What do you think? 1) What is a force?

What do you think? 1) What is a force?
2) Are any forces acting on your book / notebook as it rests on your desk? Describe them. Make a sketch showing any forces on the book. 3) What units are used to measure force? 4) Can forces exist without contact between objects? Explain. When asking students to express their ideas, you might try one of the following methods. (1) You could ask them to write their answers in their notebook and then discuss them. (2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups. The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally. Many students will be able to answer these fairly well. They may have some trouble describing the forces acting on the book. They probably know pounds as a unit of force, but they may not know that the newton is the SI unit of force.

Chapter 4 Unit Test for chapter 4 – the week before Thanksgiving break
( Period 1,2,3 November 16th ) ( Period 4,5,6 November 15th ) Packet will be collected on test day , stamped each day for completion. Physics Research Essay -- The week before finals (Period 1,2,3 December 14th ) ( Period 4,5.6 December 16th )

Physics Research Essay final draft due December 14th
Have 3 topics of interest by Monday and Must Relate to physics

Chapter 4 Preview Objectives Force Force Diagrams
Section 1 Changes in Motion Preview Objectives Force Force Diagrams

Chapter 4 Section 1 Changes in Motion Objectives Describe how force affects the motion of an object. Interpret and construct free body diagrams.

Chapter 4 Section 1 Changes in Motion Force A force is an action exerted on an object which may change the object’s state of rest or motion. Forces can cause accelerations. The SI unit of force is the newton, N. Forces can act through contact or at a distance.

Units of Force The SI unit of force is the newton (N).
Named for Sir Isaac Newton Defined as the force required to accelerate a 1 kg mass at a rate of 1 m/s2 Approximately 1/4 pound Must always pay attention to the units and convert accordingly The SI Unit for force is (N) = kg/m/s2 SI base units: (kg ⋅ m )/s2 1 N = pounds (roughly 1/4 pound) Have students determine their approximate weight in newtons to reinforce the size of the unit. When talking about problems, use both units to help them become more comfortable. For example, a N car is about a 2500 lb car.

Forces Contact forces Field forces
Pushes or pulls requiring physical contact between the objects Baseball and bat Field forces Objects create force fields that act on other objects. Gravity, static electricity, magnetism Pictured is a contact force, the bat and the ball, as well as a field force, the static electric field around charged balloon exerting a force on small pieces of paper. Ask students to identify other examples of contact forces.

Chapter 4 Force Diagrams
Section 1 Changes in Motion Force Diagrams The effect of a force depends on both magnitude and direction. Thus, force is a vector quantity. Diagrams that show force vectors as arrows are called force diagrams. Force diagrams that show only the forces acting on a single object are called free-body diagrams.

Force Diagrams, continued
Chapter 4 Section 1 Changes in Motion Force Diagrams, continued Force Diagram Free-Body Diagram In a force diagram, vector arrows represent all the forces acting in a situation. A free-body diagram shows only the forces acting on the object of interest—in this case, the car.

Drawing free body diagrams
In the early morning, a park ranger in a canoe is observing the wild on the nearby shore. The earth’s gravitational force on the ranger is 760 N downward and its gravitational force on the boat is 190 N downward. The water keeps the canoe afloat by exerting a 950 N force upward on it. Draw a free-body diagram of the canoe.

SOLUTION 1) Identify the forces acting on the object and the directions of the forces • The Earth exerts a force of 190 N downward on the canoe. • The park ranger exerts a force of 760 N downward on the canoe. • The water exerts an upward force of 950 N on the canoe.

2) Draw a diagram to represent the isolated object
2) Draw a diagram to represent the isolated object. 3) Draw and label vector arrows for all external forces acting on the object

Example 2 Suppose a sailor is resting in a hammock. The gravitational force of Earth on the hammock is 150 N. The downward force that the resting sailor exerts on the hammock is an additional 850 N. The supporting rope at one end of the hammock exerts a force of 780 N at an angle of 50.0° clockwise from the vertical. The rope at the other end exerts a force of 780 N at an angle of 50.0° counterclockwise from the vertical. Draw a free-body diagram of the hammock.

Solution 1. Identify the forces acting on the object and the directions of the forces. • Earth exerts a downward force of 150 N on the hammock. • The sailor exerts a force 850 N downward on the hammock. • Rope 1 exerts a force of 780 N at an angle of 50.0° clockwise from the vertical. • Rope 2 exerts a force of 780 N at an angle of 50.0° counterclockwise from the vertical.

Solution 2) Draw a diagram to represent the isolated object. 3) Draw and label vector arrows for all external forces acting on the object

Chapter 4 Preview Objectives Newton’s First Law Net Force
Section 2 Newton’s First Law Preview Objectives Newton’s First Law Net Force Sample Problem Inertia Equilibrium

Chapter 4 Section 2 Newton’s First Law Objectives Explain the relationship between the motion of an object and the net external force acting on the object. Determine the net external force on an object. Calculate the force required to bring an object into equilibrium.

Chapter 4 Newton’s First Law
Section 2 Newton’s First Law Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force. In other words, when the net external force on an object is zero, the object’s acceleration (or the change in the object’s velocity) is zero.

Chapter 4 Section 2 Newton’s First Law Net Force Newton's first law refers to the net force on an object.The net force is the vector sum of all forces acting on an object. The net force on an object can be found by using the methods for finding resultant vectors. Although several forces are acting on this car, the vector sum of the forces is zero. Thus, the net force is zero, and the car moves at a constant velocity.

Chapter 4 Sample Problem
Section 2 Newton’s First Law Sample Problem Determining Net Force Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. The free-body diagram below shows the forces acting on the book. Find the net force acting on the book.

Sample Problem, continued
Chapter 4 Section 2 Newton’s First Law Sample Problem, continued 1. Define the problem, and identify the variables. Given: Fgravity-on-book = Fg = 22 N Ffriction = Ff = 11 N Ftable-on-book = Ft = 18 N Unknown: Fnet = ?

Chapter 4 Section 2 Newton’s First Law Sample Problem, continued 2. Select a coordinate system, and apply it to the free-body diagram. Choose the x-axis parallel to and the y-axis perpendicular to the incline of the table, as shown in (a). This coordinate system is the most convenient because only one force needs to be resolved into x and y components. Tip: To simplify the problem, always choose the coordinate system in which as many forces as possible lie on the x- and y-axes.

Chapter 4 Section 2 Newton’s First Law Sample Problem, continued 3. Find the x and y components of all vectors. Draw a sketch, as shown in (b), to help find the components of the vector Fg. The angle  is equal to 180– 90 – 35 = 55. Add both components to the free-body diagram, as shown in (c).

Chapter 4 Section 2 Newton’s First Law Sample Problem, continued 4. Find the net force in both the x and y directions. Diagram (d) shows another free-body diagram of the book, now with forces acting only along the x- and y-axes. For the x direction: Fx = Fg,x – Ff Fx = 13 N – 11 N Fx = 2 N For the y direction: Fy = Ft – Fg,y Fy = 18 N – 18 N Fy = 0 N

Chapter 4 Section 2 Newton’s First Law Sample Problem, continued 5. Find the net force. Add the net forces in the x and y directions together as vectors to find the total net force. In this case, Fnet = 2 N in the +x direction, as shown in (e). Thus, the book accelerates down the incline.

Chapter 4 Section 2 Newton’s First Law Inertia Inertia is the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction. Newton’s first law is often referred to as the law of inertia because it states that in the absence of a net force, a body will preserve its state of motion. Mass is a measure of inertia.

Chapter 4 Section 2 Newton’s First Law Equilibrium Equilibrium is the state in which the net force on an object is zero. Objects that are either at rest or moving with constant velocity are said to be in equilibrium. Newton’s first law describes objects in equilibrium. Tip: To determine whether a body is in equilibrium, find the net force. If the net force is zero, the body is in equilibrium. If there is a net force, a second force equal and opposite to this net force will put the body in equilibrium.

Warm up 1. If the force of gravity between the sun and planets suddenly disappeared, what type of path would the planets follow? Support your answer 2. Is it correct to say that the reason an object resist change and persists in its state of motion is that it has inertia? Support your answer

Chapter 4 Preview Objectives Newton’s Second Law Newton’s Third Law
Section 3 Newton’s Second and Third Laws Chapter 4 Preview Objectives Newton’s Second Law Newton’s Third Law Action and Reaction Forces

Section 3 Newton’s Second and Third Laws
Chapter 4 Objectives Describe an object’s acceleration in terms of its mass and the net force acting on it. Predict the direction and magnitude of the acceleration caused by a known net force. Identify action-reaction pairs.

net force = mass  acceleration
Section 3 Newton’s Second and Third Laws Chapter 4 Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass. F = ma net force = mass  acceleration Units for force: mass units (kg)  acceleration units (m/s2) The units kg•m/s2 are also called newtons (N). F represents the vector sum of all external forces acting on the object, or the net force.

Increasing the force will increase the acceleration.
Newton’s Second Law Increasing the force will increase the acceleration. 1) Which produces a greater acceleration on a 3-kg model airplane, a force of 5 N or a force of 7 N? Answer: the 7 N force 2) Increasing the mass will decrease the acceleration. A force of 5 N is exerted on two model airplanes, one with a mass of 3 kg and one with a mass of 4 kg. Which has a greater acceleration? Answer: the 3 kg airplane Be sure students understand what is meant by the terms “directly proportional” and “inversely proportional.” A simulation from the Phet web site is available to help students visualize the force and the acceleration. The web address is: Choose the “Motion” simulations, then select “motion in 2D.” You can turn off the vectors and just allow students to observe the motion. Then ask the students to predict the acceleration vector. Which way will it point? Will it have a constant size? After predicting, show the acceleration vector. Next, have them predict the force vector’s direction and size. After predicting, show the force vector and both vectors. Then you can try the other motions described on the screen and ask them to observe the motion, describe the acceleration, and describe the forces. This exercise allows students to see that accelerations are caused by forces. We see the accelerations, but often do not see the forces.

Classroom Practice Problem
Space-shuttle astronauts experience accelerations of about 35 m/s2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude? Answer: 35 m/s2 x 75 kg = N

Newton’s Third Law For every action force there is an equal and opposite reaction force. The forces act on different objects. Therefore, they do not balance or cancel each other. The motion of each object depends on the net force on that object.

Forces always exist in pairs.
Newton’s Third Law Forces always exist in pairs. You push down on the chair, the chair pushes up on you Called the action force and reaction force Occur simultaneously so either force is the action force Emphasize that the action and reaction forces occur at the same time.

Hammer Striking a Nail What are the action/reaction pairs for a hammer striking a nail into wood? Force of hammer on nail = force of nail on hammer Force of wood on nail = force of nail on wood Which of the action/reaction forces above act on the nail? Force of hammer on nail (downward) Force of wood on nail (upward) Does the nail move? If so, how? Fhammer-on-nail > Fwood-on-nail so the nail accelerates downward This example is continued on the next slide.

Hammer Striking a Nail What forces act on the hammer?
Force of nail on hammer (upward) Force of hand on hammer (downward) Does the hammer move? If so, how? Fnail-on-hammer > Fhand-on-hammer so the hammer accelerates upward or slows down The hammer and nail accelerate in opposite directions. Use this example to stress the fact that the action and reaction forces do not cancel each other because they act on different objects. The best way to handle this is by drawing free body diagrams of each object next to each other. The free-body diagram for the nail is show on the previous slide. Ask students to draw the free-body diagram for the hammer. Then students can visualize the action-reaction forces and see that they do not balance each other. Each object accelerates or maintains constant motion based on the forces acting on that object.

Action and Reaction Forces
Section 3 Newton’s Second and Third Laws Chapter 4 Action and Reaction Forces Action-reaction pairs do not imply that the net force on either object is zero. The action-reaction forces are equal and opposite, but either object may still have a net force on it. Consider driving a nail into wood with a hammer. The force that the nail exerts on the hammer is equal and opposite to the force that the hammer exerts on the nail. But there is a net force acting on the nail, which drives the nail into the wood.

Action-Reaction: A Book on a Desk
Action Force The desk pushes up on the book. Reaction Force The book pushes down on the desk. Earth pulls down on the book (force of gravity). The book pulls up on Earth. Have students observe a book sitting on a desk for this slide. After students see the action force on the slide, they should be able to state the reaction force before you show it to them. Often students think the reaction force for the desk pushing up on the book is Earth pulling down on the book. Remind them that these forces act on the same object, the book, so they are not an action-reaction pair.

Action-Reaction: A Falling Book
The book pulls up on Earth. What is the result of the reaction force? Unbalanced force produces a very small upward acceleration (because the mass of Earth is so large). Action Earth pulls down on the book (force of gravity). What is the result of the action force (if this is the only force on the book)? Unbalanced force produces an acceleration of m/s2. Now, remove the book from the desk and allow it to fall to the floor. Ask students if the forces on the book are still balanced. What is the result of this unbalanced force? Acceleration. Have students calculate the acceleration of Earth. Assume the book’s mass is 2.0 kg, so the force on the book is (2.0 kg)(-9.8 m/s2) or 19.6 N downward. Therefore, the upward force on Earth is also 19.6 N. The mass of Earth is about 6 x 1024 kg, so students can calculate the upward acceleration and see how small it will be. You could also choose a falling distance and have students calculate the time required to fall the distance Earth would move upward during that time (using the equations from Chapter 2).

Problem Two students reach for a ar of mustard at the am time. One student pulls o the left with a force of 13.2 N, while the other students pulls to the right with a force of 12.9 N . If the jar has a net acceleration of 0.44 m/s2 to the left, what is the mass of the jar?

Given: F1 = 13.2 N to the left F2 = 12.9 N to the right
Two students reach for a ar of mustard at the am time. One student pulls o the left with a force of 13.2 N, while the other students pulls to the right with a force of 12.9 N . If the jar has a net acceleration of 0.44 m/s2 to the left, what is the mass of the jar? Given: F1 = 13.2 N to the left F2 = 12.9 N to the right anet = 0.44 m/s2 to the left m ( mass)= ? Use Newton's second law and solve for m Σ F = ma ΣF = F1+ F2 = F1 – F2 = 13.2 N – 12.9 N = 0.30 N ΣF = ma m=ΣF / anet = 0.30 N / 0.44 m/s2 = 0.68 kg

Chapter 4 Preview Objectives Weight Normal Force Friction
Section 4 Everyday Forces Preview Objectives Weight Normal Force Friction Sample Problem

Chapter 4 Objectives Explain the difference between mass and weight.
Section 4 Everyday Forces Objectives Explain the difference between mass and weight. Find the direction and magnitude of normal forces. Describe air resistance as a form of friction. Use coefficients of friction to calculate frictional force.

Warm UP If a force of 35 N is applied to a crate having a mass of 5 kg, what is the acceleration of the crate? A cart with a mass of 8 kg is accelerated at a rate of 2 m/s2. What is the force that was applied to the cart? 3. What is the acceleration of the object shown in the diagram below. 4. What is the acceleration of the object in problem 3 if there is a friction force of 1 N? Draw the diagram showing the forces.

Chapter 4 Section 4 Everyday Forces Weight The gravitational force (Fg) exerted on an object by Earth is a vector quantity, directed toward the center of Earth. The magnitude of this force (Fg) is a scalar quantity called weight. Weight changes with the location of an object in the universe.

Chapter 4 Weight, continued Calculating weight at any location:
Section 4 Everyday Forces Weight, continued Calculating weight at any location: Fg = mag ag = free-fall acceleration at that location Calculating weight on Earth's surface: ag = g = 9.81 m/s2 Fg = mg = m(9.81 m/s2)

Chapter 4 Section 4 Everyday Forces Normal Force The normal force acts on a surface in a direction perpendicular to the surface. The normal force is not always opposite in direction to the force due to gravity. In the absence of other forces, the normal force is equal and opposite to the component of gravitational force that is perpendicular to the contact surface. In this example, Fn = mg cos .

Normal Force

Frictional resistance force are typical proportional to the force which presses the surface together.

Friction and Surface Roughness

Friction Forces in Free-Body Diagrams
Chapter 4 Section 4 Everyday Forces Friction Forces in Free-Body Diagrams In free-body diagrams, the force of friction is always parallel to the surface of contact. The force of kinetic friction is always opposite the direction of motion. To determine the direction of the force of static friction, use the principle of equilibrium. For an object in equilibrium, the frictional force must point in the direction that results in a net force of zero.

Kinetic Friction

The Coefficient of Friction
Chapter 4 Section 4 Everyday Forces The Coefficient of Friction The quantity that expresses the dependence of frictional forces on the particular surfaces in contact is called the coefficient of friction, . Coefficient of kinetic friction: Coefficient of static friction:

Coefficient of Friction
Chapter 4 Section 4 Everyday Forces Coefficient of Friction

(N or FN ) normal force

Chapter 4 Sample Problem
Section 4 Everyday Forces Sample Problem Overcoming Friction A student attaches a rope to a 20.0 kg box of books.He pulls with a force of 90.0 N at an angle of 30.0° with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is Find the acceleration of the box.

Chapter 4 Section 4 Everyday Forces Sample Problem, continued 1. Define Given: m = 20.0 kg k = Fapplied = 90.0 N at  = 30.0° Unknown: a = ? Diagram:

Chapter 4 Section 4 Everyday Forces Sample Problem, continued 2. Plan Choose a convenient coordinate system, and find the x and y components of all forces. The diagram on the right shows the most convenient coordinate system, because the only force to resolve into components is Fapplied. Fapplied,y = (90.0 N)(sin 30.0º) = 45.0 N (upward) Fapplied,x = (90.0 N)(cos 30.0º) = 77.9 N (to the right)

Chapter 4 Section 4 Everyday Forces Sample Problem, continued Choose an equation or situation: A. Find the normal force, Fn, by applying the condition of equilibrium in the vertical direction: Fy = 0 B. Calculate the force of kinetic friction on the box: Fk = kFn C. Apply Newton’s second law along the horizontal direction to find the acceleration of the box: Fx = max

Chapter 4 Section 4 Everyday Forces Sample Problem, continued 3. Calculate A. To apply the condition of equilibrium in the vertical direction, you need to account for all of the forces in the y direction: Fg, Fn, and Fapplied,y. You know Fapplied,y and can use the box’s mass to find Fg. Fapplied,y = 45.0 N Fg = (20.0 kg)(9.81 m/s2) = 196 N Next, apply the equilibrium condition, Fy = 0, and solve for Fn. Fy = Fn + Fapplied,y – Fg = 0 Fn N – 196 N = 0 Fn = –45.0 N N = 151 N Tip: Remember to pay attention to the direction of forces. In this step, Fg is subtracted from Fn and Fapplied,y because Fg is directed downward.

Chapter 4 Section 4 Everyday Forces Sample Problem, continued B. Use the normal force to find the force of kinetic friction. Fk = mkFn = (0.500)(151 N) = 75.5 N C. Use Newton’s second law to determine the horizontal acceleration. a = 0.12 m/s2 to the right

Chapter 4 Section 4 Everyday Forces Sample Problem, continued 4. Evaluate The box accelerates in the direction of the net force, in accordance with Newton’s second law. The normal force is not equal in magnitude to the weight because the y component of the student’s pull on the rope helps support the box.

Chapter 4 Air Resistance
Section 4 Everyday Forces Air Resistance Air resistance is a form of friction. Whenever an object moves through a fluid medium, such as air or water, the fluid provides a resistance to the object’s motion. For a falling object, when the upward force of air resistance balances the downward gravitational force, the net force on the object is zero. The object continues to move downward with a constant maximum speed, called the terminal speed.

Chapter 4 Fundamental Forces There are four fundamental forces:
Section 4 Everyday Forces Fundamental Forces There are four fundamental forces: Electromagnetic force Gravitational force Strong nuclear force Weak nuclear force The four fundamental forces are all field forces.

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