Welcome to Physical Science w/ Mrs. Brown!

Unit 4-Force & Motion

Lesson 1-Force

What Is a Force?

Force and motion are closely linked. A force is a push or a pull. There are different forces that act on objects, and

most, but not all, forces will change the position of an object.

Some forces do not change an object’s motion. That’s because other forces may be acting on the object at the same time.

Some of the forces acting on the object may cancel each other out

Magnitude and Direction A force has:

Magnitude, or size-measured in an SI unit called a newton (N). 

Direction In the United States, scientists commonly use pound to

describe the unit of force. This unit can be converted to newtons with this equation: 1 pound = 4.45 newtons.

A force also has direction. Described using the words up, down, forward, backward, or

using the points on a compass for north, south, east, and west.

Force Diagrams

Forces

Net force-the sum of all forces acting on an object. When the net force on an object is zero, the object’s motion does not change. So, an object at rest will only start to move if the net force acting on it is not zero. Only a net force different from zero changes the motion of an object.

Balanced Forces- the net up/down force is equal to zero

Unbalanced forces-those with a net force that is not equal to zero.

Lesson 2-Gravitational Force

https://www.khanacademy.org/science/physics/newton-gravitation/gravity-newtonian/v/introduction-to-gravity

What Is Gravity?

Earth’s gravitational pull has constant influence, preventing us and everything on earth from floating off into space.

The earth is not the only object that exerts gravitational force. 

Gravity is a force of attraction that is universal. Every mass exerts a pull on every other mass.

Mass Versus Weight The words mass and weight are sometimes

used to mean the same thing, but they are very different.

The mass of an object is how much matter is in the object. An object’s mass on earth is the same as it would be anywhere else in the universe.

Weight, however, is a measure of gravitational force, and an object’s weight can change depending on where the object is in the universe.

Distance and Gravity

The effect of earth’s gravitational force on us is more noticeable than our effect on the earth because earth is so massive.

But the sun is even more massive than earth. Although the sun is more massive than earth, it is much

farther away. Gravitational force between objects decreases as objects

move farther apart. Gravitational force is stronger between objects that are

closer together.

The Law of Universal Gravitation

Newton greatly expanded our understanding of gravity and its importance in the universe. He defined the law of universal gravitation with these main points:1. Every object in the universe exerts a gravitational force on every other object.2. The size of the gravitational force depends on the masses of the objects.3. The size of the gravitational force depends on the distance between objectshttps://www.khanacademy.org/science/physics/newton-gravitation/gravity-newtonian/v/introduction-to-newton-s-law-of-gravitation

Lesson 3-Motion

Let’s see how science measures and describes the relationships between motion, speed, and distance.

Different Kinds of Motion

3 of the most common types of motion are: translational motion rotational motion vibrational motion

Describing Position

When we talk about a moving object, we talk about where it started and where it is going. Therefore, in order to describe the motion of an object, we must first identify its position. Often, the easiest way to describe an object's position is to compare its position to the position of other objects.

Distance vs. Displacement

Distance is how far an object travels, it does not take the direction of motion into account

Displacement describes how far and in what direction an object has moved relative to its starting point.

Calculating Displacement

You can use this mathematical formula to calculate displacement: 

d2 – d1 =Δd = displacementΔd is read “delta d” or “the change in d.”

Remember, displacement indicates distance traveled in a specific direction.

Calculating Changes in Time

Translational motion involves a change in position and also a change in time.

Just like distance, time also needs a reference point, a coordinate system, and a system of units, such as seconds. t2 – t1 = Δt = change in time

Lesson 5-Speed & Velocity

 In this lesson, you’ll find out exactly what these two words mean, and how to calculate the speed and

velocity of any moving object.

Speed So we have good idea of

what speed means: Some things move fast-high rate of speed Some things move slowly-low rate of speed.

Average Speed-distance an object travels divided by the time it takes to travel that distance. Speed takes into account both the distance

traveled and the time it takes to get there

Calculating Average Speed

Average speed is represented by the formula: s = d / tIn this formula,s represents average speedd represents total distancet represents total timeSo, if you walk 12 miles in 3 hours, d = 12 miles and t = 3 hours. Your speed is:s = d / t  s = 12 miles/ 3 hourss = 4 mph (miles per hour)

Velocity & Calculating Average Velocity

Velocity-rate that an object moves in a certain direction.  Velocity is always represented as a direction in relation to a

reference point (50 km/hour east) Average velocity is represented by the formula: v = Δx /

Δt V-represents average velocity

Δx-represents the object’s change in position, or     displacement Δt-represents change in time

To calculate an object’s average velocity, you need to know where the object started, the point where it ended, and how long it took to get there.

Lesson 6-Measuring Speed & Velocity

In this lesson, we will use the formulas for speed and velocity to learn more about the movement of

everyday objects.

Speed & Velocity Formulas

To measure the average speed of an object, you need to know the total distance it has traveled over a period of time.

To measure the average velocity of an object, you need to know its displacement over a period of time.

Formulas: s = d / t

average speed = total distance / total time Total distance does not take into account direction.

v = Δx / Δtaverage velocity = displacement / change in time Displacement is the change in position of an object in relation to a

reference point, so it always includes a direction

Velocity Includes Direction

The measurement of velocity always includes direction of motion, such as north, south, east, or west. If the movement of an object is in one dimension

(such as along a straight line), velocity can be described as either positive (+) or negative (–) in relation to a reference point.

Speed does not include direction. It always has a positive value. No matter which direction it rolls, the marble’s speed,

however, is always positive

Rearranging the Formula for Velocity

You can rearrange the formula for average velocity to solve for displacement or change in time.If you know an object’s average velocity over a period of time, you can calculate how far it moved using the following formula:

Δx = v x  tdisplacement = average velocity x change in time

If you know an object’s average velocity and displacement, you can solve for change in time using the following formula:

Δt = Δx / vchange in time = displacement / average velocity

Lesson 8: Acceleration

Let’s look at how acceleration is defined, how it’s measured, and how it relates to our understanding of

motion.

Acceleration Acceleration-how quickly velocity changes;

acceleration is occurring when objects speed up, slow down, or change direction

Acceleration is the rate of change of velocity. We say that an object is accelerating if its velocity

changes divided by timeVelocity:

involves both speed and direction. changes when speed changes. changes when direction changes, even if speed remains

constant.

Acceleration and Gravity

If you drop an object, it will fall to the ground due to gravity.

As it falls, it moves faster and faster. Acceleration due to gravity=9.8

meters/per second/per second (m/s/s) or 9.8 m/s2. 9.8 m/s2 (approximately 22 mph) faster every

second it falls.

Calculating Acceleration

The average acceleration of a moving object can be calculated using the formula a = Δv / Δt.     a represents average acceleration

   Δv represents change in velocity   Δt represents change in time or elapsed time        (the time the change in velocity takes)

Change in velocity can be calculated by subtracting the initial velocity (vi) of a moving object from its final velocity (vf).

The formula for the change in velocity is: Δv = (velocityf - velocityi)

Positive Acceleration & Deceleration

Imagine a car pulling away from a stop sign-the car’s change in velocity (velocityf – velocityi) and its acceleration both have positive values. When an object moves in one dimension (along a straight line) in the positive direction, and its velocity increases over time (it speeds up), its acceleration is positive.

Now imagine that same car slowing down as it approaches a stop sign. Its final velocity is less than its initial velocity. Therefore, if the velocity was in the positive direction, it became less positive. Its change in velocity and its acceleration have negative values.

Deceleration is a decrease in velocity over time. When deceleration occurs in motion along a line, the acceleration and velocity point in opposite directions. That means if the velocity is positive, deceleration corresponds to negative acceleration

Lesson 9: Newton's First Law of Motion

 Newton’s three Laws of Motion describe our countless interactions with forces and motion every day. In this lesson, you will explore Newton’s First Law of Motion.

Newton’s First Law of Motion In the 1600’s, Sir Isaac Newton described his

three laws of motion.Newton’s First Law of Motion:

A body at rest will remain at rest unless acted on by an external, unbalanced force. (Part 1) A body in motion will remain in motion unless acted on by an external, unbalanced force. (Part 2)

What does this mean? Let’s look at both parts:

Objects at Rest and in Motion& Inertia

Part 1: You must apply a force to get an object moving.

All objects have inertia: the mass of an object is a measure of its inertia. The bigger an object is (the more mass it has),

the more force it takes to move it.

Inertial Forces

Inertia Inertia is involved in most of the activities that you do. When you are riding in something and it starts moving, you

feel like you are being pushed backward. This is because your body tends to remain still relative to the ground, while object you riding in is moving underneath you.

A similar thing occurs when you come to a sudden stop in the same object. This time your body continues to move forward while the object under you stops. So, you feel as though you are being pushed forward. These forces you feel are not real forces; they are called inertial forces. There is no actual force pushing you backward when the object starts moving, or pushing you forward when the object suddenly stops.

Friction

Part 2: You must apply a force to get an object to stop moving once it is going.Friction: the force that opposes motion,

surface to surface force, or surface to environment (like air).

Slippery ice has much less friction than the ground.

Lesson 10: Mass and Weight

 In this lesson, you will explore how scientists define mass and weight. You will learn what mass and weight mean, how they are measured, and how they can be

calculated.

Mass and Weight

Mass and weight are different:Mass is the amount of matter an object

hasMeasured in grams (g)Mass of an object is the same no matter where in the

universe you are.

Weight is the amount of gravitational force on an object

Mass and Inertia

The mass of an object is a measure of the object’s inertia.

The greater the object’s mass, the greater its inertia. So, a more massive object is harder to move

from rest, or to change the motion of, than a less massive object.

Exploring Mass

A balance compares the mass of the object being measured with known masses by comparing the forces from gravity that each produces.

The mass of an object is the same no matter where in the universe it is measured. This is because the amount of matter in an object does not change by just moving it.

What mass vs. weight looks like…

Exploring Weight Will your weight differ on earth and on the

moon? Yes. The gravitational force acting on you is not

the same on earth as it is on the moon. That’s why your weight of the is different.

Weight, unlike mass, will be different for the same object in two places where gravity differs. An object can even have a slightly different weight

at different locations on earth. This is because the earth’s gravitational pull varies a little

Weight Is a Force Since weight is the pull of gravity on an object, it is a force

and mass is simply a quantity of matter and is not a force. Scientists define weight using the following formula: 

Weight = mass x acceleration due to gravity W = m x g

In this formula:w represents weightm represents massg represents acceleration due to gravity (g = 9.8 m/s/s (meters per second squared) or 9.8 m/s2

Measured in Newtons (N) Weight changes depending on gravitational pull

Lesson 11: Newton's Second Law of Motion

 You will learn what Newton described about the motion of an object when an unbalanced force does act on it. You will explore how force, acceleration, and mass are

related.

Newton's Second Law of Motion

When an unbalanced force actsacts on an object, the object will be accelerated. The acceleration will be proportional to the force, and will be in the same direction as the force.

Acceleration Depends on Force and Mass

Let’s explore Newton’s Second Law of Motion on and object:Increasing the force of a push on something increases its acceleration Decreasing the force would decrease the accelerationThe same force acting on a smaller mass produces a larger acceleration. Similarly, a larger mass would be accelerated less.

Expressing Force as a Formula Newton developed a mathematical formula to describe

the relationship between force, mass, and acceleration. The formula is: F = ma. 

F = unbalanced force (N)m = mass (kg)a = acceleration (m/s2)

The on-screen display shows how to determine the amount of force needed to accelerate a 2-kg mass at 3 m/s2. Take a few minutes to study how the problem is solved. Remember that the units of a newton are equivalent to kg m/s2.

***Note that in this lesson, only average force and acceleration examined. It is very difficult to examine forces and accelerations that occur in an instant in time

Lesson 12: Newton's Third Law of Motion

What happens when one object exerts a force on another object? How does the motion of each object change? 

To find out we will explore Newton’s 3rd Law!

Let’s remember some of the important concepts so far in this

unit:  A force is a push or a pull. An unbalanced force changes an object’s motion. Objects have inertia and resist forces that try to

change their motion. Friction is the force between two surfaces that

opposes motion. Unbalanced forces cause objects to accelerate

according to the equation: F = ma

Forces Occur in Pairs

Imagine a Spring. The reason it behaves as it does is because forces

always occur in pairs. Whenever you are pushing on a spring, the spring is

pushing back on you. Likewise, if you are pushing on a wall, the wall is pushing

back on you! Although you can’t see this the way you can see the spring being compressed, it’s true. Newton described force pairs in his Third Law of

Motion

Newton’s Third Law of Motion:

For every action, there is an equal but opposite reaction

A force is a push or a pull. A push or a pull involves more than one object. One object does the pushing (or pulling), while the other object gets pushed (or pulled) In other words…If one object exerts a force on a second

object, the second object exerts a force on the first object that is equal in magnitude and opposite in direction.

Some action pairs are hard to notice Sitting on a chair Pushing against a wall

Computing Acceleration &Actions and Reactions

Acceleration Formula: a = F/m

Where: a-acceleration, F-force & m-mass.**Another way of stating Newton’s Third Law of Motion is that for every action, there is an equal and opposite reaction. When two objects interact, the force exerted by the first object is called the action force. The force exerted by the second object is called the reaction force.

Lesson 13: Buoyant Forces

Have you ever wondered why a rubber duck can float in your bathtub, but a rock cannot?

 In this lesson, you will explore the force exerted by fluids. You will learn why some objects float, while others sink.

Buoyant Force Buoyant force is the force exerted on an

object which is immersed in a fluid. A fluid is any substance that flows:

Gases and liquids are fluids

Buoyant Force and Weight

In order for an object to float in a fluid, its weight (which is a force) must be

less than the buoyant force exerted by the fluid.

K12 example: a rubber duck and a rock in a swimming pool

The weight of the rubber duck is less than the buoyant force of the water, so the duck floats.

The weight of the rock is greater than the buoyant force of the water, so the rock sinks.

Fluid Pressure

Fluid Pressure Facts: (K12 example: Iceberg) When an object is placed in a fluid, pressure is exerted

on all its sides. This pressure is called fluid pressure. Pressure measures how concentrated a force is on a

particular area. The fluid pressure is the force that the fluid exerts on a

surface divided by the area of the surface. The pressure of all fluids increases as depth increases.

Archimedes’ PrincipleHow can we determine the amount of buoyant force acting

on an object?

In the third century BC, a Greek mathematician named Archimedes determined a way to measure buoyant force, known as Archimedes’ principle.

According to this principle, when an object is placed in a fluid, the buoyant force acting on it is equal to the weight of fluid that the object displaces. K12 example: rock in cylinder

Measuring Buoyant Force First, an object is lowered into a container

of water. As the object moves further down into the

container, water is displaced and flows into a catch bucket.

Finally, when the object is resting on the bottom of the container, you can see the total amount of water that has been displaced by the object.

Measuring Buoyant Force

Density=mass divided by volume or:D = m / V

Why do large ships float? Large volume in the open hull makes the ship less dense than water.

Air Exerts Buoyant Force

Air Force Facts:Air is a fluid that exerts buoyant force. However, since the density of air is so low, few substances float in it. **A notable exception to this is helium. Air is about seven times denser than helium. So, party and parade balloons filled with helium float in the air. The buoyant force exerted by the air drives them upward

We will move on to Unit 10

Next!!!