4.3 Newton’s Second and Third Laws

1. 4.3 Newton’s Second and Third Laws pp. 136 - 140 Mr. Richter

2. Agenda • Warm-Up • Check HW • Newton’s Second Law • Calculations with N2L • Newton’s Third Law

3. Objectives: We Will Be Able To… • Describe the acceleration of an object in terms of its mass and the net external force acting on it (N2L) • Predict the direction and magnitude of the acceleration caused by a known net external force. • Identify action-reaction pairs. • Explain why action-reaction pairs do not result in equilibrium.

4. Warm-Up: • Gravity pulls straight down on an object sitting on a 20° incline with 50 N of force. Draw a diagram, then calculate the components of gravity along the incline and perpendicular to the incline.

5. Newton’s Second Law (N2L)

6. Newton’s Second Law • The force required to accelerate an object depends on two quantities: • mass – how much stuff are you trying to move • acceleration – how quickly are you trying to move it

7. Newton’s Second Law • The relationship between force, mass and acceleration are quantified in Newton’s Second Law (N2L). • In other words:

8. Calculations with N2L • Problems involving N2L come in a few basic forms. • You are given 2 out of 3 of net external force, mass and acceleration and asked to find the third quantity. Easy. • You are asked to calculate acceleration beforehand, or find acceleration and then use it to determine something else about motion afterward. More involved, but also easy. • You are asked to calculate the net force first (like in the previous section), and then use that to calculate acceleration.

9. Practice Problems • A 10.0 kg object initially at rest on an incline experiences a net external force of 4.50 N down the ramp. • What is the acceleration of the object? • How fast will the object be travelling after 2.00 seconds?

10. Newton’s Third Law

11. Newton’s Third Law (N3L) • In your head, complete the following sentence: • “For every action there is… • “an equal and opposite reaction.” • This is essentially Newton’s Third Law.

12. Newton’s Third Law (N3L) • Forces only come in action-reaction pairs. • This is even true of field forces like gravity, which we will discuss later. • There are no solo forces! • It doesn’t matter which force is the action and which is the reaction, because they are equal.

13. Newton’s Third Law (N3L) • Action-reaction forces exist even when objects aren’t moving!

14. Newton’s Third Law (N3L) • If action and reaction forces are equal, how does anything move? • The hammer and the nail exert equal and opposite forces on each other. • Action-reaction forces DO NOT result in equilibrium for a single object. • The forces act on different objects. Look at the FBD of the nail itself.

15. Wrap-Up: Did we meet our objectives? • Describe the acceleration of an object in terms of its mass and the net external force acting on it (N2L) • Predict the direction and magnitude of the acceleration caused by a known net external force. • Identify action-reaction pairs. • Explain why action-reaction pairs do not result in equilibrium.

16. Homework • Due Tomorrow: • p. 133 #1-4 • p. 135 #1-3 • Due Monday: • p. 140 #1-5 • Work on Proposal