# 1. What Is Newton’s Third Law of Motion

A physics teacher pushes a cart with demonstration devices into an amphitheater (figure)). Its mass is 65.0 kg, the mass of the wagon is 12.0 kg, and the mass of the equipment is 7.0 kg. Calculate the acceleration generated when the teacher exerts a force behind 150 N on the ground. All forces that resist movement, such as friction on the car`s wheels and air resistance, amount to 24.0 N. (a) Why does an ordinary rifle kick back when shooting? (b) The barrel of a recoilless rifle is open at both ends. Describe how Newton`s third law is applied when one is dismissed. (c) Can you safely stand close to you when it is fired? If you`ve ever cut off your toe, you`ve noticed that even if your toe triggers the impact, the surface you push it on exerts a force on your toe. While the first thought that comes to mind is probably “ouch, it hurts” and not “This is a great example of Newton`s third law,” both statements are true. To understand how a super-heavy rocket can launch into space, we need to get back to basics and take a closer look at Newton`s three laws of motion. First, identify the physical principles involved. If the problem involves forces, then Newton`s laws of motion are involved, and it is important to draw the situation carefully. An example of a sketch is shown in Figure 4.10.

Then, as in Figure 4.10, use vectors to represent all forces. Label the forces carefully and make sure that their length is proportional to the size of the forces and that the arrows point in the direction in which the forces are acting. Newton`s third law of motion states that whenever a first object exerts force on a second object, the first object undergoes a force of equal but opposite magnitude in the direction of the force it exerts. Force is a push or pull that acts on an object and leads to its interaction with another object. Power is the result of interaction. Force can be divided into two categories: contact force such as frictional force and non-contact force such as gravitational force. According to Newton, when two bodies interact, exert a force on each other, and these forces are called action and reaction pairs, which is explained in Newton`s third law of motion. A physics teacher pushes a cart with demonstration equipment into a classroom, as shown in Figure 4.11. Its mass is 65.0 kg, the mass of the wagon is 12.0 kg, and the mass of the equipment is 7.0 kg. To push the cart forward, the teacher`s foot exerts a force of 150 N in the opposite direction (backwards) on the ground. Calculate the acceleration generated by the teacher.

The frictional force opposite to the movement is 24.0 N. You probably know that when you throw a ball against a wall, the ball exerts force on the wall. Similarly, the wall exerts a force on the ball, causing the ball to bounce off the wall. Similarly, the Earth pulls you down with gravitational force. What you may not realize is that you are also wielding an equal amount of power on Earth. This remarkable fact is a consequence of Newton`s third law. For the situation shown in (figure), the third law states that because the chair presses on [latex] overset{to }{C} upwards on the boy, [/latex] it presses with force on [latex] text{−}overset{to }{C}. [/latex] Similarly, it pushes down with the forces [latex] text{−}overset{to }{F} [/latex] and [latex] text{−}overset{to }{T} [/latex] on the floor or table. Finally, since the earth pulls down on the boy with force [latex] overset{to }{w}, he pulls [/latex] with force [latex] text{−}overset{to }{w} [/latex] upwards.

If this student knocked on the table in frustration, he would quickly learn the painful lesson (avoidable by studying Newton`s laws) that the table counterattacks just as harshly. We have already thought that force is a force that pushes or pulls; However, when you think about it, you realize that no push or pull ever happens alone. When you press against a wall, the wall pushes you away. This brings us to Newton`s third law. A force is a push or drag that acts on an object as a result of its interaction with another object. Strengths arise through interactions! As explained in Lesson 2, some forces result from contact interactions (normal, frictional, voltage-related and applied forces are examples of contact forces) and other forces are the result of remote interactions (gravitational, electrical and magnetic forces). According to Newton, objects A and B exert forces on each other every time they interact with each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces that result from this interaction – a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton`s third law of motion.

Formally, Newton`s third law is this: because they accelerate together, we define the system as the teacher, the car, and the equipment. The teacher pushes with a force F foot F foot of 150 N backwards. According to Newton`s third law, the ground exerts a force before F sol F sol of 150 N on the system. Since all movements are horizontal, we can assume that no net force acts in the vertical direction and that the problem becomes one-dimensional. As mentioned in the figure, friction f opposes movement and therefore acts against the direction of the F-ground. F Floor. This is exactly what happens when one object exerts force on another – each object undergoes a force that has the same force as the force acting on the other object but acting in the opposite direction. Everyday experiments, such as blunting a toe or throwing a ball, are perfect examples of Newton`s third law in action. So when you look at the upcoming launch of Artemis I, you remember Newton`s laws of motion. We can easily see Newton`s third law at work by looking at how people move. Imagine a swimmer pushing back to the side of a pool ((figure)).

She presses her feet against the pool wall and accelerates in the opposite direction of her push. The wall exerted an equal and opposite force on the swimmer. One would think that two equal and opposing forces would cancel each other out, but they don`t because they act on different systems. In this case, there are two systems that we could study: the float and the wall. If we select the float as the system of interest, as in the figure, then [latex] {F}_{text{Wall on feet}} [/latex] is an external force on that system and affects its movement. The swimmer moves in the direction of this force. In contrast, the [latex] force {F}_{text{feet on wall}} [/latex] acts on the wall, not on our system of interest. Therefore, [Latex] {F}_{text{feet on wall}} [/latex] does not directly affect the movement of the system and does not cancel [Latex] {F}_{text{wall on feet}}. [/latex] The swimmer pushes in the opposite direction in which she wants to move.

Thus, the reaction to their push goes in the desired direction. In a free-body diagram as shown (figure), we never include the two forces of an action-reaction pair; In this case, we only use [latex] {F}_{text{wall on feet}} [/latex], not [latex] {F}_{text{feet on wall}} [/latex]. Newton`s third law of motion continues to rely on the first and second laws of motion. The third law of motion states that for every action there is an equal and opposite reaction. This can be observed for dormant and accelerated objects. For example, a rest box presses on the ground due to gravitational force. In response, the soil repels what we call a “normal force,” of the same size. These forces balance each other so that no acceleration of the box occurs. copyrightCopyright © 2014 Denise W. Carlson, College of Engineering, University of Colorado Boulder; (Aircraft) 2004 Microsoft Corporation, One Microsoft Way, Redmond, WA 98052-6399 USA. All rights reserved. office.microsoft.com/en-us/images/results.aspx?qu=airplane&ex=1#ai:MP900442454|mt:2| Looking at Newton`s third law, why don`t two equal and opposing forces cancel each other out? 3.

Many people know the fact that a rifle is returned when it is fired. This decline is the result of action-reaction force pairs. A gunpowder explosion produces hot gases that expand outward, allowing the rifle to advance on the bullet. In accordance with Newton`s third law of motion, the bullet presses backwards on the rifle. The acceleration of the recoil rifle is. Newton`s first law states that any object remains in a straight line at rest or in regular motion, unless it is forced to change state by the action of an external force. This tendency to resist changes in a state of motion is inertia. There is no net force acting on an object (when all external forces cancel each other out). Then, the object maintains a constant speed. If this speed is zero, the object remains at rest. When an external force acts on an object, the speed changes due to the force. Newton`s third law represents a certain symmetry in nature: forces always occur in pairs, and one body cannot exert force on another without experiencing a force itself.

We sometimes refer vaguely to this law as “action-reaction”, where the force exerted is the action and the force experienced accordingly is the reaction. Newton`s third law has practical applications in analyzing the origin of forces and understanding forces that lie outside a system. Other examples of Newton`s third law are easy to find.