4.3 Newton’s Second and Third Laws

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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

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)
• “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