Unit 3

A. Newton’s 3rd Law and  Action/Reaction Pairs

• Newton's third law states that every action has an equal and opposite reaction

• For example, when you walk, you push the ground back and the ground pushes you forward
• The difference is that the ground has a much greater mass so its acceleration is much less
• Another example is if you push someone in a chair 
• You push the chair and the chair pushes back on you
• The reason you don't move is because you push the ground forward and it pushes you backwards
• This is similar to the tug of war/horse and buggy
• Even if nothing is in motion forces are still in action
• If a book sits on a table the book is pushing the table down and the table is pushing the apple up
• The book is always being pulled down by the Earth and the apple pulls the Earth up

B. Tug of war/horse and buggy

• Many people are under the impression that to win a tug of war battle you must pull the hardest
• This is false
• We know that from Newton's Third Law every interaction there is an equal and opposite reaction
• This means that pulling the rope harder just means the other team pulls just as hard
• The truth is that winning is not based on how hard you pull
• Rather it is based on how hard you push the ground

• The person who pushes the ground the hardest will have the ground push them back the hardest, therefore the winner is whoever pushes the ground the hardest

C. Forces in perpendicular directions

• This is best described by a box sliding down a ramp
• 
• The weight of the box is caused by gravity and will always be in the downward direction
• The box pushes up while the ramp pushes down
• When drawn the vectors will end up in the diagonal direction causing the box to accelerate down the ramp
• The steeper the box, the greater the acceleration the box will have
• This can also be seen in someone canoeing across a current going downstream
• If there is a velocity going down stream and a velocity across the velocity will end up being in the diaganol direction and the person will not canoe straight across

D. Gravity and Tides

• The force of gravity is increased as the mass of the object increases


• The more mass an object has, the more it is attracted to other object
• The force of gravity is defined by the formula: 
• As mass goes up so does the force
• They are directly proportional 
• As distance goes up, the force goes down
• They are inversely proportional
• This is why the moon has a larger impact on the tides than the sun does
• Though the sun's mass is much greater than that of the moon's,the moon is much closer
• When mass goes up by the same factor that distance does, distance  has the greater impact
• If mass goes up by a factor of 2, the force doubles
• If distance goes up by a factor of two. the force goes down to 1/4 of the original force
• The reason why distance has a greater affect is because it is squared
• The tides of the Earth are most affected by the moon
• The force of the moon on one side, and the much less force it has on the other side of the Earth is what causes what is called the tidal bulge
• The tidal bulge is caused by the moon pulling on side very hard, and very weak on the other side, because they have a greater distance
• The moon pulls one side hard, the Earth comes over some and then the oceans are spread thin causing the North and South side to have lower 
• When the moon is on the East side of the Earth, the oceans on the East and West side will be in high tide, and the North and South side will be in low tide

E. Momentum – and Impulse momentum relationship

• Momentum is a way to describe an object in motion and with relation to the object's mass
• If two objects are in motion with the same velocity but one with a higher mass, the one with the higher mass will have the higher momentum
• Momentum is also p
• p=mv
• When an object's momentum changes over a period of time, it is called an impulse
• Impulse is also equivalent to the change in momentum
• Impulse is J\
• J is also to be calculated 
• Impulse is what determines how you hit the airbags 
• Regardless of how you hit the airbags you will go from moving to not moving
• The airbags increase the time but impulse will not change
• The airbags therefore decrease the F
• The impulse of you hitting the airbags versus hitting the dashboard are the same because the change in momentum is the same:moving to at rest

F. Conservation of Momentum (Including the lab)

Within a system we know that momentum is conserved

For example, if a 5kg cart is moving at 6m/s and it hits a 1kg cart and stops, it has to have the same momentum as the other cart

• It will be moving at 30m/s
• This can also be solved by using M(a)V(a)+ M(b)V(b)= M(a)V(a)+M(b)V(b)
• 5(6)+1(0)=5(0)+1(x)
• 30+0=0+x
• 30=x
• If the objects stick together then you must use another formula such as M(a)V(a)+M(b)V(b)=M(a+b)V(ab)
• The ab at the end are not to be multiplied but is to show that the two objects are now one and stick together
• If the objects in the previous stuck together rather than the first one stopping it would be 5(6)+1(0)=6x
• 30+0=6x
• 30=6x
• x=5m/s