Newton’s Second Law Chapter 4 Net Force causing acceleration Friction Air resistance

Student Learning Outcomes Understand when Newton’s law of inertia applies and when it does not Understand what happens to objects when there is a net force (Newton’s 2 nd law) Calculate accelerations from the net force Understand a conceptual model of friction – Calculate acceleration when friction acts Understand a conceptual model of air resistance – Calculate acceleration when falling with air resistance

What can happen to an object when the net force on it equals zero? Select all that apply. 1.Speed up 2.Slow down 3.Change direction 4.Stay at rest 5.Move in a line with constant speed Give examples in this room where the net force =

What can happen to an object when the net force on it is NOT zero? Select all that apply. 1.Speed up 2.Slow down 3.Change direction 4.Stay at rest 5.Move in a line with constant speed Give a one word answer to the question, “What happens to an object when the net force on it is NOT zero?”

Accelerations Newton’s second law: – ____________ _____________ object ___________ – An object’s _______________________________ – Equation: People often say _______, but the better version is ____________ – Words: ________________________________________ – Same equation written another way: Describe this in words. Textbook pages ________________

Figure 4.2 – with extra labels, p. 59

Figure 4.11 – with extra labels, p. 64

Friction - what When does friction occur? – __________________________________________ _______________________________ In which direction does friction push? – ________________________________________. Note the word “__________” left out of your textbook! Sliding cart with scale – Start slowly, pull stronger. Maximum friction force? – Constant speed: how much friction force?

4 examples 5-kg box sits on flat floor. Maximum friction force is 100 N. Draw a picture for each of the 4 cases. – What is the net force in these cases? – What happens to the box? – Calculate acceleration. 1.You push to right with 70 N 2.You push to right with 100 N 3.You push to right with 110 N 4.You push to left with 120 N

Friction - why Why does friction occur? – ________________________________________ ______________________________ Figure 4.3:

What does friction depend on? Surprising, _________ and _______________ – How do we know? – Except for “______________ ” (or ____________ ____________________) In these cases, friction happens because ____________ ___________________________) ____________________________________ ___________________________________

___ Types of friction ______________________ friction – When ____________________________________ ___________ friction – When ___________________________________ _____________________________________ – The surfaces ___________ if there were no friction. ___________ friction (not in your textbook) – When one object __________ across a surface

Which kind of friction is weakest? 1.Static friction 2.Sliding (kinetic) friction 3.Rolling friction 4.All 3 are the same

Newton’s 3 rd law causing ________ accelerations Newton’s 3 rd law told us – _____________________________________________ _______________________________. Yet we “know” BIG truck “______” and little car “______” in a collision. Why? [Rhetorical] On next slide, I’m going to color code car and truck. In your notes, somehow “color” code them too.

Unequal accelerations, continued F truck on car makes ____________________________ – F car on truck makes ____________________________ Summary – putting it all together: Written more graphically: And what do people feel?

Simple gravity (More complicated version of gravity is our next chapter, chap. 9 – we’ll come back to 6-8) __________________________, we can say: – ______________________________ On Earth: _____________________________________ On Moon: about 1/6, 3.7 miles/hr per sec) The acceleration number is called _________ – __________________________ (symbol: ____) – ________________________________________ – _________________________________________ ________________________________________

You have two balls of equal size and smoothness, and you can ignore air resistance. One is heavy, the other much lighter. You hold one in each hand at the same height above the ground. You release them at the same time. What will happen? 1.The heavier one will hit the ground first. 2.They will hit the ground at the same time. 3.The lighter one will hit the ground first

Same objects. How does the acceleration of gravity compare? Be careful! 1.The heavier one has a larger gravitational acceleration. 2.They have the same gravitational acceleration. 3.The lighter one has a larger gravitational acceleration

Same objects. How does the force of gravity compare? Be careful! 1.The heavier one has a stronger gravitational force. 2.They have the same gravitational force. 3.The lighter one has a stronger gravitational force

Falling with air resistance Air resistance depends on _____and “frontal” __________________. One model of air resistance: – ________________________________________ Coffee filter demo – Regular – Crumpled What forces act after you drop out of a very high-up helicopter?

What is speed at the instant you drop out of very high helicopter? 1.Zero 2.10 m/s 3.Not enough information? So how strong is air resistance? What forces act? What’s the net force?

What is acceleration at the instant you drop out of very high helicopter? 1.Zero 2.10 m/s 3.10 m/s per second 4.Not enough information

If wait a long time, what happens to speed? 1.You always speed up 2.You speed up, but then slow down 3.You speed up to a certain speed

What is acceleration after waiting a long time, BUT before hitting ground? 1.More than g = 10 m/s per second 2.g 3.Less than g but more than zero 4.zero

What is acceleration in middle, before achieving terminal speed? 1. More than g = 10 m/s per second 2.G 3.Less than g but more than zero 4.zero What’s happening to the acceleration?

Jumping out of planes People in the movies often jump out of planes without a parachute, usually after the bad guy takes the last parachute. What do the good guys do to survive? Why does this work? – How does it affect __________________ ? What do parachutes do? – How does it affect ___________________?

Which encounters a greater force of air resistance if dropped at same time? 1.Falling feather 2.Falling elephant 3.Same air resistance for both 4.Not enough information

Student Learning Outcomes Understand when Newton’s law of inertia applies and when it does not Understand what happens to objects when there is a net force (Newton’s 2 nd law) Calculate accelerations from the net force Understand a conceptual model of friction – Calculate acceleration when friction acts Understand a conceptual model of air resistance – Calculate acceleration when falling with air resistance