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Questions and Answers for Topic #05

Topic 5 Concepts and Skills

Vocabulary, Variables and Equations

Review Questions

Application Questions

Word Problems

Topic #05 Concepts and Skills

Link to "Concepts and Skills"
 

Topic 5 Vocabulary:

Free Body Diagram: A diagram that shows the direction of the force operating on (acting on) each object.

Force: A push or pull exerted on an object having magnitude and direction; it may be either a contact or long range force.

Applied Force: A natural or man made force that is applied to an object.

Net Force: The vector sum of all of the forces acting on an object.

Force of Friction: The contact force that act to oppose sliding motion between two surfaces.

Force of Gravity: The long range force due to gravitational attraction between two objects. For example, the earth and another object such as you exert a gravitational pull on one another. 

Weight: A measure of the force of gravity between two objects. (The unit of weight in the metric system is the Newton, N. The unit of weight in the British [English] system is the pound, lb.) Weight can also be defined by using the definition of the force of gravity.

Displacement: The vector quantity that defines the distance and direction between two positions.

Position: The location of an object on a coordinate system such as on a map of the earth where position is recorded in terms of latitude and longitude measurement, much in the same way as a point is expressed on a graph using x and y data values.

Position Vector: The arrow on a motion diagram that is drawn from the origin to the moving object.

Velocity: The vector quantity that defines the speed and direction of a moving object. Descriptive adjectives such as instantaneous, constant and average may be used along with this term.

Acceleration: The vector quantity that defines the rate at which the speed of an object changes as well as the direction in which the change occurs. Descriptive adjectives such as instantaneous, constant and average may be used along with this term.

Air Resistance: The opposition of the atmosphere to the forward movement of an object; it also called aerodynamic drag (force), which is the resistance of air to the movement of an object. It could also be described as air friction, the friction caused by air when an object passes through it.

Terminal Velocity: The constant velocity of an object, that is achieved when the (aerodynamic) drag force equals the force of gravity and the falling object no longer accelerates because the net force acting on it is zero newtons.

Variable Symbols: The symbols used in equations to represent the measurements used in the solving of problems.
 

Review Questions:

Questions:
Section 6.1
1. A physics book is motionless on the top of a table.  If you give it a hard push with your hand, it slides across the table and slowly comes to a stop.  Use Newton’s laws of motion to answer the following questions.
a. Why does the book remain motionless before the force of the hand is applied?
b. Why does the book begin to move when your hand pushes hard enough on it?
c. Why does the book eventually come to a stop?
d. Under what conditions would the book remain in motion at constant speed?
2. Why do you have to push harder on the pedals of a single speed bicycle to start it moving than to keep it moving at a constant velocity?
3. Suppose the acceleration of an object is zero.  Does this mean that there are no forces acting on it?  Give an example supporting your answer.
4. When a basketball player dribbles a ball, it falls to the floor and bounces up.  Is a force required to make it bounce?  Why?  If a force is needed what is the agent involved?

Section 6.2
5. Before a skydiver opens his parachute, he may be falling at a velocity higher than the terminal velocity he will have after the parachute opens.
a. Describe what happens to his velocity as he opens the parachute.
b. Describe his velocity from after his parachute has been open for a time until he is about to land.
6. What is the difference between the period and the amplitude of a pendulum?
7. When an object is vibrating on a spring and passes through the equilibrium position, there is no net force on it.  Why is the velocity not zero at this point?  What quantity is zero?

Section 6.3
8. A rock is dropped from a bridge into a valley.  Earth pulls on the rock and accelerates it downward. According to Newton’s third law, the rock must also be pulling on Earth, yet Earth doesn’t seem to accelerate.  Explain.
9. All forces can be divided into just four fundamental kinds.  Name the fundamental force that best describes the following.
a. Holds the nucleus together
b. Holds molecules together
c. Holds the solar system together

 

Application Questions:

Chapter 6
Applying Concepts

Questions

10. If you are in a car that is struck from behind, you can receive a serious neck injury called whiplash.
  a.  Using Newton's laws of motion, explain what happens to cause the injury.
  b.  How does a headrest reduce whiplash?
11. Should astronauts choose pencils with hard or soft lead for making notes in space?  Explain.
12. If you find a pendulum clock running slightly fast, how can you adjust it to keep better time?
13. Review, analyze, and critique Newton's first law of motion.  Can we prove this law?  Explain.  Be sure to consider the role of friction.
14. What is the meaning of a coefficient of friction that is greater than 1?  How would you measure it?
15. Using the model of friction described in this book, would the friction between the tire and the road be increased by a wide rather than a narrow tire?  Explain.
16. From the top of a tall building, you drop two table tennis balls, one filled with air and the other with water.  Both experience air resistance as they fall.  Which ball reaches terminal velocity first?  Do both hit the ground at the same time?
17. It is often said that 1 kg equals 2.2 lb.  What does this statement mean?  What would be the proper way of making the comparison?
18. Which of the four fundamental forces makes paint cling to a wall?  Which force makes adhesive sticky?  Which force makes wax stick to a car?
19. According to legend, a horse learned Newton's laws.  When the horse was told to pull a cart, it refused, saying that if it pulled the cart forward, according to Newton's third law there would be an equal force backwards.  Thus, there would be balanced forces, and, according to Newton's second law, the cart wouldn't accelerate.  How would you reason with this horse?

 

Word Problems:

Problem Set #5:  Follow the written directions unless indicated otherwise by your instructor.

FORCE, MASS, and ACCELERATION:  F = m a

1.  An unbalanced force of 25N, E, is applied to a 12 kg mass.  What is the acceleration given to the mass?

2.  An unbalanced 16-N force is applied to a 2 kg mass.  What is the acceleration of the mass?

3.  A shot-putter exerts an unbalanced force of 140N on a shot giving it an acceleration of 19  m/s^2.  What is the mass of the shot?

4.  A 1.5 kg mass accelerates across a smooth table at 16 m/s^2.  What is the unbalanced force applied to it?

5.  An object moving with a constant velocity has an unbalanced force applied to it. If the 
unbalanced force is -20.0 N and the mass of the object is 3.75 kg, what is the acceleration of the object while this force is acting?

6.  An unbalanced force of 965N causes an object to accelerate at 54.5 m/s^2.  What is the mass of the object?

7.  Determine the acceleration that an unbalanced force of 25 N gives to a 4.0 kg mass.

8.  A racing car undergoes a uniform acceleration of 8.00 m/s^2.  If the unbalanced force causing the acceleration is 6.00 x 10^3 N, what is the mass of the racing car?

9.  A racing car has a mass of 710 kg.  It starts from rest and travels 120 m in 3.0 s.  The car undergoes uniform acceleration during the entire 3.0 s. What unbalanced force is applied to it?

10.  An artillery shell has a mass of 55 kg the shell is fired from the muzzle of a gun with a speed of 770 m/s. The gun barrel is 1.5 m long. What is the average force on the shell while it is in the gun barrel?

MASS and WEIGHT:  W = m g

11.  Determine the weights of these masses.
 a. 14 kg  b.  0.43 kg c. 0.7 kg

12.  Determine the mass of these weights. 
a. 98N  b. 80N  c. 0.98 N

13.  How much force is needed to keep a 20 N stone from falling?

14.  An economy car has a mass of 800 kg.  What is its weight?

15.  A car has a mass of 1000 kg.  What is its weight?

16.  A small yacht weighs  14,700 N.  What is its mass?

17.  A 7.5 kg object is placed on a spring scale.  If the spring scale reads 78.4 N, what is the acceleration of  gravity at that location?

18.  A car has a mass of 1200 kg.  How much would the car weigh on the moon? (the moon's gravitational acceleration is 1.6 m/s^2.

NORMAL FORCE, FRICTION, and COEFFICIENT OF FRICTION:  F(f) = m F(N)

19.  A horizontal force of 18N is necessary to pull a 52 N sled across a cement sidewalk at a constant speed.  What is the coefficient of sliding friction between the sidewalk and the metal runners of the sled?

20.  The sled in problem 19 is then placed on packed snow. If a 650N boy sits on the sled, what will be the force necessary to slide the sled at a constant speed?  The coefficient of sliding friction is 0.012 for the sled runners on packed snow.

NET FORCE and ACCELERATION:  F(net) = F(a) + F(f)  or F(net) = F(a) - F(f),  Fnet is the force that produces acceleration. (F = ma); note that the use of the addition form of the equation forces you the reader into taking responsibility for inserting negative values for scenarios where there are opposing forces, whereas using the subtraction form of the equation allows you to solve the problem using only the magnitudes of the measurements.

21.  A rubber ball weighs 49 N.  What is the acceleration of the ball if an upward force of 69N is applied?

22.   a.  What is the weight of a 20.0 kg stone?
        b.  What force is needed to accelerate the stone upward at 10 m/s^2?

23.  A rocket weighs 9800 N.
 a. What is its mass?   b. What applied force will give an acceleration of 4.0 m/s^2?

24.  An object with a mass of 22.7 kg is placed on a surface.  the mass moves horizontally at a constant speed.  The coefficient of sliding friction between the two surfaces is 0.94.  What is the force of friction?

25.  A car moving on a level highway has a mass of 4.0 x 10^2 kg.  The coefficient of friction between the tires and the highway is 0.19.  What acceleration will  a force of 2250 N produce on the car?

26.  A small rocket weighs 14.7 N.
 a.  What is its mass?
 b.  The rocket is fired from a high platform but its engine fails to burn properly.  The rocket gains a total upward force of only 10.2 N. What is the acceleration of the rocket?

27.  A force of 91 N is exerted straight up on a stone that has a mass of 0.75 kg.  Calculate
 a.  the weight of the stone.
 b.  the net force acting on the stone.
 c.  the acceleration of the stone.

28.  A rocket that weighs 7840 N on the earth is fired.  The force of propulsion is +10440 N.  Determine:
 a.  The mass of the rocket.
 b.  The acceleration of the rocket.
 c.  The velocity of the rocket at the end of 8.0 s.

29.  The instruments attached to a weather balloon have a mass of 5.0 kg.
 a.  What do the instruments weight? Rem: weight is a force.
 b.  The balloon is released on a calm day and exerts an upward force of 98N on the 
instruments.  At what rate does the balloon with the instruments accelerate? Rem: acceleration is based upon the net force.
 c.  After accelerating for 10 seconds, the weather instruments are released automatically.
What is their velocity at that instant? Rem: the final velocity after acceleration is being asked for.
 d.  What time elapses before the instruments begin to fall? Rem: in this question the last answer v(f) is now the v(i) and the upwards velocity is positive (up) while at the same time the acceleration, due to gravity, is negative (down).