This note consists of Short Youtube Lecture Videos for each subtopic. Source: Andy Masley, IB Physics Teacher, https://andymasley.com/2-mechanics/

**2.1: Motion**

Video Lectures:

- Motion Maps/Dot Diagrams
- Distance & Displacement
- Speed & Velocity
- Average vs. Instantaneous
- Position-Time Graphs
- Velocity-Time Graphs Part 1: Constant Velocity & Area Under the Curve
- Translating Between Position-Time and Velocity-Time Graphs
- Acceleration
- Velocity-Time Graphs Part 2: Changing Velocity & Slope
- Acceleration-Time Graphs
- Translating Graphs
- Kinematic Equations
- Proof of Kinematic Equations
- Free Fall
- 2D Projectile Motion Kinematics
- Air Resistance on Projectiles

**2.2: Forces**

Video Lectures:

- Forces & Free Body Diagrams
- Common Forces
- Force Webs
- Finding the Net Force
- Mass vs. Weight
- Static vs. Kinetic Friction
- Newton’s First Law
- Newton’s Second Law
- Newton’s Third Law
- Ramp Problems

**2.3: Work, Energy, and Power**

Video Lectures:

Video Lectures:

- Work
- Work & Energy
- Kinetic Energy
- Gravitational Potential Energy
- Elastic Potential Energy
- Thermal Energy
- Other Types of Energy
- Open & Closed Systems in Energy
- LOL Diagrams
- Force Distance Graphs
- 2 Example Energy Conservation Problems
- Power
- Efficiency
- Sankey Diagrams

2.4: Momentum and Impulse

2.4: Momentum and Impulse

Video Lectures:

- What is Momentum?
- Impulse & Force time Graphs
- Conservation of Momentum
- Open & Closed Systems in Momentum
- Collisions

*Definition of Momentum:*Variable: p, Unit: Ns or kgms^-1 Equation: p = mv

The momentum of an object is its mass x its velocity. An easy way to think about momentum is “how difficult it is to make an object stop.” So a truck moving at 5 m/s has more momentum than a bike moving at 5 m/s because the truck has more inertia (more mass) than the bike and would be more difficult to slow to a stop. A baseball moving at 90 m/s has more momentum than the same baseball moving at 5 m/s, because the faster object will take more force to stop. A stopped truck has no momentum because it requires no effort to stop. More specifically, momentum can be thought of as the amount of force x the amount of time required to make an object stop. For example, an object with 10 Ns of momentum could be stopped by a force of 10 Newtons applied for 1 second, 5 Newtons applied for 2 seconds, or 1 Newton applied for 10 seconds.

Worksheet – Solution Video

**Why do we care about momentum?**All concepts in physics are just tools for solving problems. Momentum is just a name we gave to Mass x Velocity because mv has very specific properties that make problem solving easy. We could give any combination of variables a name, but not all combinations of variables are useful for problem solving.

**Why is momentum useful for problem solving?**The total amount of momentum in the universe never changes (this will be proved below). Because it never changes, it is very predictable. As an example, if an object has 20 Ns of momentum, and it drops to 12 Ns of momentum, we know that some other object must have gained 8 Ns of momentum, because the missing 8 Ns cannot disappear from the universe.

**We say that a quantity is conserved if it can never be created or destroyed, so momentum is a conserved quantity.**

**Impulse**Variable: Δp Unit: Ns or kgms^-1. Equation: Δp = (p-final – p-start) = Ft

Impulse is the force applied on an object x the time it is applied for. It is also equal to the change in momentum caused by that force. We can prove this for an object that keeps its mass but changes its velocity:

Δp = (p-final – p-start) = m*(v-final) – m*(v-start) = m*(v-final – v-start) = m*Δv = m*(Δv/t)*t (we can always multiply and divide an equation by the same number) = ma*t = F*t (because F = ma).

Worksheet – Solution Video

*Proof that Momentum is Conserved:*Momentum is mv. We will assume for now that the masses of objects do not change, so the only way to change momentum is to change velocity.

We know from Newton’s Second Law that every force an object experiences is a vector equal to the object’s mass x the acceleration created by that force.

We know from Newton’s Third Law that the only way object 1 can experience a force from object 2 is if it also creates a force of the same magnitude in the opposite direction on object 2.

So Newton’s Third Law shows that Force on object 1 = -Force on object 2 (the negative indicates opposite direction). We can multiply both sides by the time the forces are applied. These times are the same, because it’s not possible for object 1 to apply a force on object 2 for a longer time than object 2 applies the reaction force back on object 1. So we get (Force on object 1)*t = (-Force on object 2)*t. Because Ft = impulse = Δp, this equation shows that Δp object 1 = -Δp object 2. This means that however much momentum is gained by object 1 will be lost by object 2, so the momentum can never just disappear. It is conserved.

__Conservation of Momentum Problems:__

**Open vs. Closed Systems in Momentum:***Momentum is always conserved, but there are objects we cannot do calculations with. For example, if a ball impacts the ground, its momentum is transferred to the Earth, but the Earth is too big and has too much momentum to accurately predict its change in motion. Momentum is still conserved, but the values in our equation become so big that we cannot do math with them. Describing systems as opened or closed is a way to deal with this problem.*

A system is a set of objects. That’s all. We can include any objects we would like in our system. Some possible systems: my coffee cup, my coffee cup and the table, my coffee cup and my table and the Great Pyramid of Giza, a bird, a bird and the Earth, a bird and the Earth and the black hole at the center of the galaxy. What I’m trying to show is that we can call any combination of objects a system.

A closed system is a system where momentum does not enter or leave the system. The objects in the system can give momentum to each other, but no outside objects can gain or lose momentum from the objects.

An open system is a system where momentum enters or leaves the system from an outside object.

**Force-Time Graphs:**Background picture: Mechanics Desktop Background https://hipwallpaper.com/mechanics-desktop-backgrounds/