Unit 3 Summary

NEWTONS THIRD LAW: Every action has an equal and opposite reaction

Just a few posts ago, I explained Newton’s Third Law in relation to Harry Potter and the platform nine and three quarters. Thinking back to that, we know that according to the law, every action has an equal and opposite reaction, and that Harry’s cart and the platform hit with the same force. An action reaction pair is a term used to describe the relationship between two interacting objects. In this case, the reaction pair is “cart hits wall, wall hits cart”

Don’t forget that the cart will experience a greater acceleration than the wall. This is because mass and acceleration are inversely proportional (Newton’s Second Law), so the object with a larger mass (wall) will have a smaller acceleration, and the other way around.

If you have ever played a game of tug of war, you may wonder what makes one team win over the other. According to the law, we know that each team is pulling the other with an equal force. However, the team that wins is the team that exerts a greater force on the GROUND. Refer to the diagram below for details…

 

Team red and team blue pull eachother with equal and opposite forces. However, the blue team pushes the ground with more force than the red team, which causes them to pull left with more force, and win.

VECTORS: A visual tool used to add up opposing forces

Say you have a weight hanging from a string, if the weight is on one side of the string, which side has more tension?

 

The longer the tension vectors (red) the more tension that piece of the string has. So there is more tension near the weight, and less on the other side. This means that the longer side of the string with more tension (left) is more suseptable to breakage.

GRAVITATIONAL FORCE: Everything with mass attracts all other objects with mass

Depends on…

àthe mass of the objects  

àdistance between the two objects. The more distance, the less force

Below is a the equation for finding the gravitational force between two objects, and the force of gravity itself.

 

Here is how you would use the equation

Force is directly proportional to mass

Force is inverse squared to distance

Double distanceà force is a ¼ of the original

Triple distanceà force is a ¹/₉ of the original

Cut d by ½à force is 4x’s greater

Cut d by ¹/₃à force is 9x’s greater

TIDES: As stated in my previous blog post, tides are created by the differences in force on opposite sides of the earth.  Once side of the earth has a greater force, since it is closest to the moon, and the other side which is farther from the moon experiences less force. These unbalanced forces created a “bulge” or “potato” around the earth (red)

Spring tides are high tides that are higher than usual, and occur when the sun, moon, and earth are all in one line. Similarly, neap tides are lower than typical low tides, which occur when the earth and moon create a right angle.

CONSERVATION OF MOMENTUM

p=mv

∆p=p_final-p_initial

the total momentum of a system remains constant before and after a collision

àmomentum can be neither created nor destroyed

P Total Before= P Total After

Before                 After

MaVa+MbVb=Ma+b(Vab)

MOMENTUM AND IMPULSE RELATIONSHIP

Impulse: the force exerted on something by a specific time interval

j=∆p

j=fx∆t

Have you ever wondered exactly how seatbelts keep us safe? This can be answered with knowledge of momentum and impulse…

Since change in momentum equals impulse, we know that the car will stop moving  regardless of how it stops.

àthe ∆p and j are constant

with airbag                J=F∆t

without airbag            J= F ∆t

As you can see above, force and change in time are inversely proportional. The seatbelt increases the time of the impulse, and therefore decreases the force. Less force, means it is less likely for you to be injured.