Solving Force Probmems Physics Mr. Maloney © 2002 Mike Maloney Objectives You will be able to diagram Force problems use FBDs to analyze and solve

Solving Solving Force Force ProbmemsProbmems

PhysicsMr. Maloney

© 2002 Mike Maloney

ObjectivesObjectives

You will be able todiagram Force problemsuse FBDs to analyze and solve force

problems.


““Picturing Force” Picturing Force” Free-Body DiagramsFree-Body Diagrams

Free Body Diagrams are vector diagrams.

We use them to graphically depict what is acting on an object.

They represent the magnitude and direction of all forces acting on an object.


Free Body Free Body DiagramsDiagrams

The diagram at right shows all the forces acting on this object.

Here we have Gravity, a Frictional Force, an Applied Force, and a Normal Force.

Objects can have any number of forces acting on them, depending on the situation.

LET’S IDENTIFY SOME


Balanced Forces (Statics) Balanced Forces (Statics) (N1L)(N1L)

There are often situations where a number of forces are acting on something, and the object has no motion – it is STATIC or in EQUILIBRIUM.

This means the NET FORCE on the object is zero, or in other words the forces balance each other out.

What does this mean mathematically?


unBalanced Forces unBalanced Forces (Dynamics) (N2L)(Dynamics) (N2L)

There are other situations where all the forces acting on something do not cancel each other out completely.

This means the NET FORCE on the object is not zero, the object will change its motion and accelerate proportional to the object’s mass.

F = m ∙ aLet’s try one of these.


Solving StrategySolving Strategy1. Determine whether it is a static problem or a

dynamic problem if you can.2. Sketch a Free Body diagram.3. Draw and label vectors representing all forces.4. Pick an “X” and “Y” direction. If there is any

motion, you should choose either X or Y to be in the direction of motion.

5. Break forces into components if necessary6. Sum the forces (F) in the X and Y directions, and

set them equal to either 0 or m∙a accordingly. (If you are unsure, use m∙a)

7. Use the summation equations (F) you have created to solve for unknown values.

Lets go back and try a few more.


More Advanced ProblemsMore Advanced ProblemsThere are usually many types of forces

acting on objects where friction and motion is occurring.

They can either be parallel to the motion, or perpendicular to it.

This next example has a block on a table where there is friction and an applied force at an angle.


Advanced ProblemsAdvanced Problems

What forces are acting parallel to the surfaces? The horizontal (x) part of the Applied Force {FA}, and

the Friction Force {Ff}

What about Perpendicular? The vertical (y) part of the Applied force {FA}, the

Weight {Fg} and the Normal force {FN}

block

surface

FA

Ff

FAx

FN

Fg

FAy


We see that an applied force can be applied at any angle. So it may act perpendicular and/or parallel to the motion.


block

surface

FA

Ff

FAx

FN

Fg

FAy


However FN is always perpendicular to the motion.

Ff is always parallel to the motion. On a flat surface Fg is perpendicular, but we saw

how it can be both on an incline. Each problem has to be looked at and analyzed

individually. So lets try some.


block

surface

FA

Ff

FAx

FN

Fg

FAy

By the Way, what is wrong with this Free

Body Diagram?


Block on hillBlock on hill Let’s go back to a block just sitting on a track. What happens when I slant the track? The cart starts to move. I did not change any

of the forces acting on the cart … so what changed?


ObjectivesObjectives

Can you …diagram Force problemsuse FBDs to analyze and solve force

problems.


APPENDIXAPPENDIX


Net ForceNet Force NET FORCE refers to the vector sum total

of all forces acting on an object. It is often expressed as F

For example, if there were two leftward forces of 10 lb each, the NET FORCE would be 20 lb leftward.

If there were one 10 lb rightward force and one 8 lb leftward force, the NET FORCE would be 2 lb rightward.

What about if the forces were in X and Y?

BACK


Static problemStatic problem

Static problems are associated with Newton’s 1st Law.

Static problems are problems in which the net force is ZERO (0).

In this case the sum of the forces in the X-direction and the Y-direction are both ZERO (0).

BACK


dynamic problemdynamic problemDynamic problems are associated

with Newton’s 2nd Law.Dynamic problems are problems in

which the net force is not ZERO. In this case the sum of the forces in the

X-direction and/or the Y-direction are not always zero, and may result in some acceleration.

BACK


x/y Componentsx/y Components

If the forces are acting in more than 2 directions (i.e. applied force at an angle) break all the forces down into vertical and horizontal components or parallel and perpendicular components using vector rules (sine and cosine).

Once they are broken up, you can Fx and Fy to solve for the unknowns.

BACK


1. 1.

A book is at rest on a table top.

FN

FgBACK TO LECTURE


2. 2.

A girl is suspended motionless in the air by two ropes attached to her.

FT

Fg

FT

BACK TO LECTURE


3. 3.

An egg is free-falling from a nest in a tree. Neglect air resistance.

FgBACK TO LECTURE


4. 4. A flying squirrel is gliding (no wing flaps)

from a tree to the ground at constant velocity. Consider air resistance.

FAIR

FgBACK TO LECTURE


5. 5. A rightward force is applied to a book in order

to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance.

FN

Fg

FAFf

BACK TO LECTURE


6. 6. A rightward force is applied to a book in order

to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance.

FN

Fg

FAFf

BACK TO LECTURE


7. 7. A college student rests a backpack upon his

shoulder. The pack is suspended motionless by one strap from one shoulder.

FA

FgBACK TO LECTURE


8. 8.

A skydiver is descending with a constant velocity. Consider air resistance.

FAIR

FgBACK TO LECTURE


9. 9.

A force is applied to the right to drag a sled across loosely-packed snow with a rightward acceleration.

FN

Fg

FAFf

BACK TO LECTURE


10. 10.

A football is moving upwards towards its peak after having been booted by the punter. Neglect air resistance.

FgBACK TO LECTURE


11. 11. Three smaller kids are pulling a rope against one

large kid. A. They are at a stand still B. The big kid is winning

BACK TO LECTURE




BACK TO LECTURE




BACK TO LECTURE


12. 12.

A car is coasting to the right and slowing down.

Ff

BACK TO LECTURE

FN

Fg


13. 13.

Mr. M is holding a book against a flat wall.

FN

Fg

FA

Ff

BACK TO LECTURE

Documents

Solving Force Probmems Physics Mr. Maloney © 2002 Mike Maloney Objectives You will be able to diagram Force problems use FBDs to analyze and solve