Week 12 (Apr. 1 - 5)

Read: Conservation of Momentum (ASGv2 Chap. 22) and The Third Law and the Power of Machines (ASGv2 Chap. 23).

Key topics: applications of newton's laws; conservation of momentum; center of mass position and velocity; elastic and inelastic collisions,

PHY 201 Lecture: Newton's laws of motion:
Quiz: There will be no quiz on week 12 since we don't have class on Monday.


  1. Force addition and Newton's Second Law (Ex. 22.1)
  2. A 1 gram spider is suspended by two threads. One attaches to the wall, the other to the ceiling. If the one attached to the wall is perfectly horizontal, and the one attached to the ceiling makes a 30 degree angle with the ceiling, then what is the tension in each thread? Solution.
  3. Ball stuck in a wedge (Ex. 22.2),
  4. Colliding Blocks (Ex. 22.4)
  5. Free Body Diagrams and projectiles (Ex. 23.1)
  6. PHY 201: Colliding steel pendulums (Ex. 22.3)

Lab: Force and rotation (Ex. 20.3). You will be guided through the lab by the "Force and Rotation Lab" . I would recommend that you watch the "Force and Rotation" lab videos from last week before coming to lab this week. These videos will greatly assist you in data collection and analysis.

Chapter 22 (9 videos). What happens when more than one force pushes or pulls on an object? In the first video below, I explain Newton's approach to force addition.

In the next video, I show how to find the acceleration of an object when two forces are pushing on it.

In the next five videos, I walk you through examples of how to use Newton's laws of motion. The first video deals with how to set up force diagrams involving weight and normal force.

Next is a problem involving tension forces exerted by strings or cords pulling on an object:

Next, a ramp problem involving friction, normal force, and weight:

What causes your car to accelerate when you hit the gas pedal? In the next video, I explain how the friction force exerted by the road on the tires is what "throws" your car forward when you hit the gas:

In the next three videos, I explain how the law of conservation of momentum follows from Newton's second and third laws of motion. What are some of the implications of the conservation of momentum?

Chapter 23 (5 videos):
In the next video, I discuss how Newton applies his laws of motion to understanding projectile motion—specifically how Galileo's kinematic equations for artillery follow form Newton's laws.

When a swinging pendulum bob collides with another, initially stationary, pendulum bob, it imparts motion to the second bob. How much? Newton describes how the conservation of momentum can be used to predict how much momentum is lost by the first bob and how much is gained by the second.

As it ends up, when two objects collide, the amount of momentum lost by the first object, and the amount of momentum gained by the second, depends on how hard or soft the two objects are: one must know the coefficient of restitution of the objects. More generally, in order to completely solve many collision problems, one must know whether the kinetic energy of the objects is conserved.

The next video, designed for PHY 201 students, explains how to solve a more complicated collision problem using the laws of conservation of momentum and conservation of energy.

Newton's laws of motion imply that you weigh as much as the entire Earth. How so? In the next and final video, Newton explains the surprising and counter-intuitive implications of his third law of motion.

Physics 1