Physics. Chapter 5 Newton s Third Law

Transcription

1 Physics Chapter 5 Newton s Third Law

2 Forces and Interactions In previous lessons, we defined a force as a push or pull. But in reality, no push or pull EVER occurs alone. They come in pairs. Some examples: Lean against a wall are you pushing on the wall or is the wall pushing on you? Both! Hit a nail with a hammer is the hammer hitting the nail or is the nail hitting the hammer? Both!

3 Forces and Interactions So we need to change our way of thinking. No need to distinguish between the exerter and the receiver of the force. The pair of forces make up a single interaction. Newton s Third Law states: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

4 Newton s Third Law Would you ever ask someone, Is it the same distance from Los Angeles to New York as it is from New York to Los Angeles? You d get some funny looks! Forces are the same way. Forces occur in force-pairs. Neither one can exist without the other. Sounds simple enough let s see if you really get it?

5 Force Pairs Tire pushes on road. Earth pulls on ball Rocket pushes on gas Earth pulls on Moon

6 Newton s Third Law People get confused easily with Newton s Third Law because we confuse acceleration with force. Most everyone knows the moon is attracted to the earth by gravity, but you get mixed answers when you ask if Earth is attracted to the moon or which pulls harder. The Reality? There is an attractive force BETWEEN the Earth and the Moon a force pair, equal and opposite.

7 Check Question Imagine a boxer capable of hitting a heavy bag with a large force, say 500N. Could that same boxer hit a piece of tissue paper with the same force?

8 Let s Work a Problem: A boxer punches a sheet of paper in midair and brings it from rest up to a speed of 25 m/s in.05s. (a) What acceleration is imparted to the paper? (b) If the mass of the paper is.003 kg, what force does the boxer exert on it?

9 Defining the System Since action and reaction forces are equal and opposite, why don t they cancel? It all depends on how you define the system. Let s look at an example:

10 Defining the System

11 Defining the System Here s another way to look at it. Think of a baseball. Trillions of inter-atomic forces at play within it. They hold the ball together, but they play no role in accelerating the ball. It takes a force external to the system to accelerate it.

12 Action and Reaction on Different Masses Two objects with different masses involved in an actionreaction force pair will undergo different accelerations. Consider a ball dropped from a tower. Action: Earth pulls on Ball Reaction: Ball pulls on Earth

13 Action and Reaction on Different Masses Or consider a cannonball fired from a cannon.

14 Newton s Laws of Motion 1. Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force. 2. The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. 3. Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

15 Vectors We ve learned the difference between a vector and scalar quantity. We represent vectors with an arrow. Length of the arrow indicates the vector s magnitude Direction of the arrow well that s obvious. We can add vectors to get their resultant. If the vectors are parallel, it s easy to find their resultant. But what if they re NOT parallel. Can we still replace two vectors with their one resultant? YES! By using the parallelogram rule!

16 Parallelogram Rule Draw a parallelogram where the 2 vectors are adjacent sides. The diagonal shows the resultant.

17 Force Vector Diagrams

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33 Velocity Vectors The vector examples we just did were all force vectors, but the parallelogram rule can be applied to other vector quantities too. Consider a plane flying due North at 80 km/h. If there is a 60 km/h crosswind blowing to the East, how fast and in what direction is the plane actually flying?

34 Velocity Vectors Here is a top view of an airplane being blown off course by wind in various directions. Using the parallelogram rule, sketch the vectors that show the resulting velocities for each case.

35 Components of Vectors The process of determining the components of a vector is call resolution. Any vector can be resolved into a horizontal and vertical component.

36 Vector Mathematics Resolving diagonal vectors into their x- and y- components:

37 Triangles to Know These triangles will show up on the SAT! * See page 64 in Barrons.

38 Example Problems You come to a wide river that moves at a steady speed of 3 mph. A man will rent you a boat that can go 5 mph. You need to get to a spot directly across the river from where you are. In which direction do you steer the boat so that you can arrive there as directly as possible? 5 mph

39 Example Problems A mass, m, is acted upon by two force vectors as shown in the diagram. Determine the magnitude and direction of the resultant force acting on mass m. 12 N F 2 = 12 N m F 1 = 16 N 16 N

40 Example Problems A block of ice of weight W slides down a slope of angle Ɵ. (a) How large is the component of the weight that acts parallel to the slope? Perpendicular to the slope?

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