unit summary

Unit summary 3

Newton’s 3^rd Law

The law states that any action has an equal and opposite reaction. We did an interesting experiment in class with weighing scales. We hooked them together and pulled, no matter how much we pulled the weight on each scale was the same. The forces were equal and opposite. The opposite forces are called an action reaction pair. An action reaction pair is the relationship between the opposite forces. For example: the apple is on the table. The table is pushing up the apple and the apple is pushing down the table. The forces in the example are equal and opposite and are an action reaction pair. In this case the earth pulls the apple down and the apple pulls the earth up. The earth has an equal and opposite force to the table pushing up, but it is not an action reaction pair because the pair can only be between two objects each affecting the other.

Tug of War

The forces of an action reaction pair are always equal and opposite so they cancel each other out creating a net force of 0. The way to move is the differences in the forces produced. For example: to win a tug of war the greater force has to be produced by the legs and the ground. No matter how hard one pulls the rope they can never exert more force than the other person according to the 3^rd law. So one must have greater force between the legs and the ground, the greater force on the ground will move the opponents forward helping you win the tug of war. The key is to remember that you can never pull harder than your opponents you must put more force on the ground than them.

Vectors

Vectors are lines with arrow heads that indicate the direction and amount of force placed on an object. The vectors can be in any direction and can be summed up to give the actual direction and amount of force. The vectors are drawn to scale so the larger they are the large amount of force they represent. Adding vectors that are parallel is simple as you add them and if they are in opposite directions subtract. If they are at a 90 degree angle then use the formula a^2+b^2=c^2. If you are adding vectors the have neither of these conditions then create parallel line and form a closed figure. Where the new parallel lines intersect is the direction and amount of force there.

Tides

The tides of the earth are low and high constantly alternating in 6 hour intervals. The tides occur because of the difference in force between the sides of the earth and the moon. The moon pulls on the water on the close side of the earth towards it and the far side is pushed from the moon. How is this possible? The moon creates a force on both sides of the earth the force on the near side is stronger than the force on the far side. At the same time the earth is pulling on both sides. The difference in net force on each side of the earth because of the force of the moon creates mirroring high and low tide on the sides of the earth. The difference in force is always opposite, meaning that the force on the far side of earth is pulling the tide away from the moon creating high tide. The strong high tides are called spring tides and the strong low tides are called neap tides.

Momentum

Momentum is equal to mass*velocity. The mass and velocity are both directly related to the momentum so if velocity increases the momentum increases and if the mass increases the momentum increases. The momentum of an object cannot be greater than the momentum of the object it is colliding with, the total momentum before the collision is the same as the total momentum after the collision. The change in momentum is called an impulse and impulse equals force*time. The force and the time are both directly proportional to the impulse, but the force is inversely proportional to the time. This means that increasing the time before something hit another object reduces the force thus reducing the damage.  

Tides

Every day the Earth experiences 4 tides, 2 high and 2 low. The tides are about 6 ours apart. As the Earth rotates faster than the Moon so the oval of tides stayes relitivly the same through out the day making a full rotation of the Earth able to have multipe high and low tides a day. The cause for the tides is the difference in force on each side of the Earth. The difference on the side closest to the Moon to the other side of the Earth. Side B (the latter) has an opposite force to side A, or an opposite force to the Moon. The difference in force pulls the tides higer up, so on the sides with no force(the low tides) must lower as all the water is being pulled to sides A and B. The times of high tides are the spring tides and the low tides are the neap tides. The neap tides are happening under a half moon and the spring tides are under a full or new moon.


 At the moment of me posting this the tide is at low tide and the beach is experiencing the spring tides. I can tell because the moon is full.

Animals and Newton's 3rd law

The horse and the cart do pull with equal and opposite forces but that is not the force that moves the cart. The cart is moved because the horses force on the ground is greater than the force of cart on the ground. The 3rd Law applies to people (and talking animals) to, mostly in the form of revenge. If you are pushed then you push the person back.

Unit 2 summary

Newton’s Second Law

Newton’s second law states that as mass increases the acceleration decreases and as mass decreases the acceleration increases. This can also be written by saying that the mass is inversely proportional to the acceleration (a~1/m). The force on an object is directly proportional to the acceleration so as the force increases the acceleration increases and if the force decreases then the acceleration decreases (a~F). These two expressions can be combined to relay Newton’s second law (a=F*1/m or a=F/m).

Mass and weight are two different things, the mass is how much stuff is in the object and the weight is the force of gravity on the object. If the mass is know we can discover the weight of the object through the formula weight=mass*Force due to gravity (w=mg).

Free Fall

In a free fall there is no air resistance and the objects weight is not taken into account. These objects are all falling at the same rate (force of gravity 10m/s^2).

            Falling down

The ball is dropped over the edge of a cliff and it takes it about 10 seconds to hit the ground at the bottom. In this the ball is falling at a constant acceleration of 10m/s^2. At 0 seconds the ball is at rest and has an a (acceleration) of 0m/s^2 and a v (velocity) of 0m/s. After you drop it at the first second the a = 10m/s^2 and the v=10m/s. After the 3^rd second the a =10m/s^2 and the v =20m/s. this continues until the ball hit the ground. The formula to find the distance it has fallen is distance=1/2gravity*time^2 (d=1/2gt^2). To find the speed it was travelling the formula is velocity=gravity*time (v=gt).

            Throwing up

With a rock you throw it up into the air and it travels a distance and falls back into your hand. What is happening here is that the initial velocity that it was given by the throw is weakened over time as gravity acts on it and falls back down to the point from which it was thrown. The distance it travels upward depends on the initial velocity. The force of gravity is placing a downward acceleration of 10m/s^2 and every second that the rock is in the air the velocity decreases by 10m/s until it is moving at 0m/s. Here it is at the top of its path and begins to fall downwards. The formulas to find the distance and the velocity are the same as falling down d=1/2gt^2 and v=gt. The only difference is that the distance formula can only measure a downward distance. If you desire to find an upward distance you must find the distance that it fell the remaining amount of time and subtract that from the total distance it fell.

Projectile motion down

Here the rock is thrown straight off the cliff with a velocity of 10m/s. The explanation above of Falling Down applies here as well as the velocity of 10m/s. The velocity is constant and so the rock falls at the same rate and is moving at a velocity of 10m/s making it land somewhere out front of the cliff not directly below it. The vertical velocity and distance is explained in the Falling Down and the horizontal velocity is found through the formula of velocity=distance/time (v=d/t) and the distance that it moves from the origin is found through the formula distance=velocity*time (d=vt). So instead of moving forward and the down like a stair case the rock moves in a diagonal arc.

Projectile motion up

Here the rock is thrown up and forward at the velocity of 10m/s. The same explanation of Throwing up is used for the vertical velocity and distance and the formula v=d/t and d=vt are used to find the horizontal velocity and distance. The path of the rock is a diagonal arc to the location of it hitting the ground.

Sky diving

Here air resistance applies and air resistance it the force of air pushing against you or an object. The higher the velocity or surface area the higher the air resistance. When you jump out of a plane your downward force is the weight of the object and you will continue to accelerate until the air resistance is equal to your downward force. Once this happens this is called terminal velocity and the acceleration stops and the velocity becomes constant. When you open your parachute the surface area dramatically increases and the air resistance does as well. Over time the air resistance will decrease and the velocity will as well until it equals the downward force. This is the second terminal velocity is at a much slower velocity because the surface area is larger.

Summary

Newton’s First law of motion

Newton’s first law states that an object in motion will stay in motion until acted on by an outside force, or that an object at rest will stay at rest until acted on by an outside force. This is a concept that cannot be displayed on earth as there are always outside forces acting on the objects. The most common forces are wind, gravity, friction, etc. The place where this concept can be see first-hand is in space. It would be quite amazing to throw a ball in space and never see it slow down.

Net Force

Net force is the total of the forces acting on an object. The amount of force that is placed on an object is measured in Newtons abbreviated (N). As an example if someone is pushing a box to the left with 10N of force, the net force of the box is 10N. If another person comes by and applies 10N of force to the right side of the box then the forces are subtracted from each other to find the net force. In this case 10-10=0, 0N in the net force of the box. Say another person walks by and adds 20N of force to the left side of the box, then the forces from each side of the box are subtracted. In this case 20+10=30N is the force on the right side of the box. The left side still only has 10N of force. So 30-10=20N of force is the net force of the box. Only the force that is applied opposite to the other force is able to be subtracted.

Equilibrium

Equilibrium is the special case where the net force of the object is 0N. At equilibrium the object can be at rest, in motion, or at rest in motion. At equilibrium the objects forces are balanced and the object can be at constant velocity or constant acceleration.

Velocity

Velocity is the speed that something travels. It is speed and the direction, it is NOT the same as speed. Velocity is normally measured in meters per second, abbreviated m/s. But velocity can also be measured in other increments as long as it is the measure unit of a distance over the time as in cm/s, mi/h, km/min, etc. When the velocity of the object remains the same it is referred to as constant velocity. Constant velocity has two formulas associated with it, these formulas only work for constant velocity. The first is to determine the velocity of the object is velocity=distance divided by time (v=d/t). The other is for finding the distance something has traveled at constant velocity it is distance =velocity multiplied by time (d=vt).

Acceleration

The acceleration of an object is the change in velocity in a period of time. Acceleration is measured by meters per second per second abbreviated m/s². Acceleration is not only forward it is also in reverse, when a car slows down it is accelerating in the opposite direction. Acceleration can be changing by either slowing down or speeding up, but when it stays at a constant there are two formulas that accompany constant acceleration. To find the velocity of an object a constant acceleration the formula is velocity =acceleration multiplied by time (v=at). The formula for the distance at a constant acceleration is the distance=one half multiplied by the acceleration multiplied by the time squared (d=1/2at²).

Using the graph

The graph equation y=mx+b is used in physics. It is mostly used in constant velocity and constant acceleration. The majority of the time the b value is about equal to 0, and all that remains is the y=mx. In the case for constant velocity the y=mx translates to d=vt, the slope m by itself translates to m=d/t. For constant acceleration the y=mx translates to d=1/2at², the slope m itself translates to m=1/2a. The slopes of the graphs are very useful to find the velocity and the acceleration.