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Physics for Kids

Understanding of Physics

In order to understand electricity and cicruits, etc, we need to set the right context. My brother already has an understanding of what distance and speed are. For me to explain what electrical energy is to him, I figured I would summarize of how we go from general physics to electricity. This is not meant to be a definition or lecture of physics; these are some analogies and relations between these physical terms.


Goal: Explain how displacement, velocity and acceleration relate to each other and how does time play a role.

Assumptions: My brother has an understanding of 'distance' and 'speed'.
  • Distance is how far you travel
  • Displacement is the shortest distance from one point to another (nice...)
  • Acceleration is the increase of speed
  • Speed is the amount of distance covered in an amount of time
  • He knows how to calculate distance, speed or time if two of the values are provided


Definition: Vector is a quantity that has both a magnitude and a direction. (Ref: I am talking about the Euclidean or "geometric" vector)


Most of the properties of an object are scalar.
  • Scalar quantities are values or properties that can be measured by "how big" it's value is.
Therefore, scalars can be measured by their magnitude.

Examples of scalar quantities are:

 Height "I am 4 feet tall"
 Money "I have $10 in my pocket"
 Apples "There are 5 apples in the basket"
 Capacity "My bottle has 6 fl.oz of water"

Magnitude and Direction

Some quantities not only have a magnitude or "size" associated with them, the quantity also needs to be specified by its direction.

Example: Giving a person directions. Let's say a person walks upto you and he asks you where the nearest gas station is. How would you answer the question?

From the diagram, we can tell that the restaurant is 2 miles away. But that's not enough to describe the directions. We also need to tell the person that the restaurant is northeast of here.

Therefore, we would tell the person that the place is 2 miles Northeast of here. The "2 miles" is known as the magnitude and "Northeast" is known as the direction.

Question: You're the captain of your army and you need to tell send a note down ordering to fire a cannon. What would write down on the note? The cannon can be rotated in any way and the power meter can be raised from 0 -> 100 depending on how far you need the cannon to go.
  • Hints: Can you just say 'crank up the power meter to 70'?
  • Answer: The note would say something like "Fire the cannon at 70% pointed at 20 degrees North of East.

Displacement, Velocity, Acceleration and Time

Let's relate the ideas of "distance" and "speed" to the quantities used in physics. The idea of vectors will finally make sense...(kinda)


Personal notes
  • How it relates to distance
  • bread crumbs vs. position
  • displacement = one's position compared to his journey
Distance is a measure of how much one has walked/traveled since he/she started.
It is not a measure of how far the person is from the starting position.

It's a scalar quantity
  • It tells us how "much" we have walked, not where or in what direction we have walked. 
  • Example: How many steps have I taken.
  • If we walked forward for two miles, then walked backwards 1 mile and walked east 1 mile. I have travelled a distance of 4 miles.
  • If I drove my car forward 10 meters, then put the car in reverse and drove 5 meters. What's the distance I have traveled?
    • Answer: 10 meters.
Distance is not very useful for the other quantities we will be working with. A more useful measurement is the displacement.

Displacement is the measure of how far one is located in comparison to his/her starting position.

It's a vector quantity
  • It tells us how much one is "displaced" or "separated" from where he/she started from.
  • Example: What's the shortest distance from where I am to where I started from.
  • If we walked forward for two miles, then walked backwards 1 mile and walked east 1 mile. I have a displacement of 1.4 miles 45 degrees N of E.
  • If I drove my car forward 10 meters, then put the car in reverse and drove 5 meters. What's my displacement after my travel?
    • Answer: 5 meters forward.


Personal Notes
  • How it relates to speed
  • How fast is the person moving vs. how is the person's position changin
Speed tells how the distance is changing with time ("distance/time")
  • i.e. how fast one is moving.
  • scalar quantity
  • It's a loose quantity and not useful when talking in terms of displacement.

Velocity tells us how displacement is changing with time ("displacement/time")
  • i.e. how one's position (with relation to the starting point) is changing with time
  • vector quantity
If my displacement keeps changing by the same amount, then velocity will be the same or constant.

If displacement is measured in meters, then velocity for a given time can be found by calculating how the distance changed in one second. (Actually, this is average velocity for 1sec)



Personal notes
-The concept of vectors should be clear by now. Discuss acceleration

Similar to velocity and displacement, acceleration describes how velocity changes over time.
  • i.e. how my 'change in displacement' is changing.
  • vector quantity
If my acceleration is positive, then my velocity is increasing over time. Therefore the amount of displacement that is changed in a given time' increases over time

When we start driving the car, we need to "accelerate".
  • To go anywhere with the car, we need our displacement to change
  • This means desire some velocity
  • To get that velocity, we need to accelerate i.e. change the velocity from 0 to our desired amount

Dimension of Time

A few examples to show how displacement, velocity and acceleration relate to each other
  • by examining how quickly we reach our destination

Driving the car example

Our goal is to reach 100 miles.

The higher our velocity is,
  • the quicker we will reach our desired displacement of 100 mi.
The higher our acceleration is,
  • the quicker we will increase our velocity. The higher our velocity is,
    • the quicker we will reach our desired displacement.
walking in a park

car racing (in one dimension)


Kinematics (Displacement, Velocity and Acceleration) gives us information about an object's position. Nothing more. These three quantities tell us about the location of an object and its relation to time.

To understand and apply/modify interaction between different objects, we need to understand dynamics of an object.

  • Force is a push or pull
  • Mass is the density of an object
  • Weight is the measure of gravitational pull on an object
  • Newtons three laws:
    • Any force or action has a reaction
    • forgot the other two
  • weight and mass affect the speed and acceleration of an object
  • Inertia is ...
  • Momentum has something to do with inertia
  • Measure of chaos: Entropy (wowwwww...)
    • Definition: A perfect excuse for not cleaning your bedroom
  • Kinetic energy is energy of motion


If we need to move an object, for example, we cannot just apply "acceleration" to the object. Acceleration is a measurement made on an object. It's not a quantity that we can use to "apply" to the object or affect the object with.
  • i.e. we cannot use a value of 'acceleration' to move an object the way we want. We can use 'acceleration' as a desired result, not a desired input
Example - How much 'effort' is needed to move the vehicle

Let's say we have two vehicles racing head to head.
  • Vehicle A is a car that weights about 1000 kg.
  • Vehicle B is a truck that weights about 5000 kg.
For both of them to move the same way and for them to reach the finish line at the same time:
  • both of them should have the same acceleration through the course of the race
    • remember that "acceleration" is a quantity we are measuring from the vehicle - not an input quantity to move the vehicle
Situation 1:
  • We apply the same quantity of "effort" using the engine for both the vehicles.
    • Pretend that we replace the "engines" with two people who can push with the same "effort".
  • Will both the vehicles have the same acceleration?
    • i.e. will both the vehicles end up move the same way and end up at the finish line at the same time?
Answer: No
  • Though both the "engines" push with the same effort, the truck will move slower that the car (therefore finishing the race late).
    • i.e. the acceleration of the truck < acceleration of the car (remember, acceleration is a measured quantity)
Why the difference in acceleration?
  • The difference between the two vehicles was their mass (Note: I say "mass" for a reason, and not "weight")
  • The heavier the mass, the "harder" it was to move the vehicle with the same acceleration using the same "effort"
    • same "effort"
    • same desired acceleratoin
    • different mass
    • result: different measured acceleration
      • heaver vehicle -> smaller acceleration
This "effort" we defined is know as force. It's a measure of how much "effort" is placed on an object.
  • i.e. the "push" felt by the object
  • The larger the force, the larger the "effort" placed on the object
Example: A person pushes the car. We refer to this effort as the force by the person on the car = F[person->car]
  • This force is a measurement of the car, not of the person
  • Note: There's another force called force by the car on the person = F[car->person]
    • This force is the measurement of the person i.e. the "push" felt by the person due to the car
    • This is called the "reaction" force
    • This is equal to F[person->car]
      • Newton's Third law!
When multiple forces are applied on an object, we call the sum of all the forces the net force.
  • This net force is a representation of all these forces.
  • Examples

The net force is what we care about when we talk about how "the position of an object is changing."
  • The sum of all forces is what moves the object. i.e. provides acceleration
  • Net Force = mass x acceleration
Therefore the acceleration of an object is equal to Force divided by the mass of the object (a = F/m)
  • Therefore, when the same force is applied to two objects
    • object with more mass has less acceleration
  • Acceleration is a measured quantity of the object's position
    • This is what we use to determine how the object is moving
  • Force is a measure of the "effort or push" experienced by the object
    • This quantifies what is causing the object to move
  • Mass is what relates our "effort to move the object" to "how the object moves"
    • F = m*a
    • acceleration is force / mass
    • For the same force: The higher the mass of the object, the less acceleration is experienced by the object
  • Mass plays a role in how objects interact and affect each other
    • Knowing the acceleration, velocity, displacement is enough to discuss how the object's position is changing with time
      • Not enough to discuss how the object interacts with other objects
    • Mass allows us to relate these quantities (a, v, d) to force, momentum, etc
      • The force, momentum, etc allows us to discuss how objects interact with other objects
To Add
Example of moving the box on the table
  • when I apply force to the box, the box starts moving
  • Review displacement, velocity and acceleration
    • velocity is positive, because displacement is increasing over time
    • velocity is not only positive but also increasing, because the change in displacement over time is increasing. [the box goes from a stop to motion...]
  • since velocity is increasing, the acceleration must be positive
  • Therefore the cause of force to the object results in acceleration of the object
  • {applied force} ---> [object of mass m] ---> {measured acceleration}
  • The "conversion" from force to acceleration depends on the mass of the object
    • heavier the object, the less acceleration obtained from an applied force


Reference: http://en.wikipedia.org/wiki/Momentum

Just like how Force takes into account the acceleration and mass of an object, momentum takes into account both the velocity and the mass. Momentum tells us how much "power" the object has.

Example: Collision with a vehicle

Energy and Power

Non-Contact forces

Gravitational Force

Electrical Force