Physics 102-002 Announcements

• Clickers – should have by now

• WebAssign – must have Account Codes by tomorrow (Tues, Feb 6)

• Exam #1 Wednesday– Covers Chapters 1-4

Class Schedule

1/17 Chapter 1 Introduction, About Science

1/22 Chapter 2 Newton’s First Law

1/24 Chapter 3 Linear Motion

1/29 Chapter 4 Newton’s Second Law

1/31 Midterm Exam 1

2/5 Chapter 5 Newton’s Third Law

2/7 Chapter 6 Momentum

Chapter 5Newton’s Third Law

• Forces and Interactions

• Newton’s Third Law– Defining your system– Action and reaction on different masses

• Summary of Newton’s Three Laws

• Vectors

Force and Interactions

Force: A push or pull exerted on an object

Examples: Hand against a wall.Book on a table.Prize fighter and tissue paperEarth and you.

Interacting objects exert forces on each other.

Force of skater B on

skater AFBA

Force of skater A on

skater BFAB

You can’t exert a force on something unless the other objects exerts and equal and opposite force on you!

Question 1

While driving down the road, an unfortunate bug strikes the windshield of a bus. Quite obviously, this is a case of Newton's third law of motion. The bug hit the bus and the windshield hit the bus. Which of the two forces is greater:

a. the force on the bug?

b. the force on the bus?

Question 2

A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is ...

a. greater than the acceleration of the bullet.

b. smaller than the acceleration of the bullet.

c. the same size as the acceleration of the bullet.

Newton’s Third Law

ON impact, the interaction forces between the blue ball and the yellow ball STOP the blue ball and MOVE the yellow ball.

To every action there is an equal and opposite reaction.

Defining the “System”System: Gives the boundaries beyond which forces are “external”

Example in the book: The orange and the apple.

Here, the orange is the system: an external force is acting on the orange, causing it to accelerate.

The apple provides the external force to accelerate the system to the right. Notice that the force exerted by the apple on the orange is still equal and opposite to the force of the orange on the apple.

If the system is the apple AND the orange: an external force is still acting on the system to accelerate it to the right. The force is provided by the friction between the apple’s “feet” and the ground.

Action and Reaction on Different Masses

am

F

The action and reaction forces are equal and opposite, no matter whether the masses are the same or not!

The earth pulls “downward” on the rock.

The falling rock pulls “upward” on the earth (with the exact same amount of force).

The force of attraction between the 2 “planets” is equal and opposite regardless of the mass difference.

(Both planets attract each other)

The force exerted on the cannonball is the same as the force exerted on the cannon. But the cannonball accerates more because of its much lower inertia (or mass).

aM

F

Cannonball: Cannon:

(Smaller mass,Larger acceleration)

(Larger mass,Smaller acceleration)

Newton’s 3 Laws: Summary

• Newton’s First LawAn object at rest tends to stay at rest and an object in

motion tends to stay in motion at constant speed and in the same direction unless acted on by an unbalanced force.

• Newton’s Second LawF = ma

• Newton’s Third LawTo every action there is an equal and opposite

reaction.

VectorsVectors have magnitude and direction. They add or subtract depending on their directions.

Parallel vectors are pretty simple:

50 N

50 N= 100 N+

50 N

50 N=+ 0 N

What if the vectors are NOT parallel:

Example: What if I walked 16 km East and 12 km NorthThe result is a NET movement of 20 km Northeast

16 km East

12

km

No

rth

20 km Northeast

Component Vectors

Resultant Vector

Vectors continued

Resultants of vectors applethttp://www.walter-fendt.de/ph14e/resultant.htm

Vector components applethttp://id.mind.net/~zona/mstm/physics/mechanics/vectors/components/vectorComponents.html

All vectors (force, velocity, acceleration, etc) add the same way.

The 30N and 40N forces add to get a resultant force of 50N.The DIRECTION of the resultant 50N force is given by the diagonal of the “parallelagram” (The Parallelagram Rule)

Nellie illustrates the parallelagram rule.Note that the 2 tensions add to more than Nellie’s weight!

Support Force Interactive Figure

Vectors continued

Velocity vectors add: The crosswind of 60 km/h adds to the planes velocity of 80 km/h to blow the plane offcourse at 100 km/h.

Some example resultant velocities.

When you throw a rock, it’s velocity has a horizontal and vertical component

Question 3The 3 boats shown are crossing a river.

Which of the boats will follow the shortest path to the opposite shore?

Velocity of the boat

Velocity of the river