FIRST

Handback quizzes

Homework discussion, and talking about cars and hills with a video.

Next time: HW turn in

HW FOR NEXT TIME: The one titled homework. Answers are in this PPT posted online.

LETS GET STARTED

Today will discuss forces at angles, and the idea of net force.

So you will: Understand how X and Y forces affect each other

indirectly. Set up net force equations for things at angles Look at difficult conceptual situations involving

forces and motion.

DO YOU GET NEWTONS 3RD LAW?

If we are looking at the amount of force of B on A compared to A on B, we know…

They are ALWAYS the same, DIFFERENT DIRECTION.

And the more INERTIA one has, the LESS it will change its current motion from the force.

LETS MAKE SURE…DEMO IT

FORCES AT ANGLES AGAIN

A force on a surface will mean that the angled force will have an effect on the normal force.

You use Sin and Cos to find the X and Y parts of the force

If there are other forces in the Y direction, then the normal force does NOT equal the weight

LETS DO ONE

You pull a cat again at an angle. There isfriction opposing your pull.

1) Draw a FBDand then another withthe X and Y components

Cat

Weight

Weight: Force of Earth on the cat

Fn

Fn: Force of ground on cat

Ff

Ff: Force of ground on cat

Pull in X

Pull: Force of person on the cat

Pull in Y

Pull X: Force of person on the cat horizontally

Pull Y Force of person on the cat vertically

Pull

QUESTIONS

1) Which is larger, the normal force or theweight?

The weight, since the cat is inequilibrium in Y. And theweight is balanced by thenormal force AND the Y pull.

Cat

Weight

Weight: Force of Earth on the cat

Fn

Fn: Force of ground on cat

Ff

Ff: Force of ground on cat

Pull in X

Pull in Y

Pull X: Force of person on the cat horizontally

Pull Y Force of person on the cat vertically

QUESTIONS

2) If you pull at a larger angle, will the weightor the normal force change?

The weight will NOT, sinceit’s the Earths pull on thecat.The normal force WILL,since you are still in equilibrium in Y,your larger pull in Y will decrease thenormal force needed from the table.

In other words…

Cat

Weight

Weight: Force of Earth on the cat

Fn

Fn: Force of ground on cat

Ff

Ff: Force of ground on cat

Pull in X

Pull in Y

Pull X: Force of person on the cat horizontally

Pull Y Force of person on the cat vertically

QUESTIONS

3) If the cat was moving at a constant velocity,would increasing the angle changethe friction?

YES! Since friction dependson normal force (Ff = µFnormal)then more of a Y pull will decreasethe normal force, resulting in less friction

In other words…

Cat

Weight

Weight: Force of Earth on the cat

Fn

Fn: Force of ground on cat

Ff

Ff: Force of ground on cat

Pull in X

Pull in Y

Pull X: Force of person on the cat horizontally

Pull Y Force of person on the cat vertically

QUESTIONS

4) Say that the coefficient of friction is µk andpulled at angle θ. Lets create2nd Law equations for X and Y

X: Lets say the accelerationwill be with the pull (my guess) so +. There is animbalance then.

X: F1x – Ff = imbalance Y: No imbalance, call down – Y: -Weight + FNormal = equilibrium

Cat

Weight

Weight: Force of Earth on the cat

Fn

Fn: Force of ground on cat

Ff

Ff: Force of ground on cat

Pull in X

Pull in Y

Pull X: Force of person on the cat horizontally

Pull Y Force of person on the cat vertically

QUESTIONS

X: F1x – Ff = imbalance

X : F1Cosθ – µFnormal = ma

Y: -Weight + FNormal = equilibrium

Y: -mg + FNormal = 0

SUMMARY

When changing the angle of a force on a surface, the normal force changes.

Which changes the friction as well.

A downward push results in MORE normal force, an upward push results in LESS normal force.

Friction is the coefficient of friction times the normal force. Use static for non-moving, kinetic for moving

DEMO OF THAT…NUTZ

LETS TRY A FEW AP PROBLEMS

Alone, with a formula sheet. Just the first, please.

WHAT THIS REINFORCED

Static friction used to get things to move.

Friction cannot make things speed up, so if friction is a larger force AND will cause a speed up, we have issues.

We needed to solve for the normal force using Y info to put in X

Lets use our knowledge to define a term….

WHAT THIS REINFORCED

25: Increasing the angle in this case is taking the burden of the weight off the normal force, so it decreases.

26: Increasing the angle will result in less forward force, which will lead to a smaller acceleration.

Draw the FBD from 40.

Now its different….What words have changed everything?

The 3 Y forces are added together. 2 are negative because they are opposite the imbalance direction.