unit 6, lesson 1 - force

ForceUnit SIX, Lesson 6.1

By Margielene D. Judan

LESSON OUTLINE

Force

Kinds of Forces

Contact Forces (Ex. Friction)

Non-contact Forces

Gravity, Weight, Law of Universal Gravitation

Magnetic Force

Electrical Force

Magnetism and Electricity

Strong and Weak Nuclear Forces

Resultant Force

FORCE – a push or pull

What are some things that force can do?

1. Can change the shape & size of things by:

• Compressing

• Stretching

• Cutting

• Tearing

• Bending

• Sliding

• Twisting

2. Put motion into a stationary (not moving) ball by rolling it.

3. Catching and throwing can stop the motion or change the direction of the ball

SPRING SCALE – measures force

invented by Robert Hooke (he also discovered

cells)

Hooke hung a 1-pound weight and

discovered that the spring was stretched to

a certain distance. Hanging a 2-pound

weight on the spring caused it to stretch

twice as far.

Given the following details, what conclusion

can you make?

Notice that a 2-pound weight can cause a

stretch twice as the 1-pound weight. If a 3-

pound weight was hung, the stretch would

be thrice and so on.

Therefore, force is directly proportional to

the length of the string (given that they

have the same stiffness).

This is known as Hooke’s Law.

Unit of Force – NEWTON (N)

-named after Sir Isaac Newton

-defined as the force that gives a mass of 1 kg

an acceleration of 1 m/s2

So:

-A force of 10 N means a 10-kg object

accelerates at 1 m/s2.





-And so on…

Main Formula for Force:

F = ma

m = mass (usually in grams, g)

a = acceleration (usually in m/s2)

Deriving into F = ma, we have

m = F/a

a = F/m

Main Formula for Force:

F = ma

m = mass (usually in kilograms, kg)

a = acceleration (usually in m/s2)

1. A man of mass 50 kg runs at an acceleration

of 1.5m/s2. Find the force he’s exerting.

1. A man of mass 50 kg runs at an acceleration

of 1.5m/s2. Find the force he’s exerting.

F = ma

F = (50kg)(1.5m/s2)

F = 75 N

2. A force of 20 N acts upon a 5 kg block.

Calculate the acceleration of the object.

2. A force of 20 N acts upon a 5 kg block.

Calculate the acceleration of the object.

a = F/m

a = (20N)/(5 kg)

a = 4m/s2

3. An object has a mass of 1,500 g. If the acceleration

of the earth is 9.8m/s2, calculate the weight of the

object.



object.

Since mass is in grams, we need to convert it into

kilograms:

1,500 g = 1.5 kg (move the decimal 3 places to the left

or divide by 1000)



object.

Weight is a force.

F = ma

F = (1.5kg)(9.8m/s2)

F = 14.7 N

4. What is the force exerted by a stationary

object (not moving) if it has a mass of 500 kg?

4. What is the force exerted by a stationary

object (not moving) if it has a mass of 500 kg?

An stationary object exerts no force since it

doesn’t accelerate (a= 0m/s2)

F = ma

F = (500kg)(0m/s2)

F = 0 N

Kinds of Forces

1.Contact Forces

2.Noncontact Forces

It would be helpful to memorize the table

1. Contact Forces

• Result from direct contact between two surfaces or

objects (ex. Friction)

FRICTION

-force that opposes motion

Disadvantages of Friction-produces heat in various parts of machines (main

disadvantage; try it by rubbing your hands)

-we have to exert more power in machines (more

power, more energy wasted like fuel in cars)

-opposes motion

-because of friction, noise is produced in machines

Advantages of FrictionWithout friction, we are handicap

-enables us to walk (w/out friction, we could not walk,

nor move)

-enables us to stop motion (w/out friction, we would

keep moving forward forever)

-We cannot fix the nail in the wood w/out it. Friction

holds the nail.

-A horse cannot pull a cart

-Without friction, you cannot open a can

Without friction, there is no motion!

Without friction, there is no force!

Force and friction, though opposite, acts

together in harmony.

Friction coexists with force. Ironic, doesn’t it?

Real-life application:

-Good exists with evil

-Opposites attract

Ways to Reduce Friction:

-using lubricants

-using wheels or rollers

-streamlining by filling,

sandpapering, or

scrubbing (common in

swimming, skydiving, and vehicles)

That is why cars,

especially racecars,

are streamlined in

form.

This allows them to

have less drag with air,

allowing them to move faster.

poor streamline

good streamline

This also

explains the

shape of fishes

and the shape

of swimmers’

strokes.

Three Types of Friction:

1. static friction – force between two non-

moving (stationary) surfaces in contact

2. sliding friction – the friction between two

sliding objects

3. rolling friction – the friction experiences from

rolling objects (rollers)

2. Noncontact Forces

• No direct contact

• “Forces acting at a distance”

• They include

1.Gravity

2.Magnetic force

3.Electrical force

4.Nuclear Force

Laboratory 2.5:

Answer p. 189.

Instead of weighing the objects, the values are given below:

oStone – 6 kilograms

oBook – 1.2 kilograms

oBlock of wood – 700 grams

Short Quiz tomorrow: (10 items)

Problem solving (F = ma; m=F/a; a=F/m) – 3 items

Force – 2 items (1 item on Hooke’s law)

Friction – 5 items

Quiz (20 pts)

1. A man has a mass of 40 kg. If the acceleration due to gravity on earth is 9.8m/s2 and on moon is 1.6m/s2, calculate the force (weight) of the man in earth and moon. (3 pts) F = ma

*with solution

Quiz (20 pts)

2. When happy, mother opens the 15-kg door with a force of 10 N. When she is angry, however, she opens the door with a force of 30 N. Calculate the acceleration of the door when mother is happy and angry. (3 pts)

a = F/m

*with solution

Quiz (20 pts)

3. A truck is stationary (not moving). If the truck is 10,000 kg in mass, calculate the force it exerts forward. (2 pts)

F = ma

Quiz (20 pts)

4. If the weight of a 10-N bar stretches the string 15 cm, how far will a 20-N bar stretch the string?

5. The unit of force was named after him.

6. A force that opposes motion.

7. Give one advantage and one disadvantage of friction. (2 pts)

8. Enumerate the three types of friction (3 pts)

Quiz (20 pts)

9. Explain why track and field runners use spike shoes instead of rollers? (2 pts)

Quiz (20 pts)

10.Which position will enable you to swim the fastest? A B

C D

Answers:

1. A man has a mass of 40 kg. If the acceleration due to gravity on earth is 9.8m/s2 and on moon is 1.6m/s2, calculate the force (weight) of the man in earth and moon. (3 pts) F = ma

Earth: Moon:

F = ma F = ma

F = (40kg)(9.8m/s2) F = (40kg)(1.6m/s2)

F = 392.0 N F = 64.0 N

Answers:

2. When happy, mother opens the 15-kg door with a force of 10 N. When she is angry, however, she opens the door with a force of 30 N. Calculate the acceleration of the door when mother is happy and angry. (3 pts) a = F/m

Happy: Angry:

a = F/m a = F/m

a = (10N)/(15kg) a = (30N)/(15kg)

a = 0.67 m/s2 a = 2.0 m/s2

Answers:

3. A truck is stationary (not moving). If the truck is 10,000 kg in mass, calculate the force it exerts forward. (2 pts)

F = ma

F = (10,000kg)(0 m/s2)

F = 0 N (anything multiplied by zero is zero!)

Any object not moving doesn’t exert any force!

Answers:

4. If the weight of a 10-N bar stretches the string 15 cm, how far will a 20-N bar stretch the string?

10𝑁

15𝑐𝑚= 20𝑁

𝑥or 10N : 15cm = 20N : x

10x = 300 N

x = 30 N

Answers:

5. The unit of force was named after him.

-Isaac Newton

6. A force that opposes motion.

-Friction

Answers:

7. Give one advantage and one disadvantage of friction. (2 pts)

8. Enumerate the three types of friction (3 pts)

Advantages of FrictionWithout friction, we are handicap

-enables us to walk (w/out friction, we could not walk,

nor move)

-enables us to stop motion (w/out friction, we would

keep moving forward forever)

-We cannot fix the nail in the wood w/out it. Friction

holds the nail.

-A horse cannot pull a cart

-Without friction, you cannot open a can

Disadvantages of Friction-produces heat in various parts of machines (main

disadvantage; try it by rubbing your hands)

-we have to exert more power in machines (more

power, more energy wasted like fuel in cars)

-opposes motion

-because of friction, noise is produced in machines

Quiz (20 pts)

9. Explain why track and field runners use spike shoes instead of rollers? (2 pts)

-Spike shoes increase the friction between the shoes and the ground, enabling a better grip for the athlete to run faster and more efficiently.

Quiz (20 pts)

10.Which position will enable you to swim the fastest?

-B. Streamline

A B

C D

2. Noncontact Forces

• No direct contact

• “Forces acting at a distance”

• They include

1.Gravity

2.Magnetic force

3.Electrical force

4.Nuclear Force

Gravitational force or Gravity

• Force of attraction between two objects. This attraction causes objects to pull on other objects.

• Weakest force among the noncontact forces

• “What goes up, must come down.”

• The force that causes objects to fall.

Gravitational force or Gravity

Gravity has been a part of

our lives. Without gravity, we

would float forever. When we

jump, we will keep on flying.

Weight

• A force

• Measure of the pull of

gravity

Mass vs. Weight

• Mass and weight are not the

same.

• Mass is a measure of inertia

or the amount of matter an

object has.

• Weight is a force which

measures the pull of gravity.

Mass vs. Weight

• The unit of mass is kg, g, etc.

• The unit of weight is N

(Newton).

Mass vs. Weight

• Mass is constant.

• Weight is changing.

Suppose your mass is 40 kg. If you’re on the earth,

your weight will be different that that of the moon, but

your mass will still be 40 kg.

Weight Formula

• Recall that F = ma

• If weight is a force, then W = F or F = W.

• The acceleration here is due to gravity, so we use g

instead of a.

• Thus, the formula is:

W = mg

Weight Formula

W = mg

The value of g depends on the place. Here

on earth, g=9.8m/s2. On the moon,

g=1.6m/s2.

Applications of Weight

Elevator

Stationary - weight is the same

Upward - weight is greater

Downward - weight is less

Free fall - zero weight (weightless)


Weight is a downward force, represented

by a vector

Take note of this principle: For every

action, there is an opposite and equal

reaction. (Newton’s 3rd law of motion)


So when the elevator goes up, you will

exert a downward reaction plus your

normal weight

+ = (greater weight)


When the elevator goes down, you will

exert an upward reaction plus your

normal weight

+ = (lesser weight)


When the elevator falls with the gravity

(9.8m/s2), you will exert an upward reaction

equal to that (9.8m/s2) plus your normal

weight

+ = 0 (forces cancel out, zero)

Stationary Upward Free fall

Tip-to-Tail Method

We connect arrows using the tip to tail

method.

The arrow formed is called the resultant.

tiptail

tip

tail tail

tip

Resultant arrow

Tip-to-Tail Method

Arrow 1 Arrow 2 Arrow 1

Arrow 2

Resultant arrow

Tip-to-Tail Method

Arrow 1 Arrow 2

Law of Universal Gravitation (L.U.G.)

• Based on stories, Newton discovered this law

because of an apple that fell from his tree.


1.Any two bodies in the universe attract each other

-The attraction between the sun and the planets causes them to remain in orbit.

-Your attraction between the earth causes you to remain intact to the ground


1.Any two bodies in the

universe attract each other

-Every matter has mass, and

anything with mass has a

force of attraction (gravity).

Therefore, you and I exert a

force of attraction to each

other.




-The reason why we cannot

feel it is because the force of

gravity from the earth is so

strong. If you like to have a

strong gravity, you need to

have a large mass.




-But large objects such as

ships have enough

gravitational force to pull

another ship. That’s why

ships must not come near to

each other to avoid collision.


2.Force is directly proportional to the product of their

masses.

Directly proportional means that:

-if one becomes larger, the other one becomes larger

also

-if one becomes smaller, the other one becomes

smaller also


2.Force is directly proportional to the product of their

masses.

So when mass increases, force also increases.

When mass decreases, force also decreases.


3.Force is inversely proportional to the square of the

distance between them.

Inversely proportional means that:

-if one becomes larger, the other one becomes

smaller

-if one becomes smaller, the other one becomes

larger




So when the distance increases, the force of gravity

decreases.

When the distance decreases, the force of gravity

increases.




stronger gravity weaker gravity




Application: L.D.R.




Application: Long Distance Relationships (LDR)

What can you say about this? Can distance really

affect a relationship? Explain.


To sum up:

• Newton's law of universal gravitation states that

any two bodies in the universe attract each other

with a force that is directly proportional to the product

of their masses and inversely proportional to the

square of the distance between them.


Mathematically, LUG can be expressed as:

g

g


G is the gravitational constant. G = 6.7 X 10-11 N⋅m2/kg2

g

g

L.U.G. Problem Solving

• What is the gravitational force between two ships with masses 1000 kg

and 2000 kg separated by a distance of 200m?

Let:

m1 = 1,000 kg

m2 = 2,000 kg

r = 200 m

G = 6.7 X 10-11 N⋅m2/kg2

𝐹𝑔 = G𝑚1𝑚2

𝑟2

𝐹𝑔 = 6.67 × 10−11(1000)(2000)

2002

𝑭𝒈 = 𝟑. 𝟑𝟑𝟓 × 𝟏𝟎−𝟗 𝑵 (Newton)

Try this!

• What is the gravitational force between two objects with masses

300,000 kg and 100,000 kg separated by a distance of 200,000 m?

Let:

m1 = 300,000 kg

m2 = 100,000 kg

r = 200,000 m

G = 6.7 X 10-11 N⋅m2/kg2

Try this!

• What is the gravitational force between two objects with masses

300,000 kg and 100,000 kg separated by a distance of 200,000 m?

Let:

m1 = 300,000 kg

m2 = 100,000 kg

r = 200,000 m

G = 6.7 X 10-11 N⋅m2/kg2

𝐹𝑔 = G𝑚1𝑚2

𝑟2

𝐹𝑔 = 6.67 × 10−11(300000)(100000)

2000002

𝑭𝒈 = 𝟓. 𝟎𝟎𝟐𝟓 × 𝟏𝟎−𝟏𝟏 𝑵

Assignment: (1 whole sheet of paper)

Calculate the force of gravity between the sun and earth given the mass of the sun is 1.989 X 1030 kg, mass of earth is 5.972 X 1024 kg, and the distance between them is 1.496 X 1011 m. (10 pts)

Make a short reaction about L.U.G. Make applications of it and relate it to spiritual life. Minimum of 5 sentences. (10 pts)

Magnetic Force

• The force exerted by:

1.a magnet

2.a moving, electrically

charged particle. (electrons)

Magnetic Force

• Every magnet has two poles:

1. North

2. South

The north pole is usually

colored red.

Magnetic Force

• Two principles are applied in

magnets:

1. Unlike poles attract

(attraction)

2. Like poles repel

(repulsion)

Electrical Force

• The attractive or repulsive

interaction between any two

charged objects.

• One of the strongest forces

(next to strong nuclear force)

Electrical Force

• This force originates in the

atom’s particles:

1.Proton – positively charged

2.Electron – negatively

charged

3.Neutron – no charge

Electrical Force

• Like in magnetism, two

principles are applied:

1. Unlike charges attract

(attraction)

2. Like charges repel

(repulsion)

Electricity and Magnetism

• As you have noticed, both

magnetism and electricity

have the same principles.

• That’s because they are

related w/ each other. In fact,

they coexist w/ each other.

And that gave rise to the

topic, electromagnetism.


• The direction of a magnetic

field is north to south always.


• Since electricity and

magnetism coexist with each

other, they are preset at the

same time. We use the right-

hand rule to determine the

direction of forces.

Nuclear Forces

• The forces that act between two nucleons.

• Strongest force

• 10 million times stronger than chemical binding

• Have two types:

1. Strong

2. Weak

Weak Nuclear Force

• Weak nuclear force is

stronger than gravity but

weaker than electromagnetic

force.

• Force that governs certain

type of radioactive decay

• Little is known about it

Strong Nuclear Force

• Strong nuclear force is the

force that holds atomic nucleus

together.

• Often referred to as binding

energy or nuclear glue.

• 10x stronger than

electromagnetic force

• 1038 times stronger than gravity

Resultant Force

• The sum of all forces

• Resultant is the single force

that has the same effect as two

or more forces.

• Equilibrant is the force equal

to the resultant but in opposite

• Can be represented by arrows

Problem Solving

• In a tug-of-war, forces are acting in opposite directions. If the players

on one end exert a force of 100 N and the players on the other end

exert a force of 90 N, what is the resultant force?

100 N 90 N

Problem Solving

• In a tug-of-war, forces are acting in opposite directions. If the players

on one end exert a force of 100 N and the players on the other end

exert a force of 90 N, what is the resultant force?

100 N 90 N

100 N – 90 N

= 10 N

Problem Solving (Try!)

• What is the resultant force based on the figure? Who will win, the blue

team, or the red team?


• What is the resultant force based on the figure? Who will win, the blue

team, or the green team?

2500 – 2200 N = 300 N

Green team wins


• What is the resultant force based on the figure? Who will win, man A or

man B? What will happen to the box?

2000 N 2000 N


• What is the resultant force based on the figure? Who will win, man A or

man B? What will happen to the box?

2000 – 2000 N = 0 N

None of them wins.

The box will remain

stationary.2000 N 2000 N

Equilibrium

• In the previous problem, forces cancel out because they are

the same. Here, force A is the equilibrant of force B and

vice-versa. The condition where the forces cancel out and

are balanced is called equilibrium.

2000 N 2000 N

Sources:

Science Links 7

http://www.citycollegiate.com/force_motionXe.htm

http://www.bbc.co.uk/bitesize/ks3/science/energy_electricity_forces/forces/activity/

http://www.citycollegiate.com/force_motionXe.htm

http://www.bbc.co.uk/bitesize/ks3/science/energy_electricity_forces/forces/activity/