12PHYS - AS91171

Finn LeSueur

2020

**Use these tips to draw a second, improved diagram**- Use arrows (force is a vector)
- Label with names
- Label with numerical values
- Start arrows from center of mass
- Length indicates magnitude
- Use dashes to indicate equally sized vectors

On your whiteboards, add these vectors (head to tail) to find the resultant (net) force acting.

For each, calculate the acceleration that a \(5kg\) object would experience.

A state of zero acceleration (constant velocity) due to \(F_{net} = 0\).

- When performing vector addition, equilibrium is seen by the vectors starting and ending at the same point.
- Vector diagrams having different start and end points indicates disequilibrium, and thus \(F_{net} \neq 0N\).

Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration. \(F=m \times a\)

**TLDR**: The acceleration created by a force depends directly upon the mass of the object.

- What is my net force?
- What is my acceleration?
- Why does it not equal \(g\)?
- What is my drag and weight forces were balanced?

- What is my net force?

\(620N\) down - What is my acceleration?

\(F=ma, a=\frac{F}{m}=\frac{620}{72}=8.61ms^{-2}\) - Why does it not equal \(g\)?

Because friction is acting against the weight force! - What if my drag and weight forces were balanced?

\(F_{net}=0\) and therefore \(a=0\) and I would be in equilibrium.

- Be able to label different forces
- Be able to label forces on an angle

Write the date and ngā whāinga ako in your book

**Friction**: Acts against the direction of motion between two in-contact moving objects.**Weight**: Acts down towards the center of mass of the object attracting you.**Applied**: Often called thrust/push/pull depending on the situation.**Drag**: Acts against the direction of motion when an object moves through a medium (e.g. air)

**Spring**: A compressed or extended spring can exert a force in the opposite direction to its displacement.**Magnetic**: A moving charged object inside an electric field will experience a magnetic force.**Tension**: A force exerted through a non-rigid object like rope**Buoyant**: A force felt due to the displacement of another medium (air, water)**Normal/Support**: A force exerted by a rigid object at \(90\degree\) to the surface (equal and opposite to the force being applied to it).

- If the surface is sufficiently strong, the support/normal force will always oppose the force acting upon it exactly
- It will always act at \(90\degree\) to the surface

Draw equal and opposite weight and normal forces on your diagram.

- It will always act at \(90\degree\) to the surface
- In this case, the normal force does not equal the weight force
- This is because not all of the weight force is acting
**perpendicular**to the surface.

- It instead is
**equal and opposite to the component of the weight force perpendicular to the surface**. - We can find it using the right triangle that is formed between the weight force exerted on the plane, and the total net force.
- \(F_{n} = F_{w}cos(\theta)\)
- The angle inside this triangle is the same as the angle of the incline.

**If**an object is in equilibrium, we can calculate a missing force by assuming \(F_{net} = 0\).- Take the three forces acting upon the object and add them through
**vector addition** - They should form a closed right-angled triangle, allowing you to find the unknown side.

Worksheet - everything up to but not including the **Work** section.

Step 1. Consider what we know about the motion of the object, and what this implies about the net force acting upon the object.

- What force do we know is
**not**acting due to the cars movement? - Therefore, what three forces
**are**acting? - Draw a force diagram illustrating these forces and their relative magnitude. Ensure you label them!

- What force do we know is
**not**acting due to the cars movement?*Thrust, because it is not accelerating.* - Therefore, what three forces
**are**acting?*Weight, friction and support.* - Draw a force diagram illustrating these forces and their relative magnitude. Ensure you label them!

- What do we know about the motion of the car?
- Therefore, what can we say about its acceleration, forces and state of equilibrium?
- Therefore, what do we know the vector diagram will look like?
- Draw a diagram.

- What do we know about the motion of the car?
*It is stationary (constant velocity).* - Therefore, what can we say about its acceleration, forces and state of equilibrium?
*\(a = 0, F_{net} = 0\) and is therefore***in equilibrium**. - Therefore, what do we know the vector diagram will look like?
*The vectors will form a closed loop.* - Draw a diagram.

- We need to calculate the
**weight force**of the car. Hint: \(F=ma\). - Recognise & state that not all of the weight force acts directly through the slope.

- Recgonise and state we need to find component of weight force acting into slope, and is equal in magnitude to the support force. Hint: \(F_{n} = F_{w}cos(\theta)\).
- Calculate \(F_{f}\) using EITHER Pythagoras OR trigonometry to find the frictional side of the triangle.

- We need to calculate the
**weight force**of the car. Hint: \(F=ma\).*\(W=F=m \times a = 1500 \times 9.8 = 14700N\). Use \(a = g\) because weight force is due to gravity.* - Recognise & state that not all of the weight force acts directly through the slope.
*Because the weight force is acting on an angle to the slop, the component perpendicular to the slope is equal & opposite to the normal force.*

- Calculate the normal force. Hint: \(F_{n} = F_{w}cos(\theta)\).

\(F_{n} = 14700 cos(12) = 14378N\) - Calculate \(F_{f}\) using EITHER Pythagoras OR trigonometry to find the frictional side of the triangle.

\(F_{f} = F_{w}sin(\theta) = 14700sin(12) = 3056N\)

OR

\(F_{f} = \sqrt(F_{w}^{2} - F_{n}^{2}) = 3056N\).

- Textbook
- New:
*Activity 10A: Newton’s Laws* - Old:
*Activity 9A: Newton’s Laws*

- New:
- Homework Booklet: Q31, Q32, Q34