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4.3 Newton’s Second and Third LawsPowerPoint Presentation

4.3 Newton’s Second and Third Laws

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Presentation Transcript

Agenda

- Warm-Up
- Check HW
- Newton’s Second Law
- Calculations with N2L
- Newton’s Third Law

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.

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.

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

Newton’s Second Law

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

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.

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?

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.

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.

Newton’s Third Law (N3L)

- Action-reaction forces exist even when objects aren’t moving!

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.

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.

Homework

- Due Tomorrow:
- p. 133 #1-4
- p. 135 #1-3

- Due Monday:
- p. 140 #1-5

- Work on Proposal

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