Conceptual Questions

4.1 Development of Force Concept


Propose a force standard different from the example of a stretched spring discussed in the text. Your standard must be capable of producing the same force repeatedly.


What properties do forces have that allow us to classify them as vectors?

4.2 Newton's First Law of Motion: Inertia


How are inertia and mass related?


What is the relationship between weight and mass? Which is an intrinsic, unchanging property of a body?

4.3 Newton's Second Law of Motion: Concept of a System


Which statement is correct? (a) Net force causes motion. (b) Net force causes change in motion. Explain your answer and give an example.


Why can we neglect forces such as those holding a body together when we apply Newton’s second law of motion?


Explain how the choice of the system of interest affects which forces must be considered when applying Newton’s second law of motion.


Describe a situation in which the net external force on a system is not zero, yet its speed remains constant.


A system can have a nonzero velocity while the net external force on it is zero. Describe such a situation.


A rock is thrown straight up. What is the net external force acting on the rock when it is at the top of its trajectory?


(a) Give an example of different net external forces acting on the same system to produce different accelerations. (b) Give an example of the same net external force acting on systems of different masses, producing different accelerations. (c) What law accurately describes both effects? State it in words and as an equation.


If the acceleration of a system is zero, are no external forces acting on it? What about internal forces? Explain your answers.


If a constant, nonzero force is applied to an object, what can you say about the velocity and acceleration of the object?


The gravitational force on the basketball in Figure 4.6 is ignored. 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?

4.4 Newton's Third Law of Motion: Symmetry in Forces


When you take off in a jet aircraft, there is a sensation of being pushed back into the seat. 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.


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?


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. Which of Newton’s laws of motion apply?


Why does an ordinary rifle recoil (kick backward) when fired? The barrel of a recoilless rifle is open at both ends. Describe how Newton’s third law applies when one is fired. Can you safely stand close behind one when it is fired?


An American football lineman reasons that it is senseless to try to out-push the opposing player, since no matter how hard he pushes he will experience an equal and opposite force from the other player. Use Newton’s laws and draw a free-body diagram of an appropriate system to explain how he can still out-push the opposition if he is strong enough.


Newton’s third law of motion tells us that forces always occur in pairs of equal and opposite magnitude. Explain how the choice of the system of interest affects whether one such pair of forces cancels.

4.5 Normal, Tension, and Other Examples of Force


If a leg is suspended by a traction setup as shown in Figure 4.30, what is the tension in the rope?

Diagram of a leg in traction.
Figure 4.30 A leg is suspended by a traction system in which wires are used to transmit forces. Frictionless pulleys change the direction of the force T without changing its magnitude.

In a traction setup for a broken bone, with pulleys and rope available, how might we be able to increase the force along the tibia using the same weight (Figure 4.30)? Note that the tibia is the shin bone shown in this image.

4.7 Further Applications of Newton's Laws of Motion


To simulate the apparent weightlessness of space orbit, astronauts are trained in the hold of a cargo aircraft that is accelerating downward at gg size 12{g} {}. Why will they appear to be weightless, as measured by standing on a bathroom scale, in this accelerated frame of reference? Is there any difference between their apparent weightlessness in orbit and in the aircraft?


A cartoon shows the toupee coming off the head of an elevator passenger when the elevator rapidly stops during an upward ride. Can this really happen without the person being tied to the floor of the elevator? Explain your answer.

4.8 Extended Topic: The Four Basic Forces—An Introduction


Explain, in terms of the properties of the four basic forces, why people notice the gravitational force acting on their bodies if it is such a comparatively weak force.


What is the dominant force between astronomical objects? Why are the other three basic forces less significant over these very large distances?


Give a detailed example of how the exchange of a particle can result in an attractive force. For example, consider one child pulling a toy out of the hands of another.