La tarea de física

PROBLEMS

EFFECTS OF MOTION

1. A tennis player accelerates the tennis ball with a mass of 75 grams, from rest to a speed of 45 m/s, if the acceleration is constant when the racket hits the ball at a distance of O. 85 meters, what is the force magnitude exerted on the ball by the racket?

3. A jet plane with a weight of2.75 x 106N is ready for taking off. If its engines give a push Of 6.35 x 106 N, what distance will it need to have a minimum speed for taking off of 285 km/h?

5. An automobile with mass2300 kg starts from rest to obtain a speed of 72 km/h in 10 seconds. Find the magnitude of the exerted force.

HORIZONTAL AND FRICTIONLESS INCLINED PLANE MOTION

7.A force of 400 N is exerted on an object with mass 30 kg to slide up on a 450 slope (figure 1.70). If the force is parallel to the plane and friction less, calculate the object's acceleration and the normal force.

9. A force of 150 N is exerted on a 100-N object makes an angle of 300º above the horizontal. If the object slides on a horizontal frictionless surface, calculate the acceleration and normal force.

11. A force of 160 N is exerted on 20-N object. The object is pushed upwards by a slope with an angle of 3C above the horizontal. If the force is parallel to the plane and frictionless, calculate the object's acceleration and the normal force.

ILL. MOTION ON HORIZONTAL AND INCLINED PLANE

13. A force of 35 N is exerted on an object to slide it with constant speed on a horizontal surface (figure 1.72). If the object with mass12 kg, what is the coefficient of kinetic friction between the object and the surface?

15. A 300 N force, parallel to an inclined plane at 400, pushes upwards a box with mass 25 kg and produces an acceleration of 1.5 m/s^2 (Figure '1.74). Calculate the coefficient of kinetic frictionμk between the box and the inclined plane.

17. A wooden box with mass 30 kg is pushed along horizontal surface by a parallel force of 135 N. If the box slides with constant speed, determine the coefficient of kinetic friction between box and surface.

19. A skier with mass 80 kg slides on a hill with a slope of 370. If the coefficient of kinetic friction is 0.12, calculate the acceleration of skier.

STATIC

21.An object with mass 14 kg suspended from a chord T1 pulled horizontally by a chord T2 and fasten to the chord T1 makes an angle of 500 with the wall. (See figure 1.77). Determine tension T1 and T2.

23. At a twin's boys party a piñata with weight 30 N is suspended from a rope that makes an angle 300 above the horizontal, as it is shown on figure 1.79. Calculate the exerted force by each child.

CONCEPT ANALYSIS

ANSWER THE FOLLOWING QUESTIONS AND EXPLAIN YOUR ANSWERS

VECTORS

1. Mention the difference between scalar and vector quantities.

2. How does the magnitude of the resultant vary between two vectors if the angle between them decreases?

3. What is the addition of three vectors that make a closed triangle?

4. What is the graphical procedure to add two concurrent vectors?

5. What is the graphical procedure to add two non-concurrent vectors?

6. Two vectors of force have magnitude of F1 8 N and F2 10 N, what is the angle for the magnitude of the resultant is maximum? And minimum?

7. Determine the magnitudes of the maximum and minimum resultant of the previous question.

8. What are the conditions where the magnitude of a vector is equal to one of its components?

9. If a vector F is sketched on a plane xy, what interval of values can take the angle if:

a. Its components x and y are positive

b. Its component x is positive and its component y is negative

c. Its components x and y are negative

d. Its component x is negative and its component y is positive

DYNAMICS

10. A resultant force F produces an acceleration a on a car. If now the resultant force is duplicated 2F, what will be the change in the acceleration of the car?

11. A resultant force F produces an acceleration a on a truck. If now it is load in such a way that double the mass. What will be the change in the acceleration of the truck?

12. A truck can accelerate 3m/s , if it loses part of its load, its mass is 1/3 of its initial mass m, what acceleration can develop the truck if the force does not change?

13. What do we impulse walk?

14. If a cannon fires a shell, how do you compare the magnitude of the force that the cannon exerts on the shell with the force that the shell exerts on the cannon? How are the cannon and the shell accelerations?

15. Suppose that you weigh in front of a sink. Using the idea of action and reaction, why is lower the weighing scale reading when you push downwards the sink? Why is it greater weighing scale reading when you pull upwards the sink from the bottom?

16. If two 40 N weights are hanged on each end of the dynamometer which is placed horizontally on the table. Will the dynamometer show a reading of 40 N or 80 N? Justify your answer.

17. Your weight change when you travel in an elevator that moves at constant speed? If the elevator accelerates, will your weight change?

18. A body on the Earth has mass of 30 Kg, What will be the mass of the body if it is taken to Saturn where the gravity is 14m/s?

19. If a truck accelerates from the rest throws the passenger backwards. Why?

20. If the truck driver stops throws the passenger forward, why?

21. A car spends more gasoline when it drives through the city than when it drives through the highway, why?

22. Explain the difference between weight and mass.

SELF-ASSESSMENT

1. The resultant of the system of forces is represented by F1 and F2 showed in Figure 1.80, the best representation is:

A) R1

B) R2

C) R3

D) R4

2. The resultant (R) of the system of forces represented by F1, F2, and F3 showed in Figure 1.81, the best representation is:

A) R1

B) R2

C) R3

D) R4

NEWTON'S LAWS

2. The inertia of an object depends on:

d) Its mass

c) Its weight

a) Its volume

b) Its density

3. If L represents length, t time and m mass, the dimensions of the force are:

a) ml/t

b) ml/t^2

c) 〖ml〗^2

d) lt/m

4. To accelerate a mass of 3 kg on a frictionless surface on the Earth is needed a force of 15 Newton, for the mass to have the same acceleration somewhere in space where the gravitational attraction of the Earth is almost null, we would need a force of:

a) 0 N

b) 3 N

c) 15 N

d) 29 N

5. If an object accelerates 6m/s2 when a resultant force of 30 N is exerted on it, to have an acceleration of 4 m/s2 the resultant exerted force must be:

a) 18 N

b) 16 N

c) 20 N

d) 21.5 N

6. If a horizontal constant net force exerts on object at rest on a table frictionless, the object:

a) always will move with constant speed

b) sometimes will accelerate

c) always will accelerate

d) always will accelerate if the force is greater than its weight

7. On the Moon the value of gravity is approximately - of the Earth. If an object on Earth has a mass of 5 kg, on the Moon it would have:

a) amass of 5 kg and weight of 5 N

b) amass of 5 kg and weight of 8 N

c) a mass of 0.51 kg and weight of 5 N

d) a mass of 0.51 kg and weight of 0.82 N

8. A frictionless force F produces an acceleration when is exerted on an object with mass m, then when it double its mass, the resultant acceleration will be:

a) a/2

b) 4a

c) 2a

d) a/6

9. A frictionless force F produces an acceleration when it is exerted on an object with mass m, then when its force is tripled, the resultant acceleration will be:

a) a/2

b) 3a

c) 2a

d) a/3

10. When there is no friction a force F produces an acceleration when acting on object with mass m, when the mass is tripled and the force increases six times, its resultant acceleration would be:

a) a/2

b) 6a

c) 2a

d) a/6

11. A block with mass m slides on an inclined plane frictionless, as it is shown on figure 1.82. The normal force exerted by the inclined plane on the block will be:

a)mg(cos 9)

b) g(sin 9)

c) mg(sin 9)

d) mg(tan 9)

12. The block showed in figure 1.82 is on an inclined plane frictionless, then its acceleration will be:

a)g(sinß)

b) g)

c) g(tanß)

d) g(cosß)

13.From the graph of acceleration versus time of an object with constant mass as shown in figure 1.83. At what interval of time the force on the object equals to zero?

a) from 2s to 4s

b) fromOs to 2s

c) from 4s to 8s

d) from 8s to 10s

14. From figure 1.83, at what interval of time the force on the object is constant and different from zero?

a) from 8s to 10s

b) from 4s to 8s

c) fromOs to 2s

d) from 2s to 4s

15. From figure 1.83, at what interval of time the force on the object decreases?

a) from 8s to 10s

b) from 4s to 8s

c) from 0s to 2s

d) from 2s to 4s

16. The mass of an astronaut in a planet where the gravity is ten times greater than the Earth, is:

Ten times greater

b) the same

c) ten g times less

d) ten g times greater

17. Is it possible to invent a technique to push a table without having it return the push?

a) yes, if somethingalso pushes it

b) yes, in space

c) no

d) a table never pushes

18. A man with weight 700 N holds a 50-N bag of tomatoes standing on a scale in an amusement park. He throws the bag upwards and before leaving his hands, a card with its weight out from the slot of the scale, what will be the weight?

a) 700 N

b) more than 750 N

c) 750 N

d) less than 750 N

19. Imagine that you are standing on a cardboard box that barely could withstand your weight. What would happen to the box if you jump vertically upward?

a) it would move to sideways

b) it would flatten

c) nothing would happen

d) it would also jump

20. An object is suspended from a chord and accelerates downward with an acceleration of 0.7 g. The tension on the chord would be:

a) equal to the object's weight

b) greater than the object's weight

c) less than the object's weight

d) equal to zero

21. A person weighs 490 N on a scale on an elevator

22. What will be the reading of the scale if the elevator is at rest?

a) ON

b) 490 N

c) 980 N

d) 590 N

23. If the elevator begins to move upward and accelerates the person upwards at 2m/s2 what will be the reading in the scale?

a) 490 N

b) 0 N

c) 590 N

d) 390 N

22. When the elevator has a convenient speed, it stops accelerating. What would be the reading in the scale when the elevator moves uniformly upwards?

a) 590 N

b) 980 N

c) ON

d) 490 N

23. If the cable bursts and the elevator freely falls, what would be the reading in the scale?

a) 980 N

b) 490 N

c) 390 N

d) 0N

24. What would be the reading of the scale if the elevator descends with a constant speed?

a) 490 N

b) 590 N

c) 390 N

d) 0N

25. If the elevator moves downward with an acceleration of 22 what will be the reading on the scale?

a) 590 N

b) 390 N

c) 0 N

d) 980 N

FRICTION FORCE

26. Why more force is needed to start walking than to keep walking?

a) because the kinetic friction force is less than the static friction

b) because the air resistance the speed of the long walk is less than the inertia needed, F= m a

c) not true

d) none of the previous is true

27. Imagine a scooter with well-oiled wheels downs. What will happen if from rest, it starts moving?

a) It will advance towards you

b) It will move back and forth

c) It will remain at rest

d) It will move quickly in opposite direction

28. Suppose that the block showed in figure 1.82 slides downwards at constant speed. The coefficient of kinetic friction, μK, between the block and the plane's surface is given by:

a) 1-cosθ

b) tanθ

c) cosθ - sinθ

d) mg(sinθ)

29. A block of mass m is pushed on a surface, as it is shown on figure 1.84. The block's speed is constant. The kinetic friction coefficient between the block and the surface is μ. Therefore the tension of the chord is given by:

T=mg/μ

T= mg

T= mg μ

None

30. Which of the following relationships, is true:

a) μs>μk

b) μs<μk

c) μs – μk = 0

d) μs = μk = 0

STATIC

31.Ifthe sum of forcesexertedonanobjectequals to zero, thentheobjectwould be:

a) at rest

b) withconstantspeed

c) with no acceleration

d) all are true

32.Ifanobject has translationalequilibrium, thentheobjectcould be:

a) uniformstraightmotion

b) at rest

c) withaccelerateduniformmotion

d) a and b are correct

33.Anobjectisdroppedverticallyupward. At thehighestpoint of itspath, theobjectwill be:

a) at equilibrium

b) not at restnorequilibrium

c) at rest and instantaneousequilibrium

d) at instantaneousrest

34.Which of thefollowingstatementsthat describes anobject at equilibriumisnot true?

a) the sum of vectors of allforcesexertedonanobjectequals to zero

b) theobjectremains at rest

c) theobjectmovesconstantlyspeed

d) theobjectmoveswithconstantacceleration

35.Anobjectmoves at constant speed. The total force exertedontheobjectis given by:

a) F= 0

b) F =mg

c) F =mv

d) F =ma

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