gravitational}\mathrm{field}\mathrm{strength}$.

The normal force on the ground for a flat surface. This equation is however only valid for the horizontal surfaces, when the surface is inclined the normal is split into two components, StudySmarter Originals.

Other types of contact forces

Of course, the normal force is not the only type of contact force that exists. Let's look at some other types of contact forces below.

Frictional force

However, do not look at friction in only a negative way because most of our daily actions are possible only due to friction! We will give some examples of this later.

Unlike the normal force, the frictional force is always parallel to the surface and in the direction that is opposite to the motion. The frictional force increases as the normal force between the objects increases. It also depends on the material of the surfaces.

Frictional force helps in controlling a moving object. In the absence of friction, objects would keep moving forever with just one push just as Newton's first law predicts, stickmanphysics.com.

Air resistance

Air resistance or drag is nothing but the friction experienced by an object as it moves through the air. This is a contact force because it happens due to the interaction of an object with air molecules, where air molecules come in direct contact with the object. The air resistance on an object increases as the speed of the object increases because it will encounter more air molecules at higher speeds. The air resistance on an object also depends on the object's shape: this is why aeroplanes and parachutes have such wildly different shapes.

Tension

Tension is the reaction force to the external pulling forces in the context of Newton's third law. This force of tension is always parallel to the external pulling forces.

The tension acts within the string and opposes the weight that it is carrying, StudySmarter Originals.

Look at the image above. The tension in the string at the point where the block is attached acts in the direction opposite to the weight of the block. The weight of the block pulls the string down, and the tension within the string acts opposite to this weight.

We have now seen some types of contact forces, but how do we differentiate between contact and non-contact forces?

Difference between contact and non-contact force

Non-contact forces are forces between two objects that don't require direct contact between the objects in order to exist. Non-contact forces are much more complex in nature and can be present between two objects separated by large distances. We've outlined the key differences between contact and non-contact force in the table below.

Now that you can clearly distinguish between these two types of forces, let's look at a few examples that include contact forces.

Examples of contact forces

Let us look at a few example situations in which the forces we talked about in the previous sections come into play.

Now let's look at how friction plays an important part in our daily lives.

Finally, let's look at a phenomenon that we regularly see in movies.

This brings us to the end of the article. Let us now go through what we've learned so far.