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Chapter 6: Forces. Layout of Chapter 6 . 6.1 . Force and Motion. 4.1 The Concepts of Force and Mass. A force is a push or a pull. Contact forces arise from physical contact . Action-at-a-distance or long-range forces do not require contact and include gravity and electrical forces.
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6.1 Force and Motion
4.1 The Concepts of Force and Mass A force is a push or a pull. Contact forcesarise from physical contact . Action-at-a-distance or long-range forcesdo not require contact and include gravity and electrical forces.
An Introduction Describing Motion video clip
Mathematically, the net force is written as where the Greek letter sigma denotes the vector sum.
Newton’s Second Law When a net external force acts on an object of mass m, the acceleration that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of the acceleration is the same as the direction of the net force.
How do we Measure force? In a Newton, of Course
SI Unit for Force This combination of units is called a newton (N).
How do we Diagram Force on an object
Arrows are used to represent forces. The length of the arrow is proportional to the magnitude of the force. 15 N 5 N
The net force on an object is the vector sum of all forces acting on that object. The SI unit of force is the Newton (N). Individual Forces Net Force 4 N 10 N 6 N
Individual Forces Net Force 5 N 3 N 4 N
Free Body Diagrams Vector arrows represent all the forces acting in a situation.
What does unbalanced really mean? • In pursuit of an answer, consider a physics book at rest on a table top. There are two forces acting upon the book. One force – the Earth's gravitational pull – exerts a downward force. The second force – the push of the table on the book (sometimes referred to as a normal force) – pushes upward on the book.
Balancing Act • Since these two forces are of equal magnitude and in opposite directions, they balance each other. The book is said to be at equilibrium. There is no unbalanced force acting upon the book and thus the book maintains its state of motion. When all the forces acting upon an object balance each other, the object will be at equilibrium; it will not accelerate. (Note: diagrams such as the one above are known as free-body diagrams and will be discussed in detail in Lesson 2.)
Balanced or Not? • To determine if the forces acting upon an object are balanced or unbalanced, an analysis must first be conducted to determine which forces are acting upon the object and in what direction. If two individual forces acting on an object are of equal magnitude and opposite direction, then these forces are said to be balanced. An object is said to be "acted upon by an unbalanced force" only when there is an individual force acting on the object which is not balanced by another force of equal magnitude and in the opposite direction . Such analyses are discussed in Lesson 2 of this unit and applied in Lesson 3.
Check your Understanding • Copy this down for information used in further examples. • Luke Autbeloe drops a 5.0 kg box of shingles (weight approximately 50.0 N) off the barn house roof into a haystack below. Upon hitting the haystack, the box of shingles encounters an upward restraining force of 50.0 N . Use this description to answer the following questions.
Example 1 • 1. Which one of the following velocity-time graphs best describes the motion of the shingles? Support your answer with sound reasoning.
Answer 1 • Graph B • The shingles experience negative acceleration until they hit the haystack. At that point the forces are balanced, so velocity becomes constant
Example 2 • 2. Which one of the following ticker tapes best describes the motion of the falling shingles from the time they are dropped to the time they hit the ground? The arrows on the diagram represent the point at which the shingles hit the haystack. Support your answer with sound reasoning.
Answer to #2 • Tape A is correct. • It shows the negative acceleration and constant velocity.
Example 3 (has many parts) • 3. Several of Luke's friends were watching the motion of the falling shingles. Being "physics types", they began discussing the motion and made the following comments. Indicate whether each of the comments is correct or incorrect. Support your answers. • A) A. Once the shingles hit the haystack, the forces are balanced and the shingles will stop.
Correct or Incorrect? • Incorrect. • They stop accelerating but do not stop moving.
Part B • B. Upon hitting the haystack, the shingles will accelerate upwards because the haystack applies an upward force.
Answer to B • Incorrect • The balanced forces on the shingles will keep velocity constant.
Example C • C. Upon hitting the haystack, the shingles will bounce upwards due to the upward force.
Answer to C • Incorrect • Forces are balanced
Example 4 • 4. If the forces acting upon an object are balanced, then the object • A. must not be moving. • B. must be moving with a constant velocity. • C. must not be accelerating. • D. none of the above.
Answer to #4 • A is possible but is not necessarily true at all times • B an object with balanced forces cannot be accelerating • C It could be at rest and staying at rest or could be in motion with constant velocity but not accelerating making C the correct answer
A free-body-diagram is a diagram that represents the object and the forces that act on it.
The net force in this case is: 275 N + 395 N – 560 N = +110 N and is directed along the + x axis of the coordinate system.
If the mass of the car is 1850 kg then, by Newton’s second law, the acceleration is
4.4 The Vector Nature of Newton’s Second Law The net force on the raft can be calculated in the following way:
Newton’s First Law An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force. The net forceis the vector sum of all of the forces acting on an object.
Looking into Newton’s 1st Law, other forces, and misconceptions of force
Misconceptions about Forces WRONG Right No, it is a contact force; therefore, once the contact is broken, the force is no longer exerted. It will continue moving with no change in velocity or direction. Inertia is a property of matter. Air exerts a huge, usually balanced force. F = ma • When a ball has been thrown, the force of the hand that threw it remains on it. • A force is needed to keep an object moving. • Inertia is a force. • Air does not exert a force • The quantity ma is a force.
6.2 Using Newton’s Forces
An Review of Newton’s Laws Video Clip
4.1 The Concepts of Force and Mass Massis a measure of the amount of “stuff” contained in an object. Weight is actually a force and can be found by using Newton’s 2nd Law W = mg
Weightless and Apparent Weight Apparent Weight Weightless Specific circumstance of acceleration = g Condition of free fall Your weight is zero but you are not without mass • The force exerted on the scale measuring your weight at any point • If there is additional force pushing down (i.e. you are in an elevator accelerating upward), your apparent weight is greater than your mass. • If there is less force pushing down on the scale (i.e. the elevator is now accelerating downward) then you have a weight less than your mass.
Looking into Friction
