29 terms

# Excelsior Physics Unit 3

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Momentum
Inertia in motion or the mass of an object multiplied by its velocity. Therefore, the faster the speed, or the larger the mass, the higher the momentum. In order to calculate momentum, you would multiply the mass times the speed. If one person is traveling at 10 m/s and another at 5 m/s, the first person has twice the momentum of the second because their speed is twice as high.
Impulse
Change in momentum= the impulse acting on it. The greater the impulse exerted on something, the greater the change in momentum. Impulse and momentum have a direct relationship. When one goes up, the other goes up the same amount. When one goes down, the other goes down the same amount.
Momentum equation
p=mv where p is momentum, m is mass in kg and v is velocity in m/s. In order to calculate equations, simply substitute the values given and use Algebra to solve for the unknown variable.
Impulse equation
I=F(change in t) where I is the impulse, F is the force in Newtons, and t is the time in seconds.
If the momentum of an object changes, either the mass or the velocity or both changed. Since mass of objects is usually constant, the velocity must change, and we know that forces cause changes in velocity. The amount of time a force acts on an object is important to its momentum.
Impact time
The time during which your momentum is brought to zero. Extending the impact time reduces the force of the impact and decreases resulting deceleration. For example, if you are a boxer in a fight, by moving away from the punch as you absorb it, you are increasing the impact time and therefore decreasing the force of impact making it hurt less. The opposite is also true. If you move into the punch, the impact time decreases and the force of impact increases and it hurts more.
Law of Conservation of Momentum
Says that in the absence of an external force, the momentum of a system remains unchanged. Says that if two objects collide, the total momentum before equals the total momentum after. The momentum of a system is conserved without the external forces acting upon it. Consider the cannon and its cannonball. The momentum before firing is zero. After the firing, the net momentum is still zero because the momentum of the cannon is equal and opposite to the momentum of the cannonball. Remembering that momentum = mass times speed, if the speeds were the same, the masses would have to be as well and vice versa.
Elastic collisions
Two objects collide and bounce. They do not become conjoined, they do not become permanently deformed, and they do not generate heat. In order to figure out problems of this type, you add the mass times the velocity of each object before the collision which will equal the mass times the velocity of each object after the collision. For example, if a billiard ball that weighs 3 kg travels at 4 m/s towards a marble that weighs 2 kg at rest, after they collide, the billiard ball is traveling at 2 m/s. How fast is the marble traveling? To solve, you multiply all the masses with the velocities and set the two before equal to the two after. Therefore,
(3kg)(4m/s)+(2kg)(0m/s)=(3kg)(2m/s)+(2kg)(v)
12 + 0 = 6 +2v
12 = 6 + 2v
6 = 2v
3 = v
Inelastic collisions
Two objects collide and become conjoined. This can be because they couple with each other or become entangled. When train cars collide and couple, there is one car sitting still and another moving towards it at 4 m/s. Once they couple, both cars move at half the velocity (2 m/s).
Elastic Collisions Equation
(M1V1)+(M2V2)=(M3V3)+(M4V4)
Inelastic Collisions Equation
(M1V1)+(M2V2)=(M1+M2)(V3)
Work
An activity involving a force and movement in the direction of the force. Measured in Joules.
Power
The rate of doing work. Measured in watts.
Energy
Measured in Joules.
Work Equation
W = Fd where W is work measured in Joules, F is Force in Newtons, and d is distance in meters. Remember that there is roughly 20 Newtons in 1 kg. Therefore, when given a problem with 240 J of work used to lift an object with a mass of 6 kg, in order to figure out how far the object could be lifted, you would need to first change the 6 kg into Newtons. To do so, multiply 6 by 20 to determine that it was 120 Newtons. Next, divide the Work by the Force to get 2 meters.
Power Equation
P = W/t where P is power, W is work in joules and t is time in seconds.
Potential Energy
The energy stored in an object because of its position/height. It is measured in Joules. Work is required to elevate objects against Earth's gravity. The potential energy due to its elevated position is called gravitational potential energy. The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity in lifting it.
Kinetic Energy
The energy of motion of an object, measured in joules.
Potential Energy Equation
PE=mgh where PE is the Potential Energy in Joules, m is the mass in kg, g is the gravitational acceleration in meters per second squared, and h is the height in meters. For example, if a 20 kg rock is sitting 3 meters off the ground, what is the Potential Energy of the object? To solve, substitute 20 for the m, 10 for the g, and 3 for the g. Multiply them all together and get 600J
Kinetic Energy Equation
KE = (mv2)/2 where KE is Kinetic energy, m is mass in kg, and v is velocity in meters per second
Law of Conservation of Energy
Says that energy cannot be created or destroyed. It can transformed, but the total amount of energy never changes. When an object sits about to to move, it is completely full of potential energy, as it moves it changes into fully kinetic energy and then as it stops it leans back into fully potential energy. However, throughout the entire process, the total energy is still the same.
A measure of the force amplification achieved by using a tool or machine.
Efficiency
Helps to measure how productive or wasteful a process is.
Efficiency Equation
Efficiency = useful work output divided by the total work output and multiplied by 100 to create a percentage. To determine how efficient something is, you simply divide the output by the input and then multiply by 100. For example, if you wanted to know how efficient a device was with an input of 200 J and an output of 50 J, you would divide 50/100 and get .25, then multiply that by 100 to get 25% efficient. The higher the percentage, the more efficient the device.
Rotation
An object spinning around itself- like turning in a circle. Most rotations occur by rotating around an axis. An axis is the straight line around which the rotation takes place.
Revolution
An object spinning around something else- like the earth revolving around the sun.
Tangential speed
The speed of something moving in a circular path. It is called tangential because the direction of motion is always tangent to the circle.
Rotational speed
The number of rotations per unit of time. If two ladybugs were sitting on a turntable, one near the center and one near the edge, their rotational speed would be equal because all parts of a rigid merry-go-round and the turn table rotate about their axis in the same amount of time, thus all parts have the same rate of rotation. However, their linear speeds would be different since one ladybug is further away from the axis of rotation. The ladybug twice as far away from the axis moves twice as fast. If it was three times as far out from the axis, it would move three times as fast and so on.
Centripetal Force
Any force that causes an object to follow a circular path. Centripetal means "center seeking" or "toward the center". The pull of the moon towards the Earth is a centripetal force, as is the motion of a car going around a corner, and a washing machine spinning the clothes to wring them dry. Additionally, when spinning a can connected to a string in a circular path, the only force acting upon the can is that of the string to pull it inward, and the outward force is on the string, not the can. If you add a ladybug to the can, the only force acting on the ladybug is the force of the can against the bug.
Centrifugal Force
An outward force attributed to circular motion. This term means "center-fleeing" or "away from the center". When whirling a can connected to a string in a circular motion, if the string breaks, the can goes off in a tangential straight line path because no force is acting on it to pull it towards the center anymore.