Understanding How Yahuah's World Works
A force is a push or pull on an object.
| Force Type | Description | Example |
|---|---|---|
| Contact Forces | Require touching | Pushing a cart |
| Non-Contact Forces | Act at a distance | Gravity, magnetism |
| Applied Force | Force applied by person/object | Kicking a ball |
| Normal Force | Surface pushing back | Table supporting book |
| Friction | Opposes motion | Brakes stopping car |
| Tension | Force through rope/string | Pulling a wagon |
The net force is the combination of all forces acting on an object.
1. What unit is used to measure force?
2. If two forces of 10N push right and 6N push left, what is the net force?
Net force = N to the
"An object at rest stays at rest, and an object in motion stays in motion with the same speed and direction, unless acted upon by an unbalanced force."
Inertia is the resistance of an object to change in motion.
1. What is inertia?
2. Why do passengers lurch forward when a car brakes suddenly?
F = m × a
Force = Mass × Acceleration
Units: Newtons (N) = kilograms (kg) × meters per second squared (m/s²)
A 10 kg box is pushed with 50 N of force. What is the acceleration?
1. A 5 kg object accelerates at 3 m/s². What force is applied?
F = N
2. A 100 N force is applied to a 20 kg object. What is the acceleration?
a = m/s²
3. What force is needed to accelerate a 2000 kg car at 4 m/s²?
F = N
"For every action, there is an equal and opposite reaction."
| Action | Reaction |
|---|---|
| You push against wall | Wall pushes back against you |
| Rocket pushes gas down | Gas pushes rocket up |
| You push water backward (swimming) | Water pushes you forward |
| Earth pulls you down | You pull Earth up (tiny!) |
| Gun pushes bullet forward | Bullet pushes gun backward (recoil) |
1. Identify the action-reaction pair: A baseball bat hits a ball.
Action:
Reaction:
Gravity is the force of attraction between any two objects with mass.
W = m × g
Weight = Mass × Gravitational acceleration
Weight is measured in Newtons (N)
| Mass | Weight |
|---|---|
| Amount of matter | Force of gravity on object |
| Measured in kg | Measured in N |
| Same everywhere | Changes with gravity |
| Scalar (no direction) | Vector (has direction: down) |
What is the weight of a 60 kg person on Earth?
1. Calculate the weight of a 25 kg object on Earth (g = 10 m/s²).
W = N
2. An object weighs 800 N. What is its mass?
m = kg
Friction is a force that opposes motion between surfaces that are in contact.
| Type | Description | Example |
|---|---|---|
| Static | Prevents motion from starting | Book sitting on slope |
| Kinetic/Sliding | Acts on moving objects | Pushing box across floor |
| Rolling | Objects rolling | Wheels on road |
| Fluid | Through liquids/gases | Air resistance |
Static friction > Kinetic friction (harder to start than keep moving)
1. Which type of friction keeps a parked car from rolling down a hill?
2. Why is it easier to push a box once it's already moving?
Speed = Distance ÷ Time
s = d/t
Units: m/s or km/h
A car travels 150 km in 3 hours. What is its average speed?
1. A runner covers 400 m in 50 seconds. What is their speed?
s = m/s
2. A car travels at 60 km/h for 2.5 hours. How far does it go?
d = km
a = (v₂ - v₁) / t
Acceleration = Change in velocity ÷ Time
Units: m/s²
A car goes from 0 to 20 m/s in 5 seconds. What is the acceleration?
1. A bike goes from 5 m/s to 15 m/s in 4 seconds. What is the acceleration?
a = m/s²
2. A car brakes from 30 m/s to 10 m/s in 5 seconds. What is the acceleration?
a = m/s² (include sign)
W = F × d
Work = Force × Distance (in direction of force)
Units: Joules (J) = Newtons × meters
P = W / t
Power = Work ÷ Time
Units: Watts (W) = Joules per second
A person pushes a box with 100 N of force for 5 meters. How much work is done?
1. How much work is done lifting a 200 N box 3 meters?
W = J
2. If 1000 J of work is done in 10 seconds, what is the power?
P = W
| Type | Description | Example |
|---|---|---|
| Kinetic (KE) | Energy of motion | Moving car |
| Potential (PE) | Stored energy | Raised object |
| Thermal | Heat energy | Fire |
| Chemical | Stored in bonds | Food, fuel |
| Electrical | Moving charges | Lightning |
| Nuclear | In atomic nucleus | Sun |
Kinetic Energy: KE = ½mv²
Gravitational PE: PE = mgh
(m = mass, v = velocity, g = gravity, h = height)
1. A 2 kg ball is 5 m high. What is its potential energy? (g = 10 m/s²)
PE = mgh = J
Energy cannot be created or destroyed - only transformed from one form to another.
Energy is never "lost" - it transforms into forms we can't use (usually heat due to friction).
Efficiency = Useful energy output ÷ Total energy input × 100%
1. What energy transformation occurs when you rub your hands together?
→
| Machine | What it Does | Example |
|---|---|---|
| Lever | Multiplies force | Seesaw, crowbar |
| Wheel & Axle | Reduces friction, multiplies force | Doorknob, car wheels |
| Pulley | Changes direction of force | Flagpole, blinds |
| Inclined Plane | Reduces force needed | Ramp, stairs |
| Wedge | Splits or separates | Axe, knife |
| Screw | Inclined plane wrapped around cylinder | Bolt, jar lid |
MA = Output Force ÷ Input Force
Or: MA = Input Distance ÷ Output Distance
MA > 1 means the machine multiplies your force!
Simple machines don't reduce work - they make it easier by:
Work in = Work out (in ideal machine)
1. A lever allows you to lift a 500 N rock with only 100 N of force. What is the MA?
MA =
2. Name the six simple machines:
, , ,
, ,
Unit 1: 1) Newtons (N); 2) 4 N to the right
Unit 2: 1) Resistance to change in motion; 2) Inertia - body keeps moving
Unit 3: 1) 15 N; 2) 5 m/s²; 3) 8000 N
Unit 4: Action: Bat hits ball; Reaction: Ball hits bat
Unit 5: 1) 250 N; 2) 80 kg
Unit 6: 1) Static friction; 2) Kinetic friction is less than static
Unit 7: 1) 8 m/s; 2) 150 km
Unit 8: 1) 2.5 m/s²; 2) -4 m/s²
Unit 9: 1) 600 J; 2) 100 W
Unit 10: 1) 100 J
Unit 11: 1) Kinetic → Thermal
Unit 12: 1) 5; 2) Lever, Wheel & Axle, Pulley, Inclined Plane, Wedge, Screw