1. Net Force
Adding force vectors

2. Net Force Forces are vectors. Forces on an object add together.
Forces can oppose each other.
Net force is the vector sum of all forces acting on a body.
The net force on a body at rest is zero.

3. Hammock Example
8 m
1 m
weight F
A hammock slung between trees 8 m apart sags 1 m when a person lies in it.
The net force acting on the person is
Equal to the weight of the person.
Equal to the tension in one cable.
Zero.
There is not enough information to answer.

4. Working with Commonly-Encountered Forces
Tension

5. Tension Forces In cables, threads, chains, etc.
Direction: along the cable, inward

6. Hammock Tension
8 m
1 m
weight F
A hammock slung between trees 8 m apart sags 1 m when a person lies in it.
The tension in one cable is
Equal to the weight of the person.
About half the weight of the person.
Zero.
Much more than the weight of the person.
There is not enough information to answer.

7. Hammock forces
Tension is the same direction as rope stretch: 4 horizontal: 1 vertical
Add up the vectors! 4 1
Here, tension is over twice the weight! 2

8. Hammock Forces forces add to zero
weight
tension
forces add to zero
Tension exceeds weight for a shallow angle!

9. Application: Lumbar Forces
Spinal curvature
standing
sitting

10. Application: Lumbar Forces
Reaching with a load
weight
weight
standing
sitting

11. Application: Lumbar Forces
Standing
load
support
tension

12. Application: Lumbar Forces
Sitting
load
support
tension
huge!