forces in two dimensions

Forces in Two Dimensions

Add vectors graphically and mathematically.

Use Newton’s laws to analyze motion when friction is involved.

Analyze force vectors in equilibrium.

Chapter

5

In this chapter you will:

Warmup

Consider a person who walks 100 m due north and then loses all sense of direction. Without knowing the direction, the person walks another 100 m. What could the magnitude of displacement be relative to the original starting point? Hint: Draw a picture using two vectors.

Section

5.1

Chapter 5: Forces in Two Dimensions

Section 5.1: Vectors

Section 5.2: Friction

Section 5.3: Force and Motion in Two Dimensions (what does two-dimensions mean?)

Chapter

5 Table of Contents

Homework for Chapter 5

Read Chapter 5

Study Guide 5, due before the Chapter Test

HW 5.A, HW 5.B: Handouts. You will need a ruler / protractor.

Vectors

Measure and report the magnitude of vectors.

Evaluate the sum of two or more vectors in two dimensions.

Determine the components of vectors.

In this section you will:

Section

5.1

VectorsSection

5.1

Vectors and Scalars

vector – A quantity with magnitude (size) and direction.

ex: displacement, velocity, acceleration, and force

Vectors are represented by arrows.

scalar - A quantity with magnitude only.

ex: distance, speed, mass, time, and volume

tip or headtail

The direction of a vector is where it points towards. You can specify the angle in degrees, as measured counter-clockwise from the x-axis.

Vectors

Vectors in Two Dimensions

Section

5.1

0°

90°

180°

270°

360°

θ = 225°

More generally, give the cardinal direction. Here, the vector’s direction is southwest, or SW.

NNE

E

SSE

NW

W

SW

Vectors

Vectors can be added by placing arrows tip to tail.

resultant - The arrow that extends fro the tail of the first vector to the tip of the last vector. It indicates both the magnitude and the direction of the vector sum.

Vectors may be added in any order (commutative operation).

A + B = B + A

Vectors may be shifted around, but as long as the keep the same size and direction, they are the same vector.

Adding Vectors

Section

5.1

Vectors

The figure below shows two force vectors on a free-body diagram.

Adding Vectors Graphically

Section

5.1

Shift vector b so a and b are tip to tail. →

vect

or b

vector a

vect

or b

vect

or b

vector a

Vectors a and b are originally drawn tail-to-tail. They can not be added this way.

Instead, shift vector b so it is tip-to-tail with vector a.

You can draw the resultant vector pointing from the tail of the first vector to the tip of the last vector and measure it to obtain its magnitude.

Vectors

Adding Vectors Graphically

Section

5.1

resu

ltant

vector b

vector a

Practice: Adding Vectors Graphically Worksheet.

Vectors

You may also use mathematical methods to determine the length or direction of resultant vectors.

If you are adding together two vectors at right angles, vector A pointing north and vector B pointing west, you could use the Pythagorean theorem to find the magnitude of the resultant, R.

Adding Vectors Mathematically

Section

5.1

R

B

A

Vectors

Finding the Magnitude of the Sum of Two Vectors

Find the magnitude of the sum of a 15-km displacement and a 25-km displacement when the angle between them is 90° and when the angle between them is 135°.

Section

5.1


Sketch the two displacement vectors, A and B, and the angle between them.

VectorsSection

5.1

AB

θ1


Identify the known and unknown variables.

VectorsSection

5.1

Known:

A = 25 km

B = 15 km

θ1 = 90°

θ2 = 135 °

Unknown:

R = ?

AB

θ1

R

Finding the Magnitude of the Sum of Two VectorsWhen the angle is 90°, use the Pythagorean theorem to find the magnitude of the resultant vector.

VectorsSection

5.1

Substitute A = 25 km, B = 15 km


When the angle does not equal 90°, use the graphical method to find the magnitude of the resultant vector:

1)Draw the vectors to scale. Pick a scale that will fit on your paper.

example: 15 km = 1.5 cm

2)Measure the resultant.

For our problem, the resultant measures 37 cm.

3)Convert back to the original units.

3.7 cm = 37 km

VectorsSection

5.1

Vectors

Are the units correct?

Each answer is a length measured in kilometers.

Do the signs make sense?

The sums are positive.


Section

5.1

Are the magnitudes realistic?

The magnitudes are in the same range as the two combined vectors, but longer. The answers make sense.

Finding the Sum of Two Vectors

VectorsSection

5.1

p. 121 Practice Problems:

1 & 2: Use the mathematical method (Pythagorean theorem and calculator) to find the magnitude of the displacement (length of the resultant).

Also, report the direction of displacement for #1. Possible answers are N, NE, E, SE, S, SW, W, NW.

3 & 4: Use the graphical method (ruler) to find the magnitude of the displacement.

Also, report the direction of displacement for #4.

Vectors

Because a vector has both magnitude and direction, you can separate it into horizontal (or x-) and vertical (or y-) components.

To do this, draw a rectangle with horizontal and vertical sides and a diagonal equal to the vector.

Draw arrow heads on one horizontal and one vertical side to make the original vector the resultant of the horizontal and vertical components.

Vector Components

Section

5.1

Fy

Fx

FLabel the vector and its x- and y- components.

vector resolution - Breaking a vector into its x- and y- components.

Practice with Vector Resolution

VectorsSection

5.1

Use a ruler to draw and label the x- and y- components of these vectors.

1. 2.

3.

4.

vd

a

F

HW 5.A Handout

Section Check

Question 1

Section

5.1

A. 3 + 4 m

B. 4 – 3 m

C. 32 + 42 m

D. 5 m

Jeff moved 3 m due north, and then 4 m due west to his friends house. What is the magnitude of Jeff’s displacement?

Section Check

Answer: D

Answer 1

Section

5.1

Reason: When two vectors are at right angles to each other as in this case, we can use the Pythagorean theorem of vector addition to find the magnitude of resultant, R.

Section Check

Answer: D

Answer 1

Section

5.1

Reason: Pythagorean theorem of vector addition states

If vector A is at right angle to vector B then the sum of squares of magnitudes is equal to square of magnitude of resultant vector.

That is, R2 = A2 + B2

R2 = (3 m)2 + (4 m)2 = 5 m

Question 2

Section

5.1 Section Check

Is the distance that you walk equal to the magnitude of your displacement? Give an example that supports your conclusion.

Answer 2

Not necessarily. For example, you could walk around the block (one km per side). Your displacement would be zero, but the distance that you walk would be 4 kilometers.

Section Check

Could a vector ever be shorter than one of its components? Equal in length to one of its components? Explain.

Question 3

Section

5.1

Answer 3

It could never be shorter than one of its components, but if it lies along either the x- or y- axis, then one of its components equals its length.

Section Check

The order in which vectors are added does not matter. Mathematicians say that vector addition is commutative. Which ordinary arithmetic operations are commutative?

Question 4

Section

5.1

Answer 4

Addition and multiplication are commutative. Subtraction and division are not.

Section Check

Which of the following actions is permissible when you graphically add one vector to another: moving the vector, rotating the vector, or changing the vector’s length?

Question 5

Section

5.1

Answer 5

Allowed: moving the vector without changing length or direction.

Section Check

In your own words, write a clear definition of the resultant of two or more vectors. Do not explain how to find it; explain what it represents.

Question 6

Section

5.1

Answer 6

The resultant is the vector sum of two or more vectors. It represents the quantity that results from adding the vectors.

Please Do Now

Write the three most important things you’ve learned and want to remember about adding vectors.

VectorsSection

5.1

Friction

Define the friction force.

Distinguish between static and kinetic friction.


Section

5.2

Friction

Push your hand across your desktop and feel the force called friction opposing the motion.

A frictional force acts when two surfaces touch.

When you push a book across the desk, it experiences a type of friction that acts on moving objects.

This force is known as kinetic friction, and it is exerted on one surface by another when the two surfaces rub against each other because one or both of them are moving.

Static and Kinetic Friction

Section

5.2

Friction

To understand the other kind of friction, imagine trying to push a heavy couch across the floor. You give it a push, but it does not move.

Because it does not move, Newton’s laws tell you that there must be a second horizontal force acting on the couch, one that opposes your force and is equal in size.

This force is static friction, which is the force exerted on one surface by another when there is no motion between the two surfaces.


Section

5.2

Friction

You might push harder and harder, as shown in the figure below, but if the couch still does not move, the force of friction must be getting larger.

This is because the static friction force acts in response to other forces.


Section

5.2

Friction

Finally, when you push hard enough, as shown in the figure below, the couch will begin to move.

Evidently, there is a limit to how large the static friction force can be. Once your force is greater than this maximum static friction, the couch begins moving and kinetic friction begins to act on it instead of static friction.


Section

5.2

Friction

Friction depends on the materials in contact. For example, there is more friction between skis and concrete than there is between skis and snow.

The normal force between the two objects also matters. The harder one object is pushed against the other, the greater the force of friction that results. Since the normal force depends on the mass of the object, greater mass means greater friction.

Surface area does NOT contribute to friction.

Give an example of low friction:

Give and example of high friction:


Section

5.2

If you pull a block along a surface at a constant velocity, according to Newton’s laws, the frictional force must be equal and opposite to the force with which you pull.

You can pull a block of known mass along a table at a constant velocity and use a spring scale, as shown in the figure, to measure the force that you exert.

You can then stack additional blocks on the block to increase the normal force and repeat the measurement.

Friction


Section

5.2

Plotting the data will yield a graph like the one shown here. There is a direct proportion between the kinetic friction force and the normal force.

The different lines correspond to dragging the block along different surfaces.

Note that the line corresponding to the sandpaper surface has a steeper slope than the line for the highly polished table.

Friction


Section

5.2

Friction

You would expect it to be much harder to pull the block along sandpaper than along a polished table, so the slope must be related to the magnitude of the resulting frictional force.

The slope of this line, designated μk, is called the coefficient of

kinetic friction between the two surfaces and relates the frictional force to the normal force, as shown below.


Section

5.2

The kinetic friction force is equal to the product of the coefficient of the kinetic friction and the normal force.

Kinetic friction force

Friction

The maximum static friction force is related to the normal force in a similar way as the kinetic friction force.

The static friction force acts in response to a force trying to cause a stationary object to start moving. If there is no such force acting on an object, the static friction force is zero.

If there is a force trying to cause motion, the static friction force will increase up to a maximum value before it is overcome and motion starts.


Section

5.2

Friction

The static friction force is less than or equal to the product of the coefficient of the static friction and the normal force.

In the equation for the maximum static friction force, μs is the

coefficient of static friction between the two surfaces, and μsFN is the maximum static friction force that must be overcome before motion can begin.


Section

5.2

Static Friction Force

Note that the equations for the kinetic and maximum static friction forces involve only the magnitudes of the forces.

The forces themselves, Ff and FN, are at right angles to each other. The table here shows coefficients of friction between various surfaces.

Although all the listed coefficients are less than 1.0, this does not mean that they must always be less than 1.0.

Static and Kinetic FrictionSection

5.2

p.129

Friction

Balanced Friction ForcesYou push a 25.0 kg wooden box across a wooden floor at a constant speed of 1.0 m/s. How much force do you exert on the box?

Section

5.2

Identify the forces and establish a coordinate system.

Draw a motion diagram indicating constant v and a = 0.

Balanced Friction ForcesSection

5.2

Draw the free-body diagram.

Balanced Friction Forces

Identify the known and unknown variables.

Section

5.2

Known:

m = 25.0 kg

v = 1.0 m/s

a = 0.0 m/s2

μk = 0.20

Unknown:

Fp = ?


The normal force is in the y-direction, and there is no acceleration.

Section

5.2

FN = Fg

= mg

Solve for the Unknown

Substitute m = 25.0 kg, g = 9.80 m/s2

FN = 25.0 kg(9.80 m/s2)

= 245 N


The pushing force is in the x-direction; v is constant, thus there is no acceleration.

Section

5.2

Fp = μkmg

Substitute μk = 0.20, m = 25.0 kg, g = 9.80 m/s2

Fp = (0.20)(25.0 kg)(9.80 m/s2)

= 49 N

Are the units correct?

Performing dimensional analysis on the units verifies that force is measured in kg·m/s2 or N.

Does the sign make sense?

The positive sign agrees with the sketch.

Is the magnitude realistic?

The force is reasonable for moving a 25.0 kg box.

Balanced Friction ForcesSection

5.2

Practice Problems p.128: 17-19.

Practice Problems p.130: 22,23.

Section

5.2 Friction

HW 5.B

Section Check

Define friction force.

Question 1

Section

5.2

Section Check

A force that opposes motion is called friction force. There are two types of friction force:

Answer 1

Section

5.2

1) Kinetic friction—exerted on one surface by another when the surfaces rub against each other because one or both of them are moving.

2) Static friction—exerted on one surface by another when there is no motion between the two surfaces.

Section Check

Juan tried to push a huge refrigerator from one corner of his home to another, but was unable to move it at all. When Jason accompanied him, they where able to move it a few centimeter before the refrigerator came to rest. Which force was opposing the motion of the refrigerator?

Question 2

Section

5.2

A. Static friction

B. Kinetic friction

C. Before the refrigerator moved, static friction opposed the motion. After the motion, kinetic friction opposed the motion.

D. Before the refrigerator moved, kinetic friction opposed the motion. After the motion, static friction opposed the motion.

Section Check

Answer: C

Answer 2

Section

5.2

Reason: Before the refrigerator started moving, the static friction, which acts when there is no motion between the two surfaces, was opposing the motion. But static friction has a limit. Once the force is greater than this maximum static friction, the refrigerator begins moving. Then, kinetic friction, the force acting between the surfaces in relative

motion, begins to act instead of static friction.

Section Check

On what does the magnitude of a friction force depend?

Question 3

Section

5.2

A. The material that the surface are made of

B. The surface area

C. Speed of the motion

D. The direction of the motion

Section Check

Answer: A

Answer 3

Section

5.2

Reason: The materials that the surfaces are made of play a role. For example, there is more friction between skis and concrete than there is between skis and snow.

Section Check

A player drags three blocks in a drag race, a 50-kg block, a 100-kg block, and a 120-kg block with the same velocity. Which of the following statement is true about the kinetic friction force acting in each case?

Question 4

Section

5.2

A. Kinetic friction force is greater while dragging 50-kg block.

B. Kinetic friction force is greater while dragging 100-kg block.

C. Kinetic friction force is greater while dragging 120-kg block.

D. Kinetic friction force is same in all the three cases.

Section Check

Answer: C

Answer 4

Section

5.2

Reason: Kinetic friction force is directly proportional to the normal force, and as the mass increases the normal force also increases. Hence, the kinetic friction force will hit its limit while dragging the maximum weight.

Force in Two Dimensions

Determine the force that produces equilibrium when multiple forces act on an object.

Learn the parallelogram method of vector addition.


Section

5.3

Equilibrants and the Parallelogram Method


Now you will use your skills in adding vectors to analyze situations in which the forces acting on an object are at angles other than 90°.

Recall that when the net force on an object is zero, the object is in equilibrium.

According to Newton’s laws, the object will not accelerate because there is no net force acting on it; an object in equilibrium is motionless or moves with constant velocity.

Equilibrium Revisited

Section

5.3

It is important to realize that equilibrium can occur no matter how many forces act on an object. As long as the resultant is zero, the net force is zero and the object is in equilibrium.

The figure here shows three forces exerted on a point object. What is the net force acting on the object?

Remember that vectors may be moved if you do not change their direction (angle) or length.



Section

5.3

The figure here shows the addition of the three forces, A, B, and C.

Note that the three vectors form a closed triangle.

There is no net force; thus, the sum is zero and the object is in equilibrium.



Section

5.3


Suppose that two forces are exerted on an object and the sum is not zero.

How could you find a third force that, when added to the other two, would add up to zero, and therefore cause the object to be in equilibrium?

To find this force, first find the sum of the two forces already being exerted on the object.

This single force that produces the same effect as the two individual forces added together, is called the resultant force.


Section

5.3


The force that you need to find is one with the same magnitude as the resultant force, but in the opposite direction.

A force that puts an object in equilibrium is called an equilibrant.


Section

5.3


The figure below illustrates the procedure for finding the equilibrant for two vectors.


Section

5.3

This general procedure works for any number of vectors.

Force in Two DimensionsExample: Forces of 6.0 N east and 8.0 N south are applied to the top of a pole. What is the equilibrant?

R = (6.0 N)2 + ( 8.0 N)2 = 10 N

Equilibrant = 10 N, NW

Section

5.3


The Parallelogram Method for Vector Addition

This is a graphical method that can only be used for adding two vectors.

1)Place the vectors tail-to-tail

2)The vectors are two sides of the parallelogram. Draw in the other two sides.

3)Draw the resultant from the tails to the tips.

Worksheet: The Parallelogram Method

Section

5.3

Section Check

If three forces A, B, and C are exerted on an object as shown in the following figure, what is the net force acting on the object? Is the object in equilibrium?

Question 1

Section

5.3

Section Check

We know that vectors can be moved if we do not change their direction and length.

The three vectors A, B, and C can be moved (rearranged) to form a closed triangle.

Since the three vectors form a closed triangle, there is no net force. Thus, the sum is zero and the object is in equilibrium. An object is in equilibrium when all the forces add up to zero.

Answer 1

Section

5.3

Section Check

What is the difference between a resultant and an equilibrant?

A resultant is the sum of two or more vectors. An equilibrant is the same magnitude as the resultant, but opposite in direction.

Question 2

Section

5.3

Section Check

Compare and contrast the tip-to-tail method for vector addition with the parallelogram method.

The tip-to-tail method can be used for adding any number of vectors. The parallelogram method can only be used to add two vectors.

In the tip-to-tail method, vectors are lined up head-to-tail. In the parallelogram method, vectors are lined up tail-to-tail.

They are both graphical methods.

Question 3

Section

5.3

Chapter 5 Review5Chapte

r

Graphical Method of Vector Addition

• add vectors tip-to-tail; the sum of the vectors is the resultant

• the resultant is from the tail of the first to the tip of the last

• vector addition is commutative

• vectors have magnitude and direction (ex: force, velocity,

displacement)

• scalars only have magnitude (ex: mass, speed, total distance)

• displacement ≠ total distance


r

Mathematical Method for Adding Vectors (use when you have a right triangle)

magnitude: R = √ A2 + B2

direction: Give the general cardinal direction ( north, west, southwest, etc.)

For vectors, you must specify both magnitude and direction.

Finding the Component of Vectors

• The x- component is the part of the vector parallel to the x-axis.• The y- component is the part of the vector parallel to the y-axis.• is labeled so the x- component is adjacent and the y-component is opposite.


r

Friction

Ff = μ FN = μ mg

• Friction only depends on μ and m (not surface area).• Ff static > Ff kinetic

Physics Chapter 5 Test Information5Chapte

r

The test is worth 50 points total.

True/False: 6 questions, 1 point each.Multiple Choice: 7 questions, 1 point each.Matching: 7 vocabulary, 1 point each.Problems: 5 questions for a total of 30 points.

Know:- graphical and mathematical methods for adding vectors- the Pythagorean Theorem- x- and y- components of vectors- how to work friction problems

forces in two dimensions

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