Applying Newton's Laws: Free Body Diagrams

Unit 4 Lesson 1E. Alexander BurtPotomac School

Action – Reaction pairs

Remember Newton's third law: forces come in pairs.

The forces are equal and opposite

The forces act on different objects.

Free Body Diagrams

We will only look at one object – therefore we will only be looking at half of each action – reaction pair.

We will draw the forces as vector arrows on the object

Magnitude (size) of the force is length of arrow

Direction of the force and direction of arrow should be the same

“Forces with names” to remember

Weight: The force caused by gravity on mass

Normal force: perpendicular to surfaces in contact.

Friction force: parallel to surfaces in contact.

Tension forces: pulling forces that run along strings and ropes.

Net Force

After you have the free body diagram:

Choose a set of coordinates

Find the x and y components of all the vectors

Find the x sum and the y sum

Notice that this is exactly the same procedure as adding vectors.

Net force is the VECTOR sum of the forces.