Free-Body Diagrams

Free-Body Diagrams

By Kyle Sheahon

For example, friction and weight act on you at all times. To figure out how (or if) things are going to move, we need to add up all the forces acting on them.

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The sum of all the forces acting on an object is called the NET FORCE.​

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We never feel one force at a time.

• Free-body diagrams help us add up all the forces in order to determine the NET Force.

• This helps us to predict how the object will move.

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Free-Body Diagrams

Let's look at the forces acting on an box sitting on a table.

• ​This box is sitting at rest.

• The force of gravity is equal to the normal force.​

• Normal Force - the force that a surface exerts on an object that is pressing on it.​

• The normal force is equal and opposite to the force pressing the object into the surface.

The book is not moving, so the forces must be equal.

• When the forces being applied to an object are equal, the object isn't moving.

• This means that these are Balanced Forces.

• Balanced Forces - forces that result in a net force (total force) of zero.​

When forces are balanced, their NET force is zero. This means the object is in equilibrium.

• Equilibrium - the state in which the net force on an object is zero.

• If the object is not in equilibrium. The object will move!

Multiple Choice

Is this a balanced force?

• Yes

• No

Draw

Circle the guy who successfully pushes the box.

Multiple Choice

What is the NET Force?

• 10 N

• 5 N

• 10 N to the right.

• 5 N to the right.

Forces are not just defined by how strong they are but the direction they are moving.

• ​Forces are vectors, which means it has both a magnitude (strength) and direction.

• You will always describe a force with its strength and direction.

• Vector - a quantity that has both a magnitude and direction.​

Let's look at the forces acting on an airplane in flight.

• ​The plane is at constant speed because the Drag force and the Thrust force are balanced.

• The plane is at constant height because the Lift force and the Weight are balanced.​

Draw

Circle the resident of Bikini Bottom who successfully pushes the box.

Fill in the Blank

What is the resulting force?

Multiple Choice

Is the rope in equilibrium?

• Yes

• No

• Not enough info.

Fill in the Blank

If the guy is pushing the box with a force of 25 N, and the friction force on the box is 15 N, what is the Net force he is pushing the box?

Fill in the Blank

Is Joe moving this box?

Multiple Select

Select all that apply.

• Balanced force

• 25 Newtons

• Positive Net Force

• Equilibrium

Fill in the Blank

If the boy in the hat pulls with a force of 28 N, and the other boy pushes with a force of 12 N, what is the NET Force?