UNIT 3 REVIEW

In this Unit, we reviewed 6 major topics

1. Newton’s 3rd Law and  Action/Reaction Pairs
2. Horse and Buggy Problems
3. Gravity and Tides
4. Momentum and Impulse Momentum
5. Conservation of Momentum

Newton's 3rd Law
Newton's 3rd Law States that every action has an equal and opposite reaction
When talking about this law, we use the equation F=ma

When there is greater mass, the acceleration will be less:

F= ma

F= ma

When mass increases, the acceleration will decrease to equal the force and when mass decreases, the acceleration will increase (as shown above)

Action Reaction Pairs
When explaining Newton's 3rd Law, we use action-reaction pairs. There are 3 rules to remember when writing these action reaction pairs:

1. Verb stays the same
2. Direction stays the same
3. Objects stay the same, but switch

Here is an example:

To explain the blue arrow, you would say:

Apple pushes Table down

To explain the yellow arrow, you would say:

Table pushes Apple up

Here is a video to explain:

Perpendicular Forces

To explain perpendicular forces, we draw vectors.  For example, how does a block slide down a ramp?

Draw lines perpendicular to the force of gravity and the support force.  The point where these lines meet becomes the direction of the F Net.

Horse and Buggy (or Tug of War)

One difficult problem is the "horse-and-buggy" problem.  The table below shows each vector you need to know to explain how a horse and buggy move forward:

Here is how you would write each reaction pair for each pair of arrows:

A: Buggy pulls Horse back, Horse pulls Buggy forward

B: Earth pushes buggy back, Buggy pushes Earth forward

C: Ground pushes Horse forward, Horse pushes Ground backward

  However how does the system, as a whole, move forward? As illustrated, the forces between the horse and ground are greater than those between the buggy and ground.  This is because the horse has more force of friction between it and the ground, therefore it overpowers the buggy's backward force, and the system moves forward.

Gravity and Tides

The Earth's natural tides can be explained through the pulls of gravity and Newton's 3rd Law.

Each day there are 4 tides in total:

• 2 high tides
• 2 low tides

You can explain these tides through Newton's 3rd Law.

Notice how, in the image below, the distance A is lesser than the distance B.

Because the distances are different, point A and point B will experience different forces from the moon's pull.

We know that Force and Distance are inversely proportional.

Therefore, 

• Since B has a greater distance, the force will be lesser,
• And since A has a lesser distance, the force will be greater.

This DIFFERENCE IN FORCE causes the water surrounding Earth to create an oval:

However, the sun also affects the tides, but less drastically, This is why we have Spring Tides and Neap Tides:

During Spring Tides, since the sun is in line with the moon, you end up getting:

• Higher high tides
• Lower low tides

Momentum and Impulse Momentum

Momentum= movement of mass

The character used for momentum is P
The equation that expresses momentum is:

P=mv

Change in momentum is the same, regardless of speed.

Impulse=force upon something and HOW LONG that force is applied
The character used for Impulse is J
The equation that expresses impulse is:

J = F * ∆ t

Momentum and impulse are connected through this equation:

J= ∆ P

Bullet-proof Vests

This means that time is inversely proportional to force when entered in an impulse equation.  Therefore, in order to decrease the force of impulse, you must increase the time of the impulse.  For example, a bullet proof vest.  To decrease the impact (F) of the bullet, the material used in a bullet-proof vest increases the change in time (∆t) of the impulse of the bullet.  Therefore, the the impact of the bullet is "absorbed" and the person wearing the vest experiences less force.

Conservation of Momentum

Conservation of momentum states that momentum can neither be destroyed nor created.

The two most important equations to remember when explaining conservation of momentum are for objects hitting and bouncing and objects and objects hitting and sticking.

When an object hits and bounces you use the following equation:

MaVa+MbVb=MaVa+MbVb

When objects stick you use the following equation:

 MaVa+MbVb=Ma+b(Vab)

I struggled most with change in tides. I overcame this confusion when I realized tides are not caused by distance, but by CHANGE in distance