Physics 211 Lecture 4, Slide 1 PHYSICS Today's Concepts: Newton’s Laws: a)Acceleration is caused by forces b)Force changes momentum c)Forces always come

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

PHYSICSPHYSICS

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

http://www.youtube.com/watch?v=HDRPazZySzE


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

A bigger mass makes this harderA bigger mass makes this harder


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



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:


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.



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.


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) 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. .




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


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.


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.



Tablecloth & DishesTablecloth & Dishes




The momentum of your carThe momentum of your car


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.


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


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.


Forces come in pairs!Forces come in pairs!

Fire-cartFire-cart


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.


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.


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.

Documents

Physics 211 Lecture 4, Slide 1 PHYSICS Today's Concepts: Newton’s Laws: a)Acceleration is caused by forces b)Force changes momentum c)Forces always come