Newton’s Third Law of Motion Newton’s Third Law of Motion deals with the interaction between pairs of objects. For example, if you hold a book up against a wall you are exerting a force on the book (to keep it there) and the book is exerting a force back at you (to keep you from falling through the book). This may sound strange, but if the book was not pushing back at you, your hand will push through the book! These two forces (the force of the hand on the book (F1) and the force of the book on the hand (F2) are called an action-reaction pair of forces. They have the same magnitude, but act in opposite directions and act on different objects (the one force is onto the book and the other is onto your hand). There is another action-reaction pair of forces present in this situation. The book is pushing against the wall (action force) and the wall is pushing back at the book (reaction). The force of the book on the wall (F3) and the force of the wall on the book (F4) are shown in the diagram. Definition: Newton’s Third Law of Motion If body A exerts a force on body B, then body B exerts a force of equal magnitude on body A, but in the opposite direction. Newton’s action-reaction pairs can be found everywhere in life where two objects interact with one another. The following worked examples will illustrate this: Worked Example 1: Newton III - seat belt Question: Dineo is seated in the passenger seat of a car with the seat belt on. The car suddenly stops and he moves forwards until the seat belt stops him. Draw a labeled force diagram identifying two action-reaction pairs in this situation. Hướng dẫn Step 1 : Draw a force diagram Start by drawing the picture. You will be using arrows to indicate the forces so make your picture large enough so that detailed labels can also be added. The picture needs to be accurate, but not artistic! Use stickmen if you have to. Step 2 : Label the diagram Take one pair at a time and label them carefully. If there is not enough space on the drawing, then use a key on the side. Worked Example 2: Newton III - forces in a lift Question: Tammy travels from the ground floor to the fifth floor of a hotel in a lift. Which ONE of the following statements is TRUE about the force exerted by the floor of the lift on Tammy’s feet? A It is greater than the magnitude of Tammy’s weight. B It is equal in magnitude to the force Tammy’s feet exert on the floor. C It is equal to what it would be in a stationary lift. D It is greater than what it would be in a stationary lift. Hướng dẫn Step 1 : Analyse the situation This is a Newton’s Third Law question and not Newton II. We need to focus on the action-reaction pairs of forces and not the motion of the lift. The following diagram will show the action-reaction pairs that are present when a person is standing on a scale in a lift. In this question statements are made about the force of the floor (lift) on Tammy’s feet. This force corresponds to F2 in our diagram. The reaction force that pairs up with this one is F1, which is the force that Tammy’s feet exerts on the floor of the lift. The magnitude of these two forces are the same, but they act in opposite directions. Step 2 : Choose the correct answer It is important to analyse the question first, before looking at the answers as the answers might confuse you. Make sure that you understand the situation and know what is asked before you look at the options. The correct answer is B. Worked Example 3: Newton III - book and wall Question: Tumi presses a book against a vertical wall as shown in the sketch. Draw a labelled force diagram indicating all the forces acting on the book. State, in words, Newton’s Third Law of Motion. Name the action-reaction pairs of forces acting in the horizontal plane. Hướng dẫn Step 1 : Draw a force diagram A force diagram will look like this: Note that we had to draw all the force acting on the book and not the action-reaction pairs. None of the forces drawn are action-reaction pairs, because they all act on the same object (the book). When you label forces, be as specific as possible, including the direction of the force and both objects involved, for example, do not say gravity (which is an incomplete answer) but rather say ’Downward (direction) gravitational force of the Earth (object) on the book (object)’. Step 2 : State Newton’s Third Law If body A exerts a force onto body B, then body B will exert a force equal in magnitude, but opposite in direction, onto body A. Step 3 : Name the action-reaction pairs The question only asks for action-reaction forces in the horizontal plane. Therefore: \ Pair 1: Action: Applied force of the girl on the book; Reaction: The force of the book on the girl. Pair 2: Action: Force of the book on the wall; Reaction: Force of the wall on the book. Note that a Newton III pair will always involve the same combination of words, like ’book on wall’ and wall on book’. The objects are ’swopped around’ in naming the pairs. Exercise E - 1. A fly hits the front windscreen of a moving car. Compared to the magnitude of the force the fly exerts on the windscreen, the magnitude of the force the windscreen exerts on the fly during the collision, is ... A/ zero. B/ smaller, but not zero. C/ bigger. D/ the same. E - 2. A log of wood is attached to a cart by means of a light, inelastic rope. A horse pulls the cart along a rough, horizontal road with an applied force F. The total system accelerates initally with an acceleration of magnitude a (figure 1). The forces acting on the cart during the acceleration, are indicated in Figure 2. A/ F1: Force of log on cart; F2: Reaction force of Earth on cart B/ F1: Force of log on cart; F2: Force of road on cart C/ F1: Force of rope on cart; F2: Reaction force of Earth on cart D/ F1: Force of rope on cart; F2: Force of road on cart E - 3. Which of the following pairs of forces correctly illustrates Newton’s Third Law? Xem thêm: Physics 11.II Force, Momentum, Impulse High School Students Studying the Sciences Physics