1. How to draw, read, and work with free body diagrams.

2. Identify your mass
This is a box, circle, or simple sketch that shows your object and is labeled with its mass.
For most objects a box or a circle is enough detail.
12 kg

3. Is it on a surface or falling?
If the object is on a level surface, draw a line representing the ground.
If it is off the ground don’t include a surface and realize that there will be no normal force.
Normal
12 kg
12 kg
12 kg
Gravity
Gravity

4. Calculate the force of Gravity
The acceleration due to gravity for any object near the surface of the earth is 9.8 m/s2
Newton’s Second Law shows the relationship between force and mass to be F=ma.
The Force of Gravity is equal to mass (given in the problem) multiplied by acceleration (9.8m/s2.)
12 x 9.8 = 117.6
Normal
12 kg
Gravity
117.6 N

5. Gravity is equal to Normal
Newton’s First Law states that an object at rest stays at rest unless acted on by an unbalanced force.
The force of gravity must be balanced by an equal and opposite force, or the object would be accelerating downwards.
The name for the force opposing gravity is the normal force.
Normal
117.6 N
12 kg
Gravity
117.6 N

6. For objects on a surface
If there is an acceleration along the ground, there must be a force.
If there is a force on a level surface, it will be opposed by a friction force.
Opposing forces
point in exactly
opposite directions.
Normal
117.6 N
Applied
Force
Friction
12 kg
Gravity
117.6 N

7. Add forces to find total force
Add together arrows point in the same direction.
Subtract arrows that point in opposite directions.
The final total is the total force.
Use the total force to find the acceleration of the object.
Normal
117.6 N
Total Force = Applied minus Friction
30N - 10N = 20N
Acceleration = Force divided by mass
a = 20/12 = 1.67m/s2
Friction
10 N
Applied Force
30 N
12 kg
Gravity
117.6 N

8. Remember Newton’s Second Law
Units to Know
Force will always be given in Newtons (N)
Acceleration will always be given in meters per second squared (m/s2)
Mass will always be given in kilograms (kg)
Remember Newton’s Second Law
Force = Mass x Acceleration (F=ma)
Used to find force to label the FBD, or to calculate the acceleration from the total force
Mass = Force ÷ Acceleration
Acceleration = Force ÷ Mass