Circle the correct answer. For those questions involving calculations, working MUST be shown to receive credit.

Transcription

1 Dynamics Assignment 3 Name: Multiple Choice. Circle the correct answer. For those questions involving calculations, working MUST be shown to receive credit. 1. Which statement is always true regarding normal forces? (A) They act to cancel out all other forces acting in the y-direction. (B) They are directed toward the surface of the earth. (C) They are equal in magnitude but opposite in direction to gravitational forces. (D) They are exerted on a surface by an object supported by the surface. 2. Study the force system diagram at the right. Which factor would NOT influence the amount of kinetic friction. (A) Applied force, F A (B) Coefficient of kinetic friction, μ k (C) Gravitational field strength, F g. (D) Normal force, F N 2. Which of the following statements concerning friction is true? (A) For two given surfaces, the coefficient of static friction is generally greater than the coefficient of kinetic friction. (B) Friction is a force that is unavoidable and serves no practical purpose. (C) Friction always acts in the direction of motion. (D) The frictional force always acts oppositely to the applied force. 3. An elevator is suspended by a cable and moves upward. Which of the following freebody diagrams best represents the forces acting on the elevator? 4. A fisher suspends a fish on a Newton spring scale in an elevator. When does the scale show the highest reading? (A) when the elevator moves downward at a constant speed. (B) when the elevator moves downward with increasing speed. (C) when the elevator moves upward with decreasing speed. (D) when the elevator moves upward with increasing speed. 5. What happens to the coefficient of friction when the force between two surfaces is tripled? (A) it triples (B) it is reduced by a factor of 3 (C) it increases by a factor of 6 (D) it is unchanged 6. Which of the following graphs best represents the relationship between the gravitational force, F, that Earth exerts and the mass, m, of an object sitting at Earth's surface, that the force is exerted upon?

2 7. Which of the following graphs best depicts the relationship between the gravitational force, F, that two masses exert on one another and the distance, d, which separates their centers of mass? 8. According to Newton's law of universal gravitation, which of the following is true regarding the gravitational force of attraction between two objects? (A) half as strong if they're moved twice as far apart (B) twice as strong if they're moved half as far apart (C) four times as strong if they're moved twice as far apart (D) four times as strong if they're moved half as far apart 9. An object has a weight of 20.0 N. An applied force of 5.0 N causes it to move uniformly. What is the coefficient of friction for the situation? (A) 0.10 (B) 0.25 (C) 4.0 (D) A kg mass is attached to a kg mass as shown in the diagram. Assuming no friction, what is the acceleration of system? (A) 0.70 m/s 2 (B) 9.80 m/s 2 ( C) 6.53 m/s 2 (D) 19.6 m/s In the diagram above, assuming no friction, what is the tension in the string? (A) 1.31 N (B) 1.96 N (C) 3.92 N (D) 9.81 N 12. A force of 1.2 N is applied to an object of mass 1.5 kg. It accelerates at 0.50 m/s 2. Determine the coefficient of kinetic friction acting on the object. Include a free-body diagram. {5} 13. A box of mass 4.5 kg is pushed across a rough surface ( K = 0.18) for a distance of 2.0 m by a constant force of 10.0 N. If the object reaches a speed of 2.0 m/s by the end of the push, what was its speed at the beginning of the push? Include a free-body diagram. {5}

3 14. Two dynamics carts are resting side by side as shown, on a level, frictionless surface. A force of 4.6 N is applied to the larger of the two. Use the information provided to find the force that cart B exerts on cart A. {5} 15. The coefficient of friction between the 3.0 kg block and the surface is The pulley is frictionless. A) Draw a FBD for each mass. {2} B) What is the acceleration of the system? {4} C) What is the tension in the string? {2} 19. A frictionless pulley is set up with two hanging masses as shown below. A) Draw a free body diagram for each mass. {2} B) What is the acceleration of the system? {4} C) What is the tension in the right hand rope while the masses move freely? {2}

4 20. A block of mass m on a frictionless surface is attached to a hanging 5.0 kg mass as shown below. The system accelerates at 2.6 m/s 2. What is the mass of the block? {4} 21. The gravitational force of attraction between two unknown masses is 4.3 N. Determine the gravitational force of attraction in each situation below. Treat each situation as a new situation. A) One of the masses is doubled but the distance between them remains the same.{2} B) One of the masses is tripled and the other is halved but the distance between them remains the same. {2} C) The masses are kept constant but the distance between them is reduced to onethird of the original distance. {2} D) One mass is quadrupled and the distance between the masses is doubled. {2} E) One mass is tripled, the other mass is reduced to one-quarter of the original mass and the distance between the masses is doubled. {4} 22. What is the gravitational force of attraction between a 25 kg table and a 11 kg chair whose centres are separated by 25 cm? {2}

5 23. The gravitational force of attraction between a 340 kg mass and a 210 kg mass is N. How far apart are the centres of the two masses? {3} 24. Two identical masses are separated by 45 cm. The gravitational force of attraction between them is N. What is the mass of each object? {4} 25. A person stands on a set of bathroom scales which have been calibrated in Newtons. The scales read N. A) What would the reading be if the same person stood on the scales on a planet where the gravitational field strength, g, is 14.0 N/kg? {2} B) If this planet had a mass of kg, what would its radius be? {3} C) What would this person weigh at an altitude of m above the planet's surface? {3}