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**Unformatted text preview: **CHAPTER 5 Applications of Newtons Laws 1* · Various objects lie on the floor of a truck moving along a horizontal road. If the truck accelerates, what force acts on the objects to cause them to accelerate? Force of friction between the objects and the floor of the truck. 2 · Any object resting on the floor of a truck will slide if the trucks acceleration is too great. How does the critical acceleration at which a light object slips compare with that at which a much heavier object slips? They are the same. 3 · True or false: ( a ) The force of static friction always equals m s F n . ( b ) The force of friction always opposes the motion of an object. ( c ) The force of friction always opposes sliding. ( d ) The force of kinetic friction always equals m k F n . ( a ) False ( b ) True ( c ) True ( d ) True 4 · A block of mass m rests on a plane inclined at an angle q with the horizontal. It follows that the coefficient of static friction between the block and the plane is ( a ) m s 1 . ( b ) m s = tan q . ( c ) m s tan q . ( d ) m s tan q . ( d ) 5* · A block of mass m is at rest on a plane inclined at angle of 30 o with the horizontal, as in Figure 5-38. Which of the following statements about the force of static friction is true? ( a ) f s > mg ( b ) f s > mg cos 30 o ( c ) f s = mg cos 30 o ( d ) f s = mg sin 30 o ( e ) None of these statements is true. ( d ) f s must equal in magnitude the component of the weight along the plane. 6 · A block of mass m slides at constant speed down a plane inclined at an angle q with the horizontal. It follows that ( a ) m k = mg sin q . ( b ) m k = tan q . ( c ) m k = 1 - cos q . ( d ) m k = cos q- sin q . ( a ) Acceleration = 0, therefore f k = mg sin q . With F n = mg cos q , it follows that m k = tan q 7 · A block of wood is pulled by a horizontal string across a horizontal surface at constant velocity with a force of 20 N. The coefficient of kinetic friction between the surfaces is 0.3. The force of friction is ( a ) impossible to determine without knowing the mass of the block. ( b ) impossible to determine without knowing the speed of the block. ( c ) 0.3 N. ( d ) 6 N. ( e ) 20 N. ( e ) The net force is zero. 8 · A 20-N block rests on a horizontal surface. The coefficients of static and kinetic friction between the surface and the block are m s = 0.8 and m k = 0.6. A horizontal string is attached to the block and a constant tension T is Chapter 5 Applications of Newtons Laws maintained in the string. What is the force of friction acting on the block if ( a ) T = 15 N, or ( b ) T = 20 N. ( a ) If m s mg > 15, then f = f s = 15 N ( b ) T > f s,max ; f = f k = m k mg 0.8 × (20 N) = 16 N; f = f s = 15 N f = f k = 0.6 × (20 N) = 12 N 9* · A block of mass m is pulled at constant velocity across a horizontal surface by a string as in Figure 5-39. The magnitude of the frictional force is ( a ) m k mg . ( b ) T cos q . ( c ) m k ( T- mg ). ( d ) m k T sin q . ( e ) m k ( mg + T sin ...

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