Unit 3 Reflection

This unit of Physics was relatively harder than the other two units this semester. First we learned about Newtons Third Law, which states that any action has an equal and opposite reaction. So if a car and a truck collide head on, then there is an equal amount of force exerted on each vehicle. This is also why if you hold a magnet out in front of a metal cart, the magnetic force of the magnet does not move the cart. Another example of Newtons Third Law is when a horse is pushing a buggy. In this example there are three action-reaction pairs; the force of the horse on the ground and the ground on the horse, the force of the horse on the buggy and the buggy on the horse, and the force of the buggy on the ground and the ground on the buggy. When the horse walks, it is pushing the ground back and the ground is pushing the horse forward. This is the same with the buggy and the ground, but since the force of the horse to the ground is greater than the buggy to the ground then the horse and buggy move forward! This can also be applied to the game of tug-o-war, the side with the greater force on the ground will be able to pull the rope to their side and win!

The next thing we learned about was vectors, a vector is a quantity having direction as well as magnitude, this may look like a line with an arrow. We used vectors to determine what direction a box would go in when on a ramp and what the force of tension would be on two ropes when holding up an object. In order to find the direction of a box on a ramp you would have to know the force of weight of the object (fweight), you would represent this with a vector pointing south of the box from the middle of the box. Then you would draw a vector north of that point with the same length as the fweight vector. After that, you would draw a line in any magnitude that is parallel to the ramp and touches the end of the vector that is north of the fweight vector. Then you would draw two perpendicular lines to the ramp and the line parallel to the ramp, one that goes through the point in the center of the box and one that starts at the bottom of the fweight vector. Lastly, you would draw a line that starts at the top of the perpendicular line on the center of the box and make it parallel to the fweight vector. Draw a vector from the point in the middle of the box to the point where that line and the perpendicular line starting at the bottom of the fweight meet and that is the direction the box is moving! This video should explain it much better than I did, it also explains how the force of tension can be found:

Next we learned about the Universal Gravitational Force which states that everything with mass attracts anything else with mass. The formula is:

 gravitational force=[(mass1xmass2)/distance (squared)]universal gravitational force or 

F=[(m1m2)/d^2]G. The universal gravitational force or G is always equal to 7x10^-11.One question that we had to answer was whether something would weigh more at the bottom of the ocean or at the top of Mt. Everest? The answer is that something would weigh more at the bottom of the ocean because of the distance, so the greater the distance the less something would weigh. Distance is a key factor in this equation.

I think that the most interesting thing that we learned about in this unit was tides. In one day there are approximately 4 tides, two low and two high. This is due t the gravitational pull of the moon, wherever the moon is there are high tides on either side of the earth, the two sides of the earth, however, will experience a different force. It takes 27 days for the moon to revolve around the earth completely. When the moon is directly in front of the sun and when the moon and the sun are on opposite sides of the earth there is a spring tide, these tides are higher than usual. When the moon is not directly in front of the sun or behind the earth but adjacent to the earth there are neap tides which are lower than usual. 

After tides we learned all about momentum and impulse! Momentum is inertia in motion, Momentum (P)=mass(m)xvelocity(v)and the units are kgm/s. Impulse(J) is the quantity of force(F) x change in time(delta t). the change in momentum is: deltaP=(mv)final-(mv)initial which is also equal to the impulse. The impulse of an object is dependent on time, so if a skateboard was moving for five seconds and a train was moving for one second, then the skateboard would have more impulse, thus more momentum. This information can also be used to explain how an airbag keeps you safe: 

P=mv

deltaP=(mv)final-(mv)initial

You go from moving to not moving in a car crash, therefor the change in momentum is the same whether you hit the dashboard or the airbags.

J=deltaP

Therefor the impulse is the same whether you hit the dashboard or the airbags.

J=F delta t

Since J is the same the airbags increase the time and the force must be less. Small force =less injury

After Momentum, we moved on to the conservation of momentum which is the change in momentum before is equal and opposite to the change in momentum after. This relates to newtons third law by this: 

[a=after b=before]

Fa=-Fb (Newtons 3rd Law)

Fa delta t= -Fb delta t

Ja= -Jb

deltaPa= -deltaPb (conservation of momentum)

This formula helped explain why car bumpers are now plastic rather than rubber. When they were rubber, cars would bounce off one another, decreasing the time of impact. When the impact time decreases, the injury is much greater, so by making the bumper plastic you increase the time of impact.

I found it most difficult to understand the conservation of momentum and how impulse worked. By making this blog I think I understand it more than I did in class! I think i overcame these difficulties by making this blog and by asking questions on review days to Mrs. Lawrence. Also, I watched did a lot of practice problems.

Although this was the hardest unit yet, I finally got a hold of it in the end. I think I could have been more diligent in class this unit, I wasn't as focussed as I could have been, but I understand the material. Next unit I hope to be more focussed than I was this unit and possibly study a little better for quizzes.