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# Contact Forces

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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.

A contact force is a force between two objects that can only exist if these objects make direct contact with each other.

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 is the reaction contact force acting on a body that is placed on any surface, due to the action force that is the weight of the body.

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(not to be confused with the upright symbolfor the newton) and given by the following equation:

.

If we measure the normal force in, the massinand the gravitational accelerationin, then the equation for the normal force on a horizontal surface in symbolic form is

or in words,

.

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

The frictional force (or friction) is the opposing force between two surfaces that are attempting to move in opposite directions.

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.

These dependencies of friction are very natural: if you push two objects together very hard, the friction between them will be high. Furthermore, materials like rubber have way more friction than materials like paper.

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.

The coefficient of friction is the ratio of the frictional force and the normal force. A coefficient of friction of one indicates that the normal force and frictional force are equal to each other (but pointed in different directions). To make an object move, the driving force must overcome the frictional force acting on it.

### 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.

The reason why there is no air resistance in space is due to the lack of air molecules there.

As an object falls, its speed increases. This leads to an increase in the air resistance it experiences. After a certain point, the air resistance on the object becomes equal to its weight. At this point, there is no resultant force on the object, so it is now falling at a constant velocity, called its terminal velocity. Every object has its own terminal velocity, depending on its weight and its shape.

Air resistance acting on an object in free fall. The magnitude of the air resistance and the velocity keep increasing until the air resistance is equal to the weight of the object, misswise.weeble.com.

If you drop a cotton ball and a metal ball of the same size (and shape) from a height, the cotton ball takes longer to reach the ground. This is due to its terminal velocity being much lower than that of the metal ball due to the lower weight of the cotton ball. Therefore, the cotton ball will have a slower falling speed, which makes it reach the ground later. However, in a vacuum, both balls will touch the ground at the same time due to the absence of air resistance!

### Tension

Tension is the force acting within an object when it is pulled from both of its ends.

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.

Tension resists the deformation of an object (e.g. a wire, string, or cable) that would be caused by external forces acting on it if the tension were not there. Thus, the strength of a cable can be given by the maximum tension it can provide, which is equal to the maximum external pulling force it can endure without breaking.

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.

 Contact force Non-contact force Contact is required for force to exist. Forces can exist without physical contact. There is no need for any external agencies: only direct physical contact is required for contact forces. There has to be an external field (such as a magnetic, electric, or gravitational field) for the force to act Types of contact forces include friction, air resistance, tension, and the normal force. Types of non-contact forces include gravity, magnetic forces, and electric forces.

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.

The normal force acts on the bag once it's placed on the surface of the table, openoregon.pressbooks.pub.

In the example above, when the bag is initially carried, the forceis used to counteract the bag's weightto carry it. Once the bag of dog food is placed on top of a table, it will exert its weighton the surface of the table. As a reaction (in the sense of Newton's third law), the table exerts an equal and opposite normal forceon the dog food. Bothandare contact forces.

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

Even when we're walking, the force of friction is constantly helping us push ourselves forward. The force of friction between the ground and the soles of our feet helps us get a grip while walking. If not for friction, moving around would have been a very difficult task.

Frictional force while walking on different surfaces, StudySmarter Originals.

The foot pushes along the surface, hence the force of friction here will be parallel to the surface of the floor. The weight is acting downwards and the normal reaction force acts opposite to the weight. In the second situation, it is difficult to walk on ice because of the small amount of friction acting between the soles of your feet and the ground. This amount of friction cannot propel us forward, which is why we can't easily start running on icy surfaces!

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

A meteor starts to burn due to the large magnitude of air resistance as it falls towards the Earth's surface, State Farm CC-BY-2.0.

A meteor falling through the Earth's atmosphere experiences a high magnitude of air resistance. As it falls at thousands of kilometres per hour, the heat from this friction burns up the asteroid. This makes for spectacular movie scenes, but this is also why we can see shooting stars!

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

## Contact Forces - Key takeaways

• Contact forces (only) act when two or more objects come in contact with each other.
• Common examples of contact forces include friction, air resistance, tension, and normal force.
• The normal force is the reaction force acting on a body that is placed on any surface due to the weight of the body.
• Always acts normal to the surface.
• The frictional force is the opposing force formed between two surfaces that are attempting to move in the same direction or opposite directions.
• Always acts parallel to the surface.
• Air resistance or drag force is the friction experienced by an object as it moves through the air.
• Tension is the force acting within an object when it is pulled from one or both its ends.
• Forces that can be transmitted without physical contact are called non-contact forces. These forces need an external field to act.

No, gravity is a non-contact force. We know this because the Earth and the Moon are gravitationally attracted to each other while they are not touching.

Yes, air resistance is a contact force. Air resistance or drag force is the friction experienced by an object as it moves through the air because the object encounters air molecules and experiences a force as a result of the direct contact with those molecules.

Yes, friction is a contact force. Friction is the opposing force formed between two surfaces that are attempting to move in opposite directions.

Yes, tension is a contact force. Tension is the force acting within an object (e.g. a string) when it is pulled from both of its ends. It is a contact force because of the direct contact between different parts of the object.

No, magnetism is a non-contact force. We know this because we can feel a magnetic repulsion between two magnets that are not touching.

## Final Contact Forces Quiz

Question

Which of these systems involves a contact force?

An object falling through a vacuum.

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Question

All forces require physical interaction between two bodies for transmission.

False.

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Question

Which of these are contact forces?

Tension.

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Question

The direction of motion and the direction of the frictional force are ...

opposite to each other.

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Question

The normal force when a book is placed on a table acts ...

perpendicular to the table's surface.

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Question

The friction force when a sliding block moves over a flat surface is ...

parallel to the surface.

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Question

The friction with air molecules when an object is free-falling causes which of the following forces?

Air resistance.

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Question

Tension acts when two ends of a string are ...

pulled apart.

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Question

The air resistance of a falling object depends on ...

the speed of the object.

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Question

The tension acting in a rope suspending an object is equal to ...

the weight of the object.

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Question

The magnitude of the normal force on an object on a horizontal surface is equal to the magnitude of the weight of that object.

True.

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Question

Which of the following is an example of a system that contains both a contact and a non-contact force?

Two magnets repelling each other.

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Question

Which of the following contact forces enables us to walk?

Friction.

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Question

Is gravity a contact force?

No.

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Question

Is tension a contact force?

Yes.

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Question

An object is stationary or moves with constant velocity if all forces on it are balanced.

True

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Question

An accelerating object has no resultant force acting on it.

False

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Question

For a stationary object on a flat horizontal plane, the normal contact force on the object is less than the weight of the object.

False

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Question

For a stationary object on a flat horizontal plane, the applied force on the object is _____ and opposite to the friction force on the object.

equal to

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Question

For a stationary object on an inclined plane, the perpendicular component of the weight is equal to and opposite to the normal force on the object.

True

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Question

For a stationary object on an inclined plane, the parallel component of the weight is _____ and opposite to the friction force on the object.

equal to

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Question

For an accelerating object on a flat horizontal plane (i.e. its speed increases), the normal contact force on the object is greater than and opposite to the weight of the object.

False

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Question

For an accelerating object on a flat horizontal plane (its speed is increasing), the applied force on the object is less than the friction force on the object.

False

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Question

For an object accelerating down an inclined plane (its speed is increasing), the perpendicular component of the weight is equal to and opposite to the normal force on the object.

True

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Question

For an object accelerating down an inclined plane (its speed is increasing), the parallel component of the weight is ______ the friction force on the object.

greater than

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Question

There is a resultant force on an object that is accelerating down an inclined plane.

True

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Question

There is no resultant force on an object that is accelerating on a flat horizontal surface.

False

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Question

The weight of an object on an inclined plane acts vertically downward.

True

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Question

We can split the weight of an object on an inclined plane into a component that is parallel to the plane and a component that is perpendicular to the plane.

True

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Question

The weight of an object on a flat horizontal surface is zero.

False

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