![]() Can you safely stand close behind one when it is fired?ġ9. Describe how Newton’s third law applies when one is fired. Why does an ordinary rifle recoil (kick backward) when fired? The barrel of a recoilless rifle is open at both ends. Which of Newton’s laws of motion apply?ġ8. Describe a situation in which one system exerts a force on another and, as a consequence, experiences a force that is equal in magnitude and opposite in direction. A device used since the 1940s to measure the kick or recoil of the body due to heart beats is the “ballistocardiograph.” What physics principle(s) are involved here to measure the force of cardiac contraction? How might we construct such a device?ġ7. Explain why you move backward in the seat-is there really a force backward on you? (The same reasoning explains whiplash injuries, in which the head is apparently thrown backward.)ġ6. When you take off in a jet aircraft, there is a sensation of being pushed back into the seat. When gravity is taken into account, what is the direction of the net external force on the basketball-above horizontal, below horizontal, or still horizontal?Ĥ.4: Newton’s Third Law of Motion: Symmetry in Forcesġ5. The gravitational force on the basketball in Figure is ignored. If a constant, nonzero force is applied to an object, what can you say about the velocity and acceleration of the object?ġ4. If the acceleration of a system is zero, are no external forces acting on it? What about internal forces? Explain your answers.ġ3. (c) What law accurately describes both effects? State it in words and as an equation.ġ2. (b) Give an example of the same net external force acting on systems of different masses, producing different accelerations. (a) Give an example of different net external forces acting on the same system to produce different accelerations. What is the net external force acting on the rock when it is at the top of its trajectory?ġ1. system can have a nonzero velocity while the net external force on it is zero. Describe a situation in which the net external force on a system is not zero, yet its speed remains constant.ĩ. Explain how the choice of the “system of interest” affects which forces must be considered when applying Newton’s second law of motion.Ĩ. Why can we neglect forces such as those holding a body together when we apply Newton’s second law of motion?ħ. Which statement is correct? Explain your answer and give an example.Ħ. ![]() 4.8: Extended Topic: The Four Basic Forces-An IntroductionĤ.3: Newton’s Second Law of Motion: Concept of a Systemĥ.4.7: Further Applications of Newton’s Laws of Motion.4.5: Normal, Tension, and Other Examples of Forces.4.4: Newton’s Third Law of Motion: Symmetry in Forces.4.3: Newton’s Second Law of Motion: Concept of a System.4.2: Newton’s First Law of Motion: Inertia.
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