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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

pp. 136 - 140

Mr. Richter

• Warm-Up

• Check HW

• Newton’s Second Law

• Calculations with N2L

• Newton’s Third Law

• 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.

• 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.

• 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

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

• In other words:

• 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.

• 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?

• “For every action there is…

• “an equal and opposite reaction.”

• This is essentially Newton’s Third Law.

• 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.

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

• 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.

• 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.

• Due Tomorrow:

• p. 133 #1-4

• p. 135 #1-3

• Due Monday:

• p. 140 #1-5

• Work on Proposal