4.3 Newton’s Second and Third Laws pp 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 (N2L)

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. 1.What is the acceleration of the object? 2.How fast will the object be travelling after 2.00 seconds?

Newton’s Third Law

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