Contact Force Examples: Different Types in Physics

Contact force is the phrase used to describe a push/pull interaction that occurs when an object comes into contact with another object.

There are several different types of contact forces. Each refers to a specific circumstance.

Air resistance force is the force exerted on an object when it comes in contact with air by traveling through it. It is friction between the air and another object or a person. When a downhill speed skier goes down a slope at a high rate of speed, the skier experiences air resistance force. Other situations where air resistance force is at play include:

• an airplane flying through the air
• a skydiver parachuting from an airplane to the ground
• riding a bicycle
  
• pitching a baseball to a batter
• passing a football to a wide receiver

Applied force occurs when a person or object directly applies force to another object, causing it to move. When you push a computer mouse that is sitting on a flat surface, this is an example of contact force between your hand and the mouse. The mouse would not move unless a force, in this case your hand, was in contact with it. Other examples include:

• using your hand to pull out a desk chair so you can sit down
• kicking a ball in a way that causes it to move
• pushing an item to knock it over
• twisting the lid on a jar to open it
• pulling or pushing on a door to open or close it

Frictional force, also known as friction, is any force that opposes motion. In other words, it is a force that tries to slow or stop something from moving. Friction causes heat. The frictional force you are most familiar with is probably the one generated when you rub your hands together to keep them warm. It happens whenever two objects slide against one another. Other examples of friction include:

• going down a slide on the playground; you slow down at the bottom because friction overcame gravity (which sped you up when you were going down)
• a cross-country skier traveling across the snow on skis attempting to overcome the sliding friction trying to slow him or her down
• pulling a piece of furniture from one spot to another such that it slides along the floor causes a great deal of friction
• sliding a glass of water across a table to another person (the glass slows then stops due to friction between the bottom of the glass and the table)
• rubbing two pieces of wood together as a method of starting a fire without using a match

Normal force is the name for the force that keeps objects in place when they are resting on an object that is stable. It is the type of force that a surface, such as a shelf or a floor, exerts on an object that is resting on it. A book that is laying flat on a desk is kept in place due to the normal force exerted by the desk. No force would be exerted if the book was not in contact with the desk, so this is a contact force. Other examples include:

• a glass of water placed on a table
• containers of food sitting on the shelf of a refrigerator
• canned goods placed on the shelf in a pantry
• a person standing on the floor or ground
• a lamp placed on an end table

A spring is an elastic object that stores mechanical energy, such as a coiled spring or rubber band. Spring force is the force created when an external force causes a spring to change its shape. When the force is removed, the spring goes back to its ordinary shape. If you've ever used a click top ink pen, you have seen spring force at work. Click the button on the top of the pen to release the writing tip, then click it again to hide it away. This action is linked to a coiled spring inside the cap of the pen. Other situations where spring force is created include:

• pressing keys on a computer keyboard
• letting out or pulling in a retractable dog leash
• a piece of elastic that changes shape and snaps when pulled and released before returning to its original shape
• the cords and harnesses used by bungee jumpers
• a slinky toy

Tension is a pulling force. It occurs when objects like wires, ropes, cables, or rods are pulled tightly from both ends. The tighter the object is being pulled in opposite directions, the greater the tension is. Applying too much tension can cause the object to break. Playing tug of war is an example of a situation in which tension force is applied to a rope. Other situations where tension force can be seen include:

• a car that is being towed behind another vehicle using chains
• the cable that water skiers hold onto while being pulled by a boat
• the cable that the chairs of a ski lift travel go up and down
• a suspension bridge, such as the Golden Gate Bridge in San Francisco
• the cable in an elevator that lifts and lowers it

Reviewing practical examples of science at work can provide context for the information taught in science classes. Now that you have reviewed some contact force examples, take the time to learn about how physics impacts objects and processes in everyday life. Start by reviewing some common examples of non-contact forces. Then, discover balanced and unbalanced forces.