Types of Forces

Types of Forces

Middle School

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Learning Objectives

• Define force and tell the difference between contact and non-contact forces.

• Identify and describe different types of contact forces like muscular and frictional forces.

• Explain gravity as a non-contact force and how it depends on mass.

• Analyze the causes and effects of friction and air resistance in daily life.

Key Vocabulary

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Force

A push or a pull on an object, which is measured in a unit called Newtons.

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Contact Force

A force that requires physical contact between objects to change an object's motion.

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Non-contact Force

A force that acts on an object without any physical contact with it, like gravity.

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Friction

A contact force that opposes motion between any two surfaces that are touching each other.

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Gravity

A non-contact force that pulls objects with mass toward one another, such as toward Earth.

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Normal Force

A support force from a stable surface on an object that is resting directly on it.

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What is a Force?

Push Force

• A push is a force that moves an object away from the source.

• This type of force requires direct contact with the object being moved.

• An example is pushing a box, which is a type of contact force.

Pull Force

• A pull is a force that brings an object closer to the source.

• The strength of a force is measured in a unit called Newtons (N).

• A spring scale is used to measure the amount of pull force.

Multiple Choice

What concept is described as a push or a pull that an object experiences as a result of interacting with another object?

• Acceleration

• Speed

• Energy

• Force

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Contact vs. Non-Contact Forces

Contact Forces

• These forces are applied only when objects are in direct physical contact.

• Contact forces are also known as mechanical forces, requiring direct interaction.

• An example is the muscular force you use to lift a book.

Non-Contact Forces

• These forces can act on objects from a distance without any touching.

• A common example is gravity, which pulls objects toward the Earth.

• Magnetic force is another example, attracting items like paper clips from afar.

Multiple Choice

Which of the following is described as a force that acts on an object without physically touching it?

• Mechanical Force

• Muscular Force

• Gravity

• Friction

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Types of Contact Forces: Applied & Muscular

Applied Force

• An applied force happens when one object directly pushes or pulls on another.

• It is a contact force that results from the interaction between two objects.

• Kicking a soccer ball or pushing a door are examples of applied forces.

Muscular Force

• This is a specific type of contact force that your body’s muscles generate.

• It is essential for performing all of your body's movements, like walking or running.

• Muscular force also powers internal body functions like breathing and your heart beating.

Multiple Choice

Which force is generated by the body to perform actions like breathing or pushing a cart?

• Applied Force

• Normal Force

• Gravity

• Muscular Force

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Types of Contact Forces

Normal Force

• ​This is a support force a surface exerts on an object resting on it.

• ​​It supports the weight of the object and prevents it from falling through the surface.

• ​For example, a table applies an upward normal force to support a book's weight.

Friction

• ​This contact force opposes motion between two surfaces that are touching each other.

• ​​It happens because even smooth surfaces have tiny bumps that resist movement.

• ​Friction helps us walk, but in machines, it can cause heat and wear.

Multiple Choice

What is the primary purpose of the normal force?

• To oppose the motion of an object

• To directly push or pull an object

• To provide support to an object resting on a surface

• To pull objects toward each other from a distance

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Air Resistance vs. Gravity

Air Resistance

• This is a frictional force that acts on objects as they move through the air.

• It is an upward force that opposes the downward pull of gravity, slowing an object down.

• A parachute traps air to create resistance, which helps to slow down a person’s fall.

Gravity

• This is a non-contact force that pulls any two objects with mass toward one another.

• On Earth, gravity is the force that pulls everything downward toward the planet’s surface.

• The strength of gravity is greater for objects that have a much larger amount of mass.

Multiple Choice

How is air resistance different from gravity, based on the descriptions?

• Air resistance is a frictional contact force, while gravity is a non-contact force.

• Gravity is a push, while air resistance is a pull.

• Gravity pulls objects up, while air resistance pulls them down.

• Air resistance only works on parachutes, while gravity affects everything.

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Common Misconceptions About Forces

Misconception	Correction
Smooth surfaces have no friction.	Even smooth surfaces have microscopic irregularities that cause friction.
Force is always a visible push or a pull.	Forces can be invisible, like gravity and magnetism.
Heavier objects always fall faster.	Gravity pulls all objects equally, but air resistance can slow some down.
The normal force is always equal to an object's weight.	It only equals the weight on a flat, horizontal surface.

Multiple Choice

How do the characteristics of two surfaces in contact influence the force of friction between them?

• The roughness of a surface has no impact on friction.

• Friction is only dependent on the mass of the objects.

• Rougher surfaces have less friction.

• Rougher surfaces have more friction.

Multiple Choice

Why does a book resting on a table not fall through it? Explain by referencing two key forces.

• The applied force on the book is balanced by the force of gravity.

• Muscular force holds the book up against the force of gravity.

• Air resistance pushes the book up, and friction holds it in place.

• Gravity pulls the book down, and the table exerts an equal and opposite normal force upward.

Multiple Choice

When a skydiver opens their parachute, they slow down. What is the correct explanation for this change in motion?

• Air resistance becomes significantly stronger than the force of gravity, causing a rapid deceleration.

• Gravity is turned off, and only air resistance acts on the skydiver.

• The parachute and gravity cancel each other out, making the skydiver stop in mid-air.

• Air resistance and gravity both start pulling the skydiver down, but at different speeds.

Multiple Choice

If you slide a heavy box across a rough floor, it eventually comes to a stop. What are the forces that explain this entire process, from the initial push to the final stop?

• The normal force from the floor becomes stronger than the applied force, stopping the box.

• The applied force gets the box moving, but the force of friction acts in the opposite direction, causing it to slow down and eventually stop.

• Gravity pulls the box down, which is the sole reason it stops moving.

• The box stops because the initial applied force runs out over time.

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Summary

• All forces are a push or pull and can be contact or non-contact.

• Contact forces require objects to touch, like friction which opposes motion.

• Air resistance is a type of friction for objects moving through the air.

• Non-contact forces like gravity act from a distance and depend on mass.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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