1. PHYSICS 197 Section 1 Chapter N4 Statics
October 18, 2017

2. Forces from Motion: Overview
Apply Newton’s
2nd law of motion
Knowing the forces involved are useful in many practical physical contexts, e.g.

3. Introduction to Statics
The simplest motion is no motion.
Statics problem: Any problem involving an object at rest.
By Newton’s 2nd law, the net external force on the object must be zero.
Simple principle: Balance the forces.

4. Clicker Question
N4T.1 A weight hangs from a string but is pulled to one side by a horizontal string, as shown in the figure. The tension force exerted by the angled string is
Less than the hanging object’s weight
Equal to the hanging object’s weight
Greater than the hanging object’s weight
Depends on other factors

5. Answer
N4T.1 A weight hangs from a string but is pulled to one side by a horizontal string, as shown in the figure. The tension force exerted by the angled string is
Less than the hanging object’s weight
Equal to the hanging object’s weight
Greater than the hanging object’s weight
Depends on other factors

6. Practice Problem

7. Demo: Sharing the Weight
In both of the cases depicted below, the same weight is supported by two strings. In which case are the students holding the strings exerting a force of greater magnitude?
Case A
Case B
Both the same

8. Demo: Sharing the Weight
In both of the cases depicted below, the same weight is supported by two strings. In which case are the students holding the strings exerting a force of greater magnitude?
Case A
Case B
Both the same
Smaller component of tension
pointing upwards. So need larger tension
to balance the weight.

9. Clicker Question
N4T.2 A person would like to pull a car out of a ditch. This person ties one end of a chain to the car’s bumper and wraps the other end around a tree so that the chain is taut. The person then pulls on the chain perpendicular to its length, as shown in the figure. The magnitude of the force that the chain exerts on the car is
Much smaller than the force the person exerts on the chain
About equal to the force the person exerts on the chain
Much bigger than the force the person exerts on the chain.
Need more information.

10. Answer Much smaller than the force the person exerts on the chain
About equal to the force the person exerts on the chain
Much bigger than the force the person exerts on the chain.
Need more information.

11. Demo: Hangover
How far out is it possible to overhang blocks from the edge of a table without them toppling?
Is it better to make
Big displacements at the bottom of the tower and smaller at the top
Uniform displacements all the way up the tower
Smaller displacements at the bottom and larger at the top?
The Leaning Tower of Lire

12. With One Block Let’s start with one block.
As long as its center-of-mass remains over the table, the block is stable.
So we can slide the block maximum half-way over the edge.

13. With Two Blocks

14. With Three Blocks

15. With Four Blocks

16. With n Blocks

17. Infinite Tower! Divergent, but very slowly! Try at home!
More details:

18. Static Problems Involving Torque
Consider a situation where multiple forces act at different positions on an extended object at rest.
These forces exert torques on that object.
If the object is at rest, its angular momentum is conserved.
So the net torque must also be zero (along with the net force, as per Newton’s 2nd law):
Recall

19. Demo: Mass Hanging from Stick

20. Demo: Mass Hanging from Stick

21. Practice Problem and Demo

22. Solution