Physics 101 EXAM 2 STUDY GUIDE

9. Which is easier to get swinging, a baseball bat held at the narrow end or a bat held closer to the massive end?

A bat held closer to the massive end is easier to get swinging

Inelastic collision

A collision in which objects become distorted, generate heat and possibly stick together.

Elastic collision

A collision in which objects rebound without lasting deformation or the generation of heat.

Law of conservation of Energy

Energy cannot be created or destroyed, it may be transformed form one form to another but the total amount of energy never changes

Mechanical energy

Energy due to the position of something of the movement of something

5. Exactly what is it that enables an object to do work?

Energy enables an object to do work

Kinetic energy

Energy that something possesses because of its motion, quantified by the relationship

Potential Energy

Energy that something possesses because of its position

25. What impulse occurs when the same force of 10 N acts on the cart for twice the time? Impulse = Ft

I = (10N) (5 s) = 50 N . s

27. How much impulse stops a 50 kg carton that is sliding at 4 m/s when it meets a rough surface?

I = mv = (50kg)(4 m/s) = 200kg . m/s = 200 N . s

21. What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?

The efficiency is 100%

13. Is it correct to say that if no net impulse is exerted on a system then no change in the momentum of the system will occur?

Yes, the statement is correct.

29. Show that 2.4 J of work is done when a force of 2.0 N moves a book 1.2 m. W=fd

(2.0 N)(1.2m) = 2.4 N . m = 2.4 J

33. Show that about 786 W of power is depended when a 500 N barbell is lifted 2.2 m above the floor in 1.4 s P=W/t

(500N)(2.2m) / 1.4 s = 786 W

37. Calculate the kinetic energy of an 84 kg scooter moving at 10 m/s KE= 1/2 mv(squared)

1/2(84kg) (10m/s)squared = 4200 kg (m/s) squared = 4200 J

45. To tighten a bolt, you push with a force of 80 N at the end of a wrench handle that is 0.25 m from the axis of the bolt. A. What torque are you exerting? B. You move your hang inward to be only 0.10 m from the bolt. To achieve the same torque, show that you should exert 200 N of force C. Do your answers depend on the direction of your push relative to the direction of the wrench handle?

A. Torque = force x lever arm = (0.25m) (80N) = 20 N . m B. Force = 200 N, then (200 N) (0.10 m) = 20 N . m C. Yes. These answers assume that you are pushing perpendicular to the wrench handle.

45. In terms of impulse and momentum, why do air bags in cars reduce the risk of injury?

Air bags lengthen the time of impact, thereby reducing the force of impact.

51. When you catch a fast moving baseball with your bare hand, why is it important to extend your hand forward for the catch?

An extended hand allows more time for reducing the momentum of the ball to zero, resulting in a smaller force of impact on your hand.

43. If a person's speed doubles and all else remains the same, what will be the person's angular momentum?

Angular momentum = mvr = (80kg) (6m/s) (2m) = 960 kg . m2/s

19. If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much?

As force is increased, distance is decreased by the same factor.

77. Why is it important to secure file cabinets to the floor, especially cabinets with heavy loads in the drawers?

Because the CG of the cabinet can easily be shifted beyond the support base of the cabinet. When this happens, the torque that is produced causes the cabinet to topple over.

17. Railroad car A rolls ar a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?

Car B has the speed of car A before the collision.

41. Calculate the force of friction that keeps an 80 kg person sitting on the edge of a horizontal rotating platform when the person sits 2 m from the center of the platform and has a tangential speed of 3 m/s. Centripetal force: F = mv(squared) /r

F = (80kg) (3 m/s)squared/2 m = 360 N

31. A car carrying a 75kg test dummy crashes into a wall at 25 m/s and is brought to rest in 0.1 s. Show that the average force exerted by the seat belt on the dummy is 18750N.

Ft = mv F = mv/t so (75kg)(25 m/s) / .1 s = 18750 N

9. Why is it advantageous to roll with the punch in boxing?

If you roll with the punch, the impact occurs over a long time, which means a less forceful punch.

15. An apple hanging from a limb has potential energy because of its height. If it falls, what becomes of this energy just before it hits the ground? When it hits the ground?

Immediately before the apple hits the ground, its initial PE becomes KE. When it hits the ground its energy becomes thermal energy.

3. What are two ways to increase impulse?

Impulse can be increased by increasing the force or increasing the time of application.

41. Marshall pushes crates starting from rest across the floor for 3s with a net force of, 100N, 75N, 50N. Rank from greatest to least for: Impulses delivered Changes in momentum Final Speeds Momenta in 3s

Impulse delivered : ABC Changes in momentum: ABC Final Speeds: CBA Momenta in 3s: ABC

Impulse - Momentum relationship

Impulse is equal to the change in the momentum of the object that he impulse acts upon. Ft=mv

Law of conservation of momentum

In the absence of an external force, the momentum of a system remains unchanged. Hence, the momentum before an event involving only internal forces is equal to the momentum after the event.

7. A car is raised a certain distance in a service station lift and therefore has potential energy relative to the floor. If it were raised twice as high, how much more potential energy would it have?

It would have twice the PE because the distance is two times greater

55. A lunar vehicle is tested on Earth at a speed of 10 km/h. When it travels as fast on the moon, is its momentum more less, or the same?

Its momentum is the same. (its weight might change, but not mass)

7. When you are struck by a moving object, is it favorable that the object makes contact with you over a short time or over a long time? Explain.

Less force occurs if momentum is decreased over a long time.

35. A 5kg fish swimming at 1 m/s swallows an absentminded 1kg fish swimming toward it at a speed that brings both fish to a halt immediately after lunch. Show that the speed of the approaching smaller fish before lunch mush have been 5m/s.

Momentum before = Momentum after (5kg) (1m/s) + ( 1kg) v = 0 5m/s + v = 0 v = -5 m/s

15. When a cannonball is fired, momentum is conserved for the system cannon + cannonball. Would momentum be conserved for the system if momentum were not a vector quantity? Explain.

Momentum would not be conserved if force, and therefore impulse, was not a vector quantity.

51. The mass and speed of the three vehicles, A = 800 kg, 1.0 m/s, B = 1000kg, 2.0 m/s, C = 90 kg, 8.0 m/s. Rank them from greatest to least in: Momentum KE Work done to bring them up to their respective speeds

Momentum: BAC KE: CBA Work done: CBA

25. If you are not wearing a seat belt in a car that rounds a curve, and you slide across your seat and slam against the car door, what kind of force is responsible for your slide. Centripetal, centrifugal, or no force?

No force is responsible, you tend to move forward in a straight line and the car curves you

Flip Card, use voi

Ooooo ya ooooooo right there oooooo

23. What is the momentum of a 50 kg carton that slides at 4 m/s across an icy surface? Momentum = mv

P = mv = (50kg)(4m/s) = 200 kg . m/s

5. What is rotational inertia, and how is it similar to inertia as studied in earlier chapters?

Rotational inertia is the resistance to a change in rotational motion. It is similar to plane inertia, which is a resistance to change in velocity

19. Suppose a ball of putty moving horizontally with 1kg*m/s of momentum collides with and sticks to an identical ball of putty moving vertically with 1kg*m/s of momentum. What is the magnitude of their combined momentum.

Since they are same magnitude vectors at right angles to each other, the combined momentum in 2kg m/s

13. How does speed affect the friction between a road and a skidding tree?

Speed has little or no effect on friction

29. What is the law of inertia for rotating systems in terms of angular momentum?

The angular momentum of a system remains constant when no net torque acts

29. Bowling Barry asks how much impulse is needed to stop a 10kg bowling ball moving at 6 m/s. What is your answer?

The bowling ball has a momentum of (10kg)(6m/s) = 60 kg . m/s which has the magnitude of the impulse to stop it. That's 60 N . s

81. A 0.5kg cart on an air track moves 1.0 m/s to the right, heading toward a 0.8kg cart moving to the left at 1.2 m/s. What is the direction of the two cart system's momentum?

The direction of momentum is to the left because the momentum of the 0.8 kg cart is greater. By magnitude, net momentum = (0.5) (1) - (0.8) (1.2) = - 0.46.

23. Is it an inward force or an outward force that is exerted on the clothes during the spin cycle of an automatic washing machine?

The force on the clothes is inward

5. How is the impulse-momentum relationship related to newton's second law?

The impulse momentum relationship is derived from Newton's second law.

13. What is meant by the "lever arm" of a torque?

The lever arm is the shortest distance between the applied force and rotational axis

Tangential speed

The linear speed tangent to a curved path, such as in circular motion

61. When an apple falls from a tree and strikes the ground without bouncing, what becomes of its momentum?

The momentum of the falling apple is transferred to the earth. Interestingly, when the apple is released, the earth and the apple move toward each other with equal one oppositely directed momenta.

Rotational speed

The number of rotations or revolutions per unit of time

Torque

The product of force and lever arm distance, which tends to produce or change rotation

Impulse

The product of the force acting on an object and the time during which it acts.

Work

The product of the force and the distance moved by force

Momentum

The product of the mass of an object and its velocity.

Energy

The property of a system that enables it to do work

Rotational inertia

The property of an object to resist any change in its state of rotation

17. What is recycled Energy?

The reemployment of energy that otherwise would be wasted

1. Which has a greater momentum: a heavy truck at rest or a moving skateboard?

The skateboard, because only it is moving.

15. If you toss a stick into the air, it appears to wobble all over the place. Specifically, about what place does it wobble?

The stick wobbles, spins really, about its center of gravity

Power

The time rate of work

3. A tapered cup rolled on a flat surface makes a circular path. What does this tell you about the tangential speed of the rim of the wide end of the cup compared with that of the rim of the narrow end?

The wide end has a greater tangential speed than the narrow end

39. Calculate the torque produced by the same 50 N force when a pipe extends the length of the wrench to 0.5 m. Torque = lever arm x Force

t = 0.5 m x 50 N = 25 m . N