CHAPTER 10 THE NATURE OF FORCE

Chapter Preview Questions 1. How do you know an object is in motion? a. It has inertia. b. It has mass. c. It is changing position. d. It has a reference point.

Chapter Preview Questions 1. How do you know an object is in motion? a. It has inertia. b. It has mass. c. It is changing position. d. It has a reference point.

Chapter Preview Questions 2. What is velocity? a. speed b. speed in a given direction c. miles per hour d. change in speed over time

Chapter Preview Questions 2. What is velocity? a. speed b. speed in a given direction c. miles per hour d. change in speed over time

Chapter Preview Questions 3. Which is the best definition of acceleration? a. change in velocity b. change in reference point c. increase in speed d. decrease in speed

Chapter Preview Questions 3. Which is the best definition of acceleration? a. change in velocity b. change in reference point c. increase in speed d. decrease in speed

Chapter Preview Questions 4. If an object starts out at rest and accelerates to 100 m/s, what is its initial speed? a. -100 m/s b. 0 m/s c. 100 m/s d. 32 m/s

Chapter Preview Questions 4. If an object starts out at rest and accelerates to 100 m/s, what is its initial speed? a. -100 m/s b. 0 m/s c. 100 m/s d. 32 m/s

What causes an object’s velocity to change? You drop a tennis ball and a baseball at the same time. Why do the balls fall to the floor?

centri- Latin Word Origins Latin Origin Meaning Key Term Center Centripetal force

com- Latin Word Origins Latin Origin Meaning Key Term Together, with Compression

jacere Latin Word Origins Latin Origin Meaning Key Term Throw Projectile

premere Latin Word Origins Latin Origin Meaning Key Term Press Compression

pro- Latin Word Origins Latin Origin Meaning Key Term Forward, before Projectile

tensus Latin Word Origins Latin Origin Meaning Key Term Stretch Tension

Apply It! Look at pro- and jacere and predict the meaning of projectile. Revise your definition as needed as you read the chapter. Look for words with these Latin origins as you read the chapter. Sample: I see that pro- means “forward” and that jacere means “to throw.” By combining the two I get the definition of projectile.

End of Chapter Preview

Section 1: The Nature of Force How is a force described? How do balanced and unbalanced forces affect an object’s velocity?

I. What is Force Force- a push or pull Magnitude (strength) and direction Newton- si unit Net force- combined forces Same direction + speed boat (60mph) + current downstream (5mph)- 65mph Dif direction – boat (60) – current (5) = 55mph Balanced force = 0 net force = same amt in opposite directions- NO MOVEMENT Unbalanced force = MOVEMENT

Combining Force Vectors The strength and direction of the individual forces determine the net force.

Unbalanced Forces Unbalanced forces acting on an object result in a net force and cause a change in the object’s velocity.

Balanced Forces Balanced forces acting on an object do not change the object’s velocity.

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End of Section 1: The Nature of Force

Section 2: Friction, Gravity, and Elastic Forces What factors determine the strength of the friction force between two surfaces? What factors affect the gravitational force between two objects? Why do objects accelerate during free fall? When is matter considered to be elastic?

II. Friction, Gravity and Elastic Forces Friction- 2 surfaces exert pressure on e/o Strength = types of surfaces and how much force they have on e/o Rougher surface = more friction More pushing force = more friction Static- not moving- book on table- strongest Sliding- slide over e/o- slide book across table-2nd Rolling- rolls across surface- skateboard-4th Fluid- solid object moves through liquid- surfing-3rd

Types of Friction

Gravity- force that pulls objects towards e/o Gravity acts everywhere and on everything=Law of universal gravitation Free Fall- force of gravity alone causes an obj to accelerate Every second ojb falls its velocity increases by 9.8 m/s Air Resistance= fluid friction of air acts on obj falling Greater surface area = greater resistance Projectile- an obj that is thrown will fall just as fast as obj dropped, will hit ground at same time. Elastic forces- if it returns to its original shape after it is squeezed or streched Compression- squeeze ex. sponge Tension- stretch/pull, ex. rubberband

Gravity Between Objects The force of gravity between objects increases with greater mass and decreases with greater distance.

Mass and Weight The gravitational force exerted on a person or object at the surface of a planet is known as weight.

Free Fall The graph shows how the speed of an object in free fall changes with time. Use the graph to answer the following questions.

Free Fall Interpreting Graphs: What is the speed of the object at 1 second? At 3 seconds? 9.8 m/s; 29.4 m/s.

Free Fall Calculating: Calculate the slope of the graph. What does this number represent? The slope is 9.8. The speed increases by 9.8 m/s each second.

Free Fall Predicting: Use the slope that you calculated in Step 2 to predict the object’s speed at 6 seconds. 58.8 m/s

Free Fall Drawing Conclusions: The graph has a constant slope. What does the slope tell you about the object’s motion? The object’s speed increases at a constant rate.

Air Resistance Falling objects with a greater surface area experience more air resistance.

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Click the Video button to watch a movie about free fall.

End of Section 2: Friction, Gravity, and Elastic Forces

Section 3 and 4: Newton’s First and Second and Third Laws What is Newton’s first law of motion? What is Newton’s second law of motion? What is Newton’s third law of motion? How can you calculate the momentum of an object? What is the law of conservation of momentum?

III. Newtons Laws 1st law- obj at rest will stay at rest (book on table wont move until force acts on it, pull or push), obj. in motion will stay in motion unless force acts on it (train will stop with rolling and sliding friction) Inertia- is the tendency of an obj to resist change in motion. MORE MASS = MORE INERTIA 2nd Law- A= net force (mass x acceleration)/mass 3rd Law- obj. exerts force on another, then the 2nd obj exerts a = force in the opposite direction on the 1st obj

Calculating Force A speedboat pulls a 55-kg water-skier. The skier to accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration. Read and Understand What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0 m/s2

Calculating Force A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration. Plan and Solve What quantity are you trying to calculate? The net force (Fnet) = __ What formula contains the given quantities and the unknown quantity? a = Fnet/m or Fnet = m x a Perform the calculation. Fnet = m x a = 55 kg x 2.0 m/s2 F = 110 kg • m/s2 F = 110 N cccc

Calculating Force A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration. Look Back and Check Does your answer make sense? A net force of 110 N is required. This does not include the force that overcomes friction.

Calculating Force Practice Problem What is the net force on a 1,000-kg object accelerating at 3 m/s2? 3,000 N (1,000 kg x 3 m/s2)

Calculating Force Practice Problem What net force is needed to accelerate a 25-kg cart at 14 m/s2? 350 N (25 kg x 14 m/s2)

Click the PHSchool.com button for an activity about Newton’s laws. More on Newton’s Laws Click the PHSchool.com button for an activity about Newton’s laws.

Newton’s Third Law of Motion Newton’s third law of motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.

Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s, or a 4.0-kg sledgehammer swung at 0.9 m/s? What information are you given? Mass of smaller sledgehammer = 3.0 kg Velocity of smaller sledgehammer = 1.5 m/s Mass of larger sledgehammer = 4.0 kg Velocity of larger sledgehammer = 0.9 m/s

Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? What quantities are you trying to calculate? The momentum of each sledgehammer = __ What formula contains the given quantities and the unknown quantity? Momentum = Mass x Velocity Perform the calculation. Smaller sledgehammer = 3.0 km x 1.5 m/s = 4.5 kg•m/s Larger sledgehammer = 4.0 km x 0.9 m/s = 3.6 kg•m/s

Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Look Back and Check Does your answer make sense? It is possible for the 3.0-kg hammer to have more momentum than the 4.0-kg one because it has a greater velocity.

Calculating Momentum A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum? Golf ball: 0.045 kg x 16 m/s = 0.72 kg•m/s Baseball: 0.14 kg x 7 m/s = 0.98 kg•m/s The baseball has greater momentum.

Calculating Momentum What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s? 0.27 kg•m/s (0.018 kg x 15 m/s = 0.27 kg•m/s)

Conservation of Momentum - In the absence of friction, momentum is conserved when two train cars collide.

Momentum Activity Click the Active Art button to open a browser window and access Active Art about momentum.

End of Section 3 and 4: Newton’s Laws

Section 5: Rockets and Satellites How does a rocket lift off the ground? What keeps a satellite in orbit?

IV. Rockets and Satellites Satellite- obj that orbits another obj Planets Rockets- lift off with = and opposite force pushing off the ground Artificial satellites orbit around Earth continuously fall toward Earth, but because Earth is curved, they travel around it- falling projectile Centripetal force- force that causes obj to move in circular path Centripetal = “center-seeking”

What Is a Satellite? A projectile follows a curved path. The horizontal and vertical motions act independently.

What Is a Satellite? The faster a projectile is thrown, the father it travels before it hits the ground. A projectile with enough velocity moves in a circular orbit.

What Is a Satellite? Depending on their uses, artificial satellites orbit at different heights.

End of Section 5: Rockets and Satellites

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