# Forces

Dynamics

Reference - Forumla Sheet

## Content Overview

Activities

Inertia Stations Lab - Card flick, coin catch, tablecloth trick, hoop dreams, daruma otoshi

Egg Drop Water Demo - Can we safely swipe the supports out from underneath a few eggs?

Clever Dumbbell Demo - Investigating the behavior of inertia on a heavy mass

Pushing Things Around - PhET Computer Simulation

Newtonian Shot - If we launch two different masses, which will go further and faster?

Spring Scale Investigation of Newton's 3rd Law Lab

Fan Cart & Drone - Observe the third law by pushing on air

(Optional, Time Permitting) Center of Mass Demos - Chair pick-up challenge, one-hoof horse, cantilevered bridge, weighted wheel, loaded dice, balanced bottle holder, flying eagle

Topics

Newton's Laws, 1st 2nd & 3rd

Free Body Diagrams

Weight & Apparent Weight

## Students are Expected to Understand

Measuring and calculating forces and related values

Drawing and interpreting free body diagrams

Resolve common contradictions related to forces

### Some Notes on Forces

• Newton's 1st law deals with the ability of mass to remain at the same velocity. Anything with mass resists acceleration, and the more mass it has, the more it resists. We call this inertia. Objects without any interference from net forces will continue to move the same way it has before, whether at rest or moving quickly. This also includes spinning
• Newton's 2nd law explains how mass and net force affect the ability to change an object's velocity. Think about it. Who would be easier to push? Mr. Ho or Mr. Ahn, the bigger teacher next door in Room 12? Right, smaller masses like Mr. Ho are easier to accelerate. And the bigger the push, the bigger the acceleration.
• Newton's 3rd law focuses on the idea that every force has an equal and opposite force. It's like a mirror, and these pairs of forces cannot exist without the other.

After studying the pattern behind moving things, we begin looking into what causes things to move. Forces is also an important foundation unit that later topics will rely on. Forces may also be referred to as Dynamics.

*These videos are to be used as a support and are not a replacement for in-class experiences

# Inertia

What many also call Newton's 1st Law, it describes how objects keep at a constant velocity. Whether that's 0 m/s or 5 m/s, it's a way to explain how things in our universe like to stay doing what they're already doing. In this case, we see a truck move forward while the carts, without anything to push them forward with the truck, maintain a steady velocity of 0 m/s. What could these dock loaders have done to prevent this from happening?

# Free Body Diagram

What is the net force of this object along the y-axis? If the forces are equal and opposite along an axis, there is no net force. This means that there is no acceleration along that axis

What is the net force of this object along the x-axis? The forces along this axis are not balanced, so there is a net force. Add up all of your forces together to find your net force, and remember that anything left or down is negative.

Given a mass for this object, how would you find its acceleration? Once you know your net force and mass, use Newton's 2nd Law to solve for this

# Force Pairs

Also called Newton's Third Law, the idea is that every force, every push has an exact copy in the other direction. In the image here to the left, Wall-E wishes to move in space with Eve, but without anything to push off, Wall-E is stuck... until Wall-E finds a fire extinguisher. The gas from the canister pushes Wall-E around. Which direction does Wall-E point the extinguisher in order to move in the desired direction? What direction does your foot push so that you move forward?

## Zero-G Flights

Part of astronaut training now involves sending people in planes to simulate the experience of microgravity. Previously, they put astronauts underwater or on pulleys to reduce their weight

## Along a Parabolic Path

Nicknamed the vomit comet, the plane flies sharply upward making you feel heavy. If you were to stand on a scale during this part of the flight, you would weigh almost twice as much!

## Change in Apparent Weight

At the top of the flight, you will have over 20 seconds of weightlessness as you and the plane are in a free fall together. Without anything pushing on you - not even the plane - you are free to float about