1. Unit Two: Dynamics
Newton’s Third Law

2. Newton’s Third Law Summary
When one object exerts a force on a second object, the second object exerts a force on the first that is equal in magnitude but opposite in direction.
These forces are called action-reaction forces.
Ex: If you push against a wall, you don’t go through it as the wall “pushes back”.
Only the forces on an object determine its acceleration.

3. Newton’s Third Law
• With equal and opposite forces, how does anything ever move?
Example: Picking up a ball:
Ball exerts an equal force on your hand, but this is not on the ball and does not appear in the free body diagram

5. Example
Suppose you are floating around in space (many km from any planet so that you feel no gravity) outside of your spaceship. You get frustrated and decide to kick your spaceship. Does your foot hurt?

6. Solution
Yes, your foot will hurt. Even though there is no gravity, Newton’s Third law still applies. If you kick the spaceship, it applies an equal and opposite force on your foot.

7. Newton’s Third Law Worksheet
*** Homework: Complete this sheet.
Complete the online comment/question about the lab.

8. System of Masses (Summary)
When two or more masses are attached by a string or rope and hang over a pulley system, there is a system of masses.
Some assumptions that must be made:
- Strings only exert pulling forces.
- The tension in the string is the same throughout its length.
- A frictionless pulley changes the direction of a string without diminishing its tension.
- Strings do not stretch.
- The strings’ mass is negligible.

9. Tension – Quick Reminder
Tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object.
It is measured in Newtons
It is measured parallel to the string on which it applies

10. Example 1 (Atwood Machines)
Two spheres of masses 1.5 kg and 3.0 kg are tied together by a light string looped over a frictionless pulley (called an Atwood machine). They are allowed to hang freely. What will be the acceleration of each mass?

11. Ask yourself how the system will move:
First, we know that mass m is falling and dragging mass M off the table. The force of kinetic friction opposes the motion of mass M. However, we know that friction is negligible here because it is a smooth surface!
We also know, since both masses are connected by a nonstretching rope, that the two masses must have the same speed and the same acceleration.

12. Example 2
A 2.0 kg mass, placed on a smooth, level table is attached by a light string passing over a frictionless pulley to a 5.0 kg mass hanging freely over the edge of a table.
A) Draw a free body diagram of the masses
B) Calculate the tension in the string
C) Calculate the
acceleration of the
2.0 kg mass

13. Answer

14. Answer continued
The masses move together – so the accelerations are the same!
The forces are slightly different. How?

15. http://schools. hwdsb. on
Systems of Masses Worksheet