Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 11

Physics 211Physics 211Lecture 4Lecture 4

Today's Concepts:Today's Concepts:

Newton’s Laws:Newton’s Laws:a)a) Acceleration is caused by forcesAcceleration is caused by forcesb)b) Force changes momentumForce changes momentumc)c) Forces always come in pairsForces always come in pairsd)d) Good reference framesGood reference frames

SR1SR1

SR3SR3SR4SR4

SR2SR2

SR5SR5

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 22

The directions of force, velocity, and accelerationThe directions of force, velocity, and acceleration

Inertial/non-inertial reference frames.Inertial/non-inertial reference frames.

Third newtons lawThird newtons law

i understood it alli understood it all

Nothing, lets go on to Rotation!!Nothing, lets go on to Rotation!!

none. This is old material. I did 98% of this in high school: We none. This is old material. I did 98% of this in high school: We just didn't call things derivatives. But I would be interested in an just didn't call things derivatives. But I would be interested in an example where the mass of an object changes.example where the mass of an object changes.

This actually isn't something that was discussed in the This actually isn't something that was discussed in the prelecture, but something that has always bothered me is the prelecture, but something that has always bothered me is the derivation of the centripetal force on a banked ramp (it's inverse derivation of the centripetal force on a banked ramp (it's inverse tangent of v^2/r or something along those lines. tangent of v^2/r or something along those lines.

Stuff you asked about:Stuff you asked about:

centrifugal forcecentrifugal force

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 33

Acceleration is caused by force.Acceleration is caused by force.

A bigger mass makes this harderA bigger mass makes this harder

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 44

Preflight/ActPreflight/Act

Suppose the net force on some object is parallel to the x axis. Which of Suppose the net force on some object is parallel to the x axis. Which of the following statements best describes the motion of the object:the following statements best describes the motion of the object:

A)A) Its velocity is parallel to the x axisIts velocity is parallel to the x axisB)B) Its acceleration parallel to the x axisIts acceleration parallel to the x axisC)C) Both its velocity and its acceleration are parallel to the x axisBoth its velocity and its acceleration are parallel to the x axisD)D) Neither its velocity or its acceleration need be parallel to the x axisNeither its velocity or its acceleration need be parallel to the x axis

B) Acceleration is in the same direction as the net force, B) Acceleration is in the same direction as the net force, but the net force tells us nothing about its velocity. but the net force tells us nothing about its velocity.

C) acceleration is in the same direction as the force, and C) acceleration is in the same direction as the force, and velocity is the same direction as acceleration, so all must velocity is the same direction as acceleration, so all must be parallel be parallel

Vote againVote again

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 55

Preflight/ActPreflight/Act

Suppose the net force on some object is parallel to the x axis. Which of Suppose the net force on some object is parallel to the x axis. Which of the following statements best describes the motion of the object:the following statements best describes the motion of the object:

A)A) Its velocity is parallel to the x axisIts velocity is parallel to the x axisB)B) Its acceleration parallel to the x axisIts acceleration parallel to the x axisC)C) Both its velocity and its acceleration are parallel to the x axisBoth its velocity and its acceleration are parallel to the x axisD)D) Neither its velocity or its acceleration need be parallel to the x axisNeither its velocity or its acceleration need be parallel to the x axis

Your preflight results:Your preflight results:

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 66

Act

5M5MMM

FF aa

A force F is applied to a small block, that pushes a larger block. The two blocks accelerate to the right. Compare the NET FORCE on the block w/ mass M, to the net force on the block with mass 5M.

A) FM < F5M

B) FM = F5M

C) FM > F5M

F ma

Net ForceSame acceleration, so larger

mass has larger net force.

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 77

Preflight/ActPreflight/Act

You are driving a car with constant speed around a horizontal circular track.You are driving a car with constant speed around a horizontal circular track.

The net force acting on your carThe net force acting on your car

A)A) Points radically inward toward the center of the circular trackPoints radically inward toward the center of the circular trackB)B) Points radically outward, away from the center of the circular trackPoints radically outward, away from the center of the circular trackC)C) Points forward in the same direction your car is movingPoints forward in the same direction your car is movingD)D) Points backward, opposite to the direction your car is movingPoints backward, opposite to the direction your car is movingE)E) Is zero.Is zero.

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 88

You are driving a car with constant speed around a horizontal circular track.You are driving a car with constant speed around a horizontal circular track.The net force acting on your carThe net force acting on your car

A)A) Points radically inward toward the center of the circular trackPoints radically inward toward the center of the circular trackB)B) Points radically outward, away from the center of the circular trackPoints radically outward, away from the center of the circular trackC)C) Points forward in the same direction your car is movingPoints forward in the same direction your car is movingD)D) Points backward, opposite to the direction your car is movingPoints backward, opposite to the direction your car is movingE)E) Is zero.Is zero.

A) the acceleration is pointing in, so the force must too A) the acceleration is pointing in, so the force must too . .

B) the force points outward, explaining why your body shifts B) the force points outward, explaining why your body shifts to the right when making a left turn. to the right when making a left turn.

E) Because there is no acceleration, the force is 0. . E) Because there is no acceleration, the force is 0. .

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 99

Preflight/ActPreflight/Act

You are driving a car with constant speed around a horizontal circular track.You are driving a car with constant speed around a horizontal circular track.

The net force acting on your carThe net force acting on your car

A)A) Points radically inward toward the center of the circular trackPoints radically inward toward the center of the circular trackB)B) Points radically outward, away from the center of the circular trackPoints radically outward, away from the center of the circular trackC)C) Points forward in the same direction your car is movingPoints forward in the same direction your car is movingD)D) Points backward, opposite to the direction your car is movingPoints backward, opposite to the direction your car is movingE)E) Is zero.Is zero.

Centripetal acceleration is always pointed inward, and the Centripetal acceleration is always pointed inward, and the net force points in the same direction as acceleration. net force points in the same direction as acceleration.

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1010

Wineglass demoWineglass demo

Aside: Centripetal acceleration and forceAside: Centripetal acceleration and force

1) Objects moving in a circle always have a 1) Objects moving in a circle always have a component of acceleration, called centripetal, component of acceleration, called centripetal, which is toward the center of the circle.which is toward the center of the circle.**

2) Centripetal acceleration must be caused by a force:2) Centripetal acceleration must be caused by a force: - Friction, gravity – whatever force keeps it - Friction, gravity – whatever force keeps it moving in a circle. moving in a circle. - This force is often called the “centripetal force”- This force is often called the “centripetal force”

** They can have also have tangential acceleration if their speed is not constantThey can have also have tangential acceleration if their speed is not constant

3) There is no “new” kind of force here.3) There is no “new” kind of force here.

4) There is no such thing as centrifugal force.4) There is no such thing as centrifugal force.

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1111

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1212

Tablecloth & DishesTablecloth & Dishes

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1313

Preflight/ActPreflight/Act

You are driving a car with constant speed around a horizontal circular track.You are driving a car with constant speed around a horizontal circular track.

The momentum of your carThe momentum of your car

A)A) Points radically inward toward the center of the circular trackPoints radically inward toward the center of the circular trackB)B) Points radically outward, away from the center of the circular trackPoints radically outward, away from the center of the circular trackC)C) Points forward in the same direction your car is movingPoints forward in the same direction your car is movingD)D) Points backward, opposite to the direction your car is movingPoints backward, opposite to the direction your car is movingE)E) Is zero.Is zero.

The momentum points in the same direction as the The momentum points in the same direction as the velocity, which is forward. velocity, which is forward.

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1414

Ice-puckIce-puck Fake Forces: Coriolis (YouTube)Fake Forces: Coriolis (YouTube)

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1515

This effect can This effect can make cannons make cannons miss their targetmiss their targetif you don’t takeif you don’t takeit into account.it into account.

Hurricane CatrinaHurricane Catrina

And makes hurricanes And makes hurricanes rotate CCW (CW) in the rotate CCW (CW) in the Northern (Southern) Northern (Southern) hemisphere.hemisphere.

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1616

Forces come in pairs!Forces come in pairs!

Fire-cartFire-cart

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1717

A small guy and a large football player moving at the same speed collide head-on.

Which person experiences the larger force during the collision?

A. The small guy.B. The football player.C. They experience the same force.

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1818

A small guy and a large football player moving at the same speed collide head-on.

Which person experiences the larger acceleration during the collision?

A. The small guy.B. The football player.C. The accelerations are the same.

Physics 211 Lecture 4, Slide Physics 211 Lecture 4, Slide 1919

A small guy moving at a high speed collides with a stationary large football player. Now, which person experiences the larger force during the collision?

1. The small guy experiences the larger force.2. The football player experiences the larger

force.3. Both experience the same force.