Physics Exam #2

Kinetic energy

(KE)- The energy of motion; equal (nonrelativistically) to the mass multiplied by the square of the speed, multiplied by the constant ½ KE= ½ mv^2.

Potential energy

(PE)- The energy of position; usually related to the relative position of two things, such as a stone and Earth (gravitational PE), or an electron and a nucleus (electric PE).

Work

(W)- The product of the force on an object and the distance through which the object is moved (when force is constant and motion is in a straight line in the direction of the force); measured in joules. Work= force x distance.

15. How much power is required to do 100 J of work on an object in a time of 0.5 sec?

- (100 J)/ (0.5 s) = 200 W

12. How many joules of work are done on an object when a force of 10 N pushes it a distance of 10 m?

- 10N x 10 m = 100 J

20. What is the kinetic energy of a 4 kg football traveling at 10 m/s?

- 2 x 100= 200

17. A boulder is raised above the ground so that its potential energy relative to the ground is 200 J. Then it is dropped. What is its kinetic energy just before it hits the ground?

- 200 J

18. What will be the kinetic energy of an arrow having a potential energy of 50 J after it is shot from a bow?

- 50 J - A drawn arrow has 50 J of stored energy due to the stretch of the bow and string. When released, this energy is converted into kinetic energy such that the arrow will have 50 J of kinetic energy upon being fired. Of course, this assumes no energy is lost to sir resistance, friction or any other non- conservative forces and that the arrow is shot horizontally.

41. If a moving object doubles its speed, by how much does its (A) momentum and (B) kinetic energy change?

- A) Since momentum is directly proportional to velocity, it will have twice as much momentum as it originally had. B) Kinetic energy is doubled.

Define angular momentum and describe the conditions under which it (a) remains the same and (b) changes.

- Angular Momentum- The product of a body's rotational iner-tia and rotational velocity about a particular axis. For an object that is small compared with the radial distance, its magnitude is the product of mass, speed, and radial distance from the spin axis, Angular momentum= mvr. Angular Momentum is defined as the product of rotational inertia and rotational velocity: Angular momentum = rotational inertia x rotational velocity. Just as an external net force is required to change the linear momentum of an object, and external net torque is required to change the angular momentum of an object. We can state a rotational version of Newton's first law (the law of inertia): An object or system of objects will maintain its angular momentum unless acted upon by an external net torque.

Describe center of gravity. (Pg. 140-141)

- Center of Gravity (CG)- The point at the center of an object's weight distribution where the force of gravity can be considered to act. The center of gravity is the average location of the weight of an object. We can completely describe the motion of any object through space in terms of the translation of the center of gravity of the object from one place to another, and the rotation of the object about its center of gravity if it is free to rotate.

Describe center of mass.

- Center of Mass- The point at the center of an object's mass distribution where all its mass can be concentrated. For everyday conditions, it is the same as the center of gravity. The center of mass is the point at which all the mass can be considered to be "concentrated" for the purpose of calculating the "first moment", i.e., mass times distance. For two masses this distance is calculated from. -The difference between Center of Gravity and Center of Mass: - Center of gravity is the point in a body around which the resultant torque due to gravity forces vanish. That means that for any rigid body, the two points are the same, because you can model rigid bodies in free fall as if gravity acted only on the center of mass, and forces on the center of mass make no torque. * Since weight and mass are proportional, center of gravity and center of mass refer to the same point of an object.

. Give examples of centripetal force.

- Centripetal force is a force on an object directed to the center of a circular path that keeps the object on the path. It's value is based on three factors: 1) the velocity of the object as it follows the circular path; 2) the object's is distance from the center of the path; and 3) the mass of the object. Centripetal force is easily calculated as long as you know the mass m of the object, its distance r from the center and the tangential velocity v. The following equation is based on the metric system; note that the centripetal force f is measured in Newton's. One Newton is approximately 0.225 lb. Do you remember riding on the merry-go-round as a kid? Did you ever stand at the very edge of the merry-go-round and hold on tight to the railing as your friends pushed the wheel faster and faster? Maybe you remember that the faster the wheel turned, the harder it became to hold on. You might not have known it at the time, but you were creating a balance between two forces, one real and one apparent, in order to stay on that circular path. Merry-go-rounds are a perfect example of how a force is used to keep an object moving in a circular path. Your body wanted to fly off the merry-go-round in a straight line, but your hands exerted an opposing force to keep you on. The tendency for your body to fly off the merry-go-round is called centrifugal force. It isn't a real force, but an apparent one. The force you used with your hands to stay on the ride is real, and it is called centripetal force. Let's learn more about it.

36. Why do gymnasts use floor mats that are very thick and spongy?

- Gymnasts use floor mats because the cushion in the mat extends the time and lessens the force.

22. What is impulse? Formula?

- Impulse- The product of force and the time interval during which the force acts. An impulse produces a change in momentum. Impulse = Ft= Δ(mv)

Define kinetic energy and describe the work-energy theorem. (Pg. 114-115)

- Kinetic Energy (KE)- The energy of motion; equal (nonrelativistically) to mass multiplied by the square of the speed, multiplied by the constant ½. KE= ½ mv^2 - Work- Energy Theorem- When a car speeds up, its gain in kinetic energy comes from the work done on it. Or, when a moving car slows, work is done to reduce its kinetic energy. Work=ΔKE. Work equals change in kinetic energy. This is the work-energy theorem. The work in this equation is the net work- that is, the work based on the net force.

34. To calculate momentum of an object, you multiply its _____________ and ___________.

- Mass and Velocity

Define mechanical energy. (Pg.113)

- Mechanical Energy- Energy due to the position or the movement of something; potential or kinetic energy (or a combination of both).

33. In an inelastic collision, momentum is conserved. (Pg.99)

- Momentum

21. What is momentum? Formula?

- Momentum- Inertia is motion. The product of the mass and the velocity of an object (provided the speed is much lass than the speed of light). Momentum has magnitude and direction and therefore is a vector quantity. Also called linear momentum and abbreviated p=mv.

23. HOW are impulse, momentum, Newton's 2nd Law all related? You can use formulas- be logical in your explanation!!!

- Newton's 2nd law states that if no net force is exerted on a system, no acceleration occurs. This relates to momentum because no acceleration= no change in velocity or momentum (Mass x velocity). Also, no net force= no net impulse and thus no change in momentum. Impulse = the change in momentum (force x time interval)

16. What are the two main forms of mechanical energy?

- PE and KE

19. What is the potential energy of a 10 kg box 10 m above the floor?

- PEgrav= mass x g x height m= represents the mass of the object h= height of the object g= gravitational pull 10 x 10= 100

Define potential energy. (Pg.113)

- Potential Energy (PE)- The energy of position; usually related to the relative position of two things, such as a stone and Earth (gravitational PE), or an electron and a nucleus (electric PE)

44. Define and describe power. (Pg.112)

- Power -The rate at which work is done or energy is transformed; equal to the work done or the energy transformed divided by time; measured in watts. Power= work/time. Power= work done/ time interval

13. What is power?

- Power-The rate at which work is done or energy is transformed; equal to the work done or the energy transformed divided by time; measured in watts. Power= work/ time.

14. In which situation is more power required: Slowly lifting a book bag full of books up the stairs or quickly lifting the same book bag full of books up the same stairs?

- Quickly

Describe rotational speed.

- Rotational speed (sometimes called angular speed) is the number of rotations or revolutions per unit of time. All parts of the rigid merry-go-round and turntable turn about the axis of rotation in the same amount of time. Thus, all parts share the same rate of rotation, or the same number of rotations or revolutions per unit of time. It is common to express rotational rates in revolutions per minute (RPM).

Describe the work-energy theorem.

- The energy associated with the work done by the net force does not disappear after the net force is removed (or becomes zero), it is transformed into the Kinetic Energy of the body. We call this the Work-Energy Theorem.

Distinguish between rotate and revolve.

- The person in the horse is revolving around the center, however rotation of the horse and person would occur if the person in the horse spins. The Earth, spinning on its axis, revolves around the Sun. The Moon, rotating on its axis, revolves around the Earth.

State the law of conservation of energy. (Pg. 117)

- The study of various forms of energy and their transformations from one form into another has led to one of the greatest generalizations in physics- the law of conservation of energy: Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes. - Definition of the Conservation of Energy- The Principle that energy cannot be created or destroyed. It may be transformed from one form into another, or transferred from one object to another, but the total amount of energy never changes. Work input= work output

Define and describe torque. (Pg. 139)

- Torque- The product of force and the lever-arm distance, which tends to produce rotational acceleration. Torque= lever- arm distance x force. - Torque= lever arm x force

. Define and describe work. (Pg.110- 111)

- Work (W)- The product of the force on an object and the distance through which the object is moved (when force is constant and motion is in a straight line in the direction of the force); measured in Joules. Work = force x distance. Work is a transfer of energy. Work= force x distance W=Fd

11. What is the momentum of an 6 kg bowling ball rolling at 2m/s?

- p= m x v m= is the mass v= velocity 6 kg x 2m/s= 12 kg x m/s

26. Understand the Conservation of Energy and be able to solve problems using the Conservation of Energy.

-Conservation of Energy- The principle that energy cannot be created or destroyed. It may be transformed from one form into another, or transferred from one object to another, but the total amount of energy never changes.

Give an example in which rotational speed changes but angular momentum does not.

-Everyone has seen the classic "scratch spin" in figure skating, where the skater draws her arms and a leg in and speeds up tremendously. This is the result of conservation of angular momentum: as the skater reduces her rotational inertia by pulling her arms and leg in, her rotation speed must increase to maintain constant angular momentum. Angular momentum conservation plays a VERY important role in all figure skating routines.

Describe how to find the center of gravity of an irregularly shaped object.

-The center of gravity is a geometric property of any object. The center of gravity is the average location of the weight of an object. We can completely describe the motion of any object through space in terms of the translation of the center of gravity of the object from one place to another, and the rotation of the object about its center of gravity if it is free to rotate. In flight, rockets rotate about their centers of gravity.

Centripetal force

A center- directed force that causes an object to follow a curved or circular path.

Elastic collision

A collision in which colliding objects rebound with no lasting deformation or heat generation

Inelastic collision

A collision in which the colliding objects become distorted and/ or generate heat during the collision, and possibly stick together.

Machine

A deices for increasing (or decreasing) a force or simply changing the direction of a force.

Equilibrium

In general, a state of balance. For mechanical equilibrium, the state in which no net force and no net torques act. In liquids, the state in which evaporation equals condensation. More generally, the state in which no net change of energy occurs.

Conservation of momentum

In the absence of a net external force, the momentum of an object or system of objects is unchanged.

Momentum

Inertia in motion. The product of the mass and the velocity of an object (provided the speed is much less than the speed of light). Momentum has magnitude and direction and therefore is a vector quantity. Also called linear momentum and abbreviated p=mv.

Tangential speed

Linear speed along a curved path.

1. Distinguish between mass and momentum. Which is inertia and which is inertia in motion?

Mass is inertia while momentum is inertia in motion. Momentum combines mass and weight.

10. Distinguish between an elastic and an inelastic collision.

No kinetic energy is lost during an elastic collision. Objects are distorted and generate heat during an inelastic collision.

Rotational inertia

Reluctance or apparent resistance of an object to change its state of rotation, determined by the distribution of the mass of the object and the location of the axis of rotation or revolution.

Energy

That which can change the condition of matter. Although energy is commonly defined as the ability to do work, it is actually describable only by examples.

8. Which impulse is greatest?

The impulse is greatest when catching then throwing.

2. When the force of impact on an object is extended in time, does the impulse increase or decrease?

The impulse will increase.

3. In a car crash, why is it a good idea for an occupant to extend the time during which the collision takes place?

The larger the time of impact, the smaller the force required to change the occupant's momentum.

Rotational speed

The number of rotations or revolutions per unit of time; often-measured in rotations or revolutions per second or minute.

Center of mass (CM)

The point at the center of an object's mass distribution where all its mass can be considered to be concentrated. For everyday conditions, it is the same as the center of gravity.

Center of gravity (CG)

The point at the center of an object's weight distribution where the force of gravity can be considered to act.

Conservation of energy

The principle that energy cannot be created or destroyed. It may be transformed from one form into another, or transferred from one object to another, but the total amount of energy never changes.

Angular momentum

The product of a body's rotational iner-tia and rotational velocity about a particular axis. For an object that is small compared with the radial distance, its magnitude is the product of mass, speed, and radial distance from the spin axis, Angular momentum= mvr.

Torque

The product of force and the lever-arm distance, which tends to produce rotational acceleration. Torque= lever-arm distance x force.

9. What does it mean to say that momentum is conserved?

The total momentum before the collision equals the total momentum after the collision.

Conservation of energy for machines

The work output of any machine at steady state cannot exceed the work input.

Conservation of angular momentum

When no external torque acts on an object or a system of objects, no change of angular momentum takes place. Hence, the angular momentum after the event.

6. Do you experience an impulse when you catch a ball of the same speed?

Yes

7. Do you experience an impulse when you catch it and then throw it out again?

Yes

5. When you throw a ball, do you experience an impulse?

Yes.

Efficiency

in a machine, the ratio of useful energy output to total energy input, or the percentage of the work input that is converted to the work output. Efficiency= useful energy output/total energy input

Impulse

the product of force and the time interval during which the force acts. An impulse produces a change in momentum. Impulse= Ft=Δ(mv).

Work- energy theorem

the work done on an object is equal to the kinetic energy gained by the object. Work= ΔKE