ch04f - Chapter 4 Newtons Laws CHAPTER 4 Newtons Laws Note:...

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Chapter 4 Newton’s Laws CHAPTER 4 Newton’s Laws Note: For all problems we shall take the upward direction as positive unless otherwise stated. 1* ·· How can you tell if a particular reference frame is an inertial reference frame? If Newton’s first law is obeyed, the reference frame is an inertial reference frame. 5* · If an object is acted upon by a single known force, can you tell in which direction the object will move using no other information? No; only its acceleration is known (assuming one knows the mass). 9* · A particle of mass m is traveling at an initial speed v 0 = 25.0 m/s. It is brought to rest in a distance of 62.5 m when a net force of 15.0 N acts on it. What is m ? ( a ) 37.5 kg ( b ) 3.00 kg ( c ) 1.50 kg ( d ) 6.00 kg ( e ) 3.75 kg a = v 2 /2 s = F / m ; m = v 2 /2 Fs m = 3.00 kg; ( b ) 13* · A force F = 6 N i - 3 N j acts on an object of mass 1.5 kg. Find the acceleration a . What is the magnitude a ? a = F / m = (4 i - 2 j ) m/s 2 ; a = (16 + 4) 1/2 m/s 2 = 4.46 m/s 2 . 17* · A 4-kg object is subjected to two forces, F 1 = 2 N i - 3 N j and F 2 = 4 n i - 11 N j . The object is at rest at the origin at time t = 0. ( a ) What is the object’s acceleration? ( b ) What is its velocity at time t = 3 s? ( c ) Where is the object at time t = 3 s? ( a ) Find the net force, F = F 1 + F 2 a = F / m ( b ) v = a t ( c ) r = v av t ; v av = 1/2 v (3 s) F = (2 i + 4 i ) N - (3 j + 11 j ) N = (6 i - 14 j ) N a = (1.5 i - 3.5 j ) m/s 2 v = (4.5 i - 10.5 j ) m/s r = (6.75 i - 15.75 j ) m 21* · On the moon, the acceleration due to gravity is only about 1/6 of that on earth. An astronaut whose weight on earth is 600 N travels to the lunar surface. His mass as measured on the moon will be ( a ) 600 kg. ( b ) 100 kg. ( c ) 61.2 kg. ( d ) 9.81 kg. ( e ) 360 kg. ( c ) The mass is (600 N)/(9.81 m/s 2 ) = 61.2 kg, and remains the same on the moon.

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Chapter 4 Newton’s Laws 25* ·· Caught without a map again, Hayley lands her spacecraft on an unknown planet. Visibility is poor, but she finds someone on a local communications channel and asks for directions to Earth. “You are already on Earth,” is the reply, “Wait there and I’ll be right over.” Hayley is suspicious, however, so she drops a lead ball of mass 76.5 g from the top of her ship, 18 m above the surface of the planet. It takes 2.5 s to reach the ground. ( a ) If Hayley’s mass is 68.5 kg, what is her weight on this planet? ( b ) Is she on Earth? ( a ) Use s = 1/2 at 2 to find accel. of gravity, g ' g ' = (2 × 18/2.5 2 ) m/s 2 = 5.76 m/s 2 w = mg ' w = (68.5 5.76) N = 395 N ( b ) Evidently, she is not on Earth. 29* · A baseball player hits a ball with a bat. If the force with which the bat hits the ball is considered the action force, what is the reaction force? ( a ) The force the bat exerts on the batter’s hands. ( b ) The force on the ball exerted by the glove of the person who catches it. ( c ) The force the ball exerts on the bat. ( d ) The force the pitcher exerts on the ball while throwing it. ( e
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This note was uploaded on 11/25/2010 for the course PHYSICS 4A taught by Professor Eduardo during the Spring '09 term at DeAnza College.

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ch04f - Chapter 4 Newtons Laws CHAPTER 4 Newtons Laws Note:...

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