# Force

11SCI - Mechanics

2019

# Mahi Tuatahi

Brainstorm situations where force is involved, things that cause force and how it can be used in a Physics context on the board!

# Force

Force has lots of applications in our world! Everything from cars, to aeroplanes, tug-of-war, sports and even bio-mechanics!

## Defining Force

Force is a push or a pull and is measured in Newtons (N).

Forces have a size (1, 2, 3, 4) and a direction (left, right, up, down, $$40^{\circ}$$).

## How Forces Act

Consider you sitting on your seat. What forces are acting upon you?

Draw a box to represent yourself, with arrows coming out of the box to represent the forces. Make sure to label them!

## Force Diagram

• Longer arrows indicate more force, and vice versa
• The arrow points in the direction the force is acting
• Arrows should be labelled $$F_{name}$$

## Balanced Forces

Think and discuss with the people around you:

Sitting on your chair, are the forces acting on you balanced or unbalanced? How do you know? What does it feel like?

## Balanced Forces

• If a force is balanced, the object is in equilibrium and does not accelerate
• If the forces are unbalanced, the object is not in equilibrium and does accelerate

### Vertical and Horizontal Forces

• Vertical and horizontal forces are separate. They do not affect each other.
• We can balance them to find out the net force in the vertical and horizontal directions.
• If we compare them and they are the same, then forces are balanced. If they are different, forces are unbalanced.

# sciPAD Page 33-35

## Force & Weight

\begin{aligned} Force = mass \times acceleration \\ F = ma \end{aligned}
• Force acting upon a mass will create an acceleration
• An acceleration acting upon a mass is felt as a force
• ^ Gravity (acceleration) acting upon mass (you) is felt as a force (weight)
• Weight is a force

## Weight vs Mass

Mass Weight
Measured in $$kgs$$ Measured in Newtons
Is constant Changes with acceleration/gravity
Cannot be measured directly Can be measured using a scale

## Your Weight

• The acceleration due to gravity on Earth is $$10\frac{m}{s^{2}}$$.
• Calculate your weight on Earth using $$F=ma$$
• Use this website to visualise your weight on other planets: https://exploratorium.edu/ronh/weight/

## Mahi Tuatahi

Complete the questions on the last page of the worksheet from last period!

## Types of Forces

• Down: Weight (due to gravity)
• Up
• Lift (planes, birds, things with wings)
• Bouyancy (floating in water)
• Support (rigid physical connections)
• Faster: Push/thrust forces (rockets, rugby)
• Slower: Generally called resistive forces
• Drag (water/air)
• Friction (hard materials)

## Force Diagrams

• Equal and opposite forces should be the same length
• Greater forces have longer arrows
• Accelerating means unbalanced forces
• Balanced (zero net force) means no acceleration (constant velocity)

## Task

1. sciPAD page 34
2. sciPAD page 36
3. Use the QR code at the back to find the answers and mark your work!

## Ngā Whāinga Ako

1. Describe the relationship between mass and acceleration of an object which is acted upon by a given unbalanced force (called net force).
2. Use the relationship $$F = ma$$ to calculate the net force, mass or acceleration of an object.
3. Understand forces acting during terminal velocity

## Net Force

• Net force is the resultant force in the x and y directions
• If the forces are equal and opposite, there is zero net force
• If there is zero net force, acceleration is zero
• We cannot know if the object is stationary or moving at a constant velocity without more information from the question

## Terminal Velocity

• Friction forces increase as the velocity of an object increases
• When friction and weight forces are balanced, the object stops accelerating
• This is terminal velocity! Source: http://misswise.weebly.com/terminal-velocity.html

## Terminal Velocity

• The object starts from rest at the top of the jump
• Its velocity increases due to the acceleration due to gravity (weight force)
• The friction force, drag, increases with velocity until they become balanced
• The velocity becomes constant Source: https://hardphysics13.wordpress.com/terminal-velocity/

## Task

Question 18 and then Question 17 from the mahi kāinga booklet

## Mahi Tuatahi

1. Do you recall $$v=\frac{d}{t}$$? calculate the velocity of a cyclist that travels $$100m$$ in $$15s$$.
2. What is the difference between mass and weight?
3. Calculate the weight of a $$1.5kg$$ mass on Earth.
4. After $$10s$$ a runner has accelerated from rest to $$3m/s$$

## L16: Mahi Kāinga Booklet Q19

• To answer this question, carefully read the question and the things that you should include in your answer.
• A good way to structure it would be for each point:
• Draw a force diagram
• Explain the forces (net force, balanced, unbalanced, horizontal, vertical)
• Explain the acceleration of the rocket due to the forces, and thus its speed
• Explain what is causing friction, and the relative size of those friction forces

## Mahi Kāinga Booklet Q20

• Right is constant velocity, left is moving to the left but slowing down.
• For each bird:
• Describe the net force (balanced, unbalanced, direction)
• Relate this to the acceleration of the bird and therefore the velocity