1. 2.1.2A Free Body Diagrams and Net Force
Drawing Pictures
2.1.2A Free Body Diagrams and Net Force

2. Free Body Diagrams VECTOR diagrams Show ALL FORCES acting on an object
Must be properly labeled
Equilibrium FN Fg

3. Determining Net Force equilibrium  net force = 0  acceleration = 0
MOTIONLESS or CONSTANT VELOCITY
net force ≠ 0
SPEEDING UP or SLOWING DOWN

4. Determining Net Force – Top View
Consider each dimension separately then combine using Pythagorean Theorem. X Y
2 kg object
+7.0 N
F1 = +7.0 N
+7.0 N
0 N
Net force = +7.0 N
a = 3.5 m/s2
Not in notes

5. Determining Net Force – Top View
Consider each dimension separately then combine using Pythagorean Theorem. X Y
2 kg object
+7.0 N
-3.0 N
F2 = -3.0 N
F1 = +7.0 N
+4.0 N
0 N
Net force = +4.0 N
a = 2.0 m/s2
Not in notes

6. Determining Net Force – Top View
Consider each dimension separately then combine using Pythagorean Theorem. X Y
2 kg object
F3 = +3.0 N
+7.0 N
-3.0 N
+3.0 N
F2 = -3.0 N
F1 = +7.0 N
+4.0 N
+3.0 N
Net force = 5.0 N
a = 2.5 m/s2
Not in notes

7. Determining Net Force – Top View
Consider each dimension separately then combine using Pythagorean Theorem. X Y
2 kg object
F3 = +3.0 N
+7.0 N
-3.0 N
+3.0 N
-5.0 N
F2 = -3.0 N
F1 = +7.0 N
+4.0 N
-2.0 N
Net force = 4.5 N
a = 2.25 m/s2
F4 = -5.0 N
Notes

8. Example #1
A 30 newton force and a 20 newton force act concurrently on an object.
What are the minimum/maximum net force can these two forces produce? At what angle between the forces does each happen?
What net force is produced when the angle between the two forces is 90°?
min = 10 N occurs at 180°
max = 50 N occurs at 0°
36 N

9. Determining Net Force – Side View
Consider each dimension separately.
On a flat surface total vertical force = 0. X Y
2 kg object
+8.0 N
-Fg
+FN
F = +8.0 N
+8.0 N
0 N
frictionless
Net force = +8.0 N
a = 4.0 m/s2
Not in notes

10. Determining Net Force – Side View
Consider each dimension separately.
On a flat surface total vertical force = 0.
Hor.
Vert.
2 kg object
+8.0 N
-2.0 N
-Fg
+FN
Ff = -2.0 N
F = +8.0 N
+6.0 N
0 N
Net force = +6.0 N
a = 3.0 m/s2
Notes

11. Example #2
A 50 kilogram object is pushed along a flat, frictionless surface with a constant force of 100 newtons.
Sketch this object. Include all forces with labels and quantities.
What rate of acceleration will the crate experience?

12. Determining Net Force – Vertical
Object moving straight up or down  total horizontal force = 0.
Hor.
Vert.
20 N object in freefall
-20 N
0 N
-20 N
Fg = -20 N
Net force = -20 N
a = m/s2
Not in notes

13. Example #3
What is the net force acting on a 3000 newton rocket if its engine produces an upward thrust of 4500 newtons?
Fthrust
1500 N upward Fg

15. End of 2.1.2A - PRACTICE