# 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

• 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)

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!

## 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

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