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PHY131H1F Summer – Class 4 Today: •  Circular Motion •  Forces •  Free Body Diagrams •  Newton’s Second Law •  Newton’s First Law

Test on May 27 evening •  Test will be Thursday, May 27 from 6:30pm to

7:50pm in EX100.

•  There is no class in BA1160 that day, but there are Practicals, as normal.

•  Test will cover Chs.1-6, Error Analysis Tutorial and Practicals Sessions 1-4 Material.

•  Please bring a non-communicating calculator and a single 2-sided aid sheet which you have prepared yourself with helpful equations, etc.

•  I will provide any numerical constants you may need on the test.

Relative Motion •  A helpful notation: •  vTG = velocity of

•  vPT = velocity of

•  vPG = velocity of

v = v + v

Inner subscripts disappear

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A.  between North and East, more than 25 m/s. B.  between North and East, less than 25 m/s. C.  between South and East, more than 25 m/s. D.  between South and East, less than 25 m/s. E.  between South and East, exactly 25 m/s.

In Class Discussion Question 1. Car A is driving with velocity relative to the ground vAG = 25 m/s, East. Car B is driving with velocity relative to the ground vBG = 15.8 m/s, North. What is the velocity of Car A relative to Car B?

Projectile Motion Review Projectile motion is made up of two independent motions:

The kinematic equations that describe these two motions are

•  Average:

•  Standard Deviation:

•  This class: let’s use g =

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Monkey and Hunter Demonstration The classic problem: “A monkey hanging from the branch of a

tree is spotted by a hunter. The monkey sees that the barrel of the gun is pointed directly at him. At the exact instant the gun is fired, the monkey lets go of the branch. Will the bullet (A) go above the monkey, (B) go below the monkey, or (C) hit the monkey?

Our demonstration uses a blow gun. The gun is aimed directly at the Roman Centurion, which is held up by an electromagnet. As the bullet leaves the muzzle of the blow gun, it breaks a connection that releases the magnet.

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In-Class Discussion Question 3: A carnival has a Ferris wheel

where some seats are located halfway between the center and the outside rim. Compared with the seats on the outside rim, the inner cars have

A.  Smaller angular speed and greater tangential speed B.  Greater angular speed and smaller tangential speed C.  The same angular speed and smaller tangential speed D.  Smaller angular speed and the same tangential speed E.  The same angular speed and the same tangential speed

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In class discussion question 4.

•  Satellites in orbit are accelerating toward the earth, so they must be getting closer and closer to our planet

A.  True B.  False C.  [bad question: satellites are not

accelerating toward the earth.]

Centripetal Acceleration

•  A bike wheel of diameter 1.0 m turns 20 times per second. What is the magnitude of the centripetal acceleration of a yellow dot on the rim?

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Nonuniform Circular Motion •  Any object traveling along a curved path has

•  If, as it is traveling in a circle, it is speeding up or slowing down

•  The total acceleration is

Note:

•  Up until now, we have been studying kinematics, a description of

•  In Chapter 5 we begin to study dynamics, which is

Quick Review Quiz: The fan blade is slowing down. What are the signs of ω and α? [Let’s define, as Knight often does, positive to be counter-clockwise.]

Α. ω is positive and α is positive. B. ω is negative and α is positive. C. ω is positive and α is negative. D. ω is negative and α is negative.

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In class discussion question

•  Satellites in orbit are accelerating toward the earth, so they must be getting closer and closer to our planet

A.  True B.  False C.  [bad question: satellites are not

accelerating toward the earth.]

•  1643- •  According to wiki, was a

“physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian and one of the most influential people in human history.”

•  In Philosophiæ Naturalis Principia Mathematica, published , he described

and the , laying the

groundwork for classical mechanics.

Isaac Newton

Newton’s first law is also known as the law of inertia. If an object is at rest,

If it is moving,

If no force:

If object is forced:

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In Class Discussion Question. A fan attached to a cart causes it to accelerate at 2 m/s2. Suppose the same fan is attached to a second cart with smaller mass. The mass of the second cart plus fan is half the mass of the first cart plus fan. The acceleration of the second cart is

A.  16 m/s2. B.  8 m/s2. C.  4 m/s2. D.  2 m/s2. E.  1 m/s2.

In Class Discussion Question Three forces act on an object. In which direction does the object accelerate?

What is a force? •  A force is •  A force is

•  A force requires

•  A force is either a is the only long-

range force we will deal with in PHY131. •  Important contact forces are:

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Tactics: Drawing force vectors

A Short Catalog of Forces

is 9.8 m/s2, toward the centre of the Earth (down).

A Short Catalog of Forces

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A Short Catalog of Forces

A Short Catalog of Forces

A Short Catalog of Forces

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In Class Discussion Question. Which is true? “Friction A.  always causes objects to slow down.” B.  always causes objects to speed up.” C.  can cause objects to speed up or slow

down, depending on the situation.”

Before Next Class: •  Read Chapter 6 of Knight. •  Complete MasteringPhysics.com Problem Set 3

due by May 24 at 11:59pm •  Do Suggested End-of-chapter Exercises and

Problems from Knight: Ch.4: 5, 11, 15, 21, 27, 53, 59 and 83

•  Ch.5: 3, 7, 13, 25, 35 and 53