4.3 Newton’s Second and Third Lawspp. 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 (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