Forces

Mahi Tuatahi

Draw a diagram indicating all the forces acting upon you

Muscle Contraction & Light-Headedness

Pātai: Vector Reminder

1. Draw a vector diagram for each situation.
2. Illustrate and label the resultant force.
3. Calculate the acceleration each experiences given \(m=5kg\).

Equilibrium

A state of zero acceleration (constant velocity) due to \(F_{net} = 0N\). E.g. vector diagram starts and ends at the same spot.

• Vector diagrams having different start and end points indicates disequilibrium, and thus \(F_{net} \neq 0N\).

Whakatika

1. What is my net force?
   \(620N\) down
   
2. What is my acceleration?
   \(F=ma, a=\frac{F}{m}=\frac{620}{72}=8.61ms^{-2}\)
   
3. Why does it not equal \(g\)?
   Because friction is acting against the weight force!
   
4. What if my drag and weight forces were balanced?
   \(F_{net}=0\) and therefore \(a=0\) and I would be in equilibrium.
   

Akoranga 22 Mahi Tuatahi

Quizizz!

Ngā Whāinga Ako

1. Be able to label different forces
2. 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).

Whakatika

Calculating a Missing Force

• If an object is in equilibrium, we can calculate a missing force by knowing \(F_{net} = 0\) and drawing a vector diagram
• They should form a closed right-angled triangle, allowing you to find the unknown side with Pythag/trig.

Task/Ngohe

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

Case Study

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

Question 1

• 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!

Question 1 Whakatika

• What force do we know is not acting due to the cars movement?
  Thrust, because it is parked.
  
  
• 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!

Question 2

• 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.

Question 2 Whakatika

• 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 diagram.
  
  
• Draw a diagram.

Question 3

• 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.
  
  

Question 3 Continued

• 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.

Question 3 Whakatika

• 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.
  

Question 3 Whakatika Continued

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

Practise / Whakawai

• Textbook
  • New: Activity 10A: Newton’s Laws
  • Old: Activity 9A: Newton’s Laws
• Homework Booklet: Q31, Q32, Q34