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4.3 Newton’s Second and Third Laws. pp. 136 - 140 Mr. Richter. Agenda. Warm-Up Check HW Newton’s Second Law Calculations with N2L Newton’s Third Law. Objectives: We Will Be Able To….

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4 3 newton s second and third laws

4.3 Newton’s Second and Third Laws

pp. 136 - 140

Mr. Richter


Agenda
Agenda

  • Warm-Up

  • Check HW

  • Newton’s Second Law

  • Calculations with N2L

  • Newton’s Third Law


Objectives we will be able to
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
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
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 law1
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
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
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
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 n3l1
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 n3l2
Newton’s Third Law (N3L)

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


Newton s third law n3l3
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
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
Homework

  • Due Tomorrow:

    • p. 133 #1-4

    • p. 135 #1-3

  • Due Monday:

    • p. 140 #1-5

  • Work on Proposal