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Quick Check 1. A single engine plane has a mass of 1500 kg and acceleration of 0.4 m/s 2 . What is the thrust, or unbalanced force, from the engine? 2. What is the mass of a rocket that accelerates at 2.0 m/s 2 and has a thrust of 25 000 N? 3. Find the acceleration of a passenger jet that has a mass of 250 000 kg and a total engine thrust of 50 000 N. Multiple Forces The example and problems in the Quick Check above involve only one unbalanced force. In other situations, like Sample Problem 4.2.1 below, there are more forces to consider. Remember that, in these problems, it is important to identify the forces that create the unbalanced force. Figure 4.2.1 shows four different forces acting on a block. The two vertical forces are the force of gravity and the normal force (the force exerted by the floor on the block). The two horizontal forces are the applied force and the force of friction. The two vertical forces balance each other because the force of gravity and the normal force equal each other in size and act in opposite directions. The unbalanced force is the difference between the applied force and the friction force opposing the applied force. 170 Chapter 4 Newton’s Laws of Motion — DRAFT © Edvantage Interactive 2011 ISBN 978-0-9864778-3-6
Sample Problem 4.2.1 — Multiple Forces Acting on a Body A 45.0 N block is being pushed along a floor, where the coefficient of kinetic friction is 0.333. If a force of 25.0 N is applied, at what rate will the block accelerate? The mass of the block is 4.60 kg. �� Figure 4.2.1 Forces acting on block What to think about 1. Find the force of friction. 2. Find the unbalanced force. � 3. Find the acceleration. �� How to Do It F f = µF n = µF g = (0.333) × (45.0 N) = 15.0 N unbalanced force = applied force – friction force F = 25.0 N – 15.0 N = 10.0 N a = F = 10.0 N m 4.60 kg = 2.17 m/s2 © Edvantage Interactive 2011 ISBN 978-0-9864778-3-6 Chapter 4 Newton’s Laws of Motion — DRAFT 171
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Page 7 and 8: Measuring Inertia Quick Check As a
Page 9 and 10: Investigation 4-1 Investigating Ine
Page 11 and 12: Problem 5: Where will the string br
Page 13: 4.2 Newton’s Second Law of Motion
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Page 21 and 22: Quick Check The first force is call
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