FORCE AND MOTIONChapter 8

FORCE AND MOTION

Motion is such an everyday, ordinary thing that anywhere you look, you will see objects in motion. You look out a window, you see the leaves or branches of trees moving in the wind. Cars and buses move along the streets. Airplanes cross the skies. You see people and animals move about everywhere. Water flows in rivers and ocean waves rush to the shore. Even the planets move around the sun through space.

All these objects may move in various ways. This chapter will introduce you to different kinds of simple motion. It will explain why things move the way they do using Newton’s laws of motion. You will understand certain factors which affect speed, velocity and acceleration of moving objects.

DESCRIBING MOTIONLesson 1

DESCRIBING MOTION

When you drop a stone, it moves in a quick downward motion. A leaf falling from a tree on a windy day may show motion in another way. A bird flying in the sky moves differently as well. Not one of these motions is exactly like any other motion described. When do you say an object is moving or not moving?

To know if an object is moving or not, there should be a frame of reference. A frame or point of reference is a place or position from which an object’s motion may be observed and described. If a ball falls from a table, the ball is moving if you used the table as your point of reference. On the other hand, the table has moved if you use the ball as your frame of reference.

Motion is described in reference to the position from which the observer is viewing the motion. Thus, whether an object is moving or not moving depends upon the frame of reference from which you are observing the object. Have you ever experienced riding in a car or bus and felt the vehicle to be moving when it is not? What causes you to feel the movement?

REMEMBERING SCIENCE FACTS

Motion is a change in position.A frame is a position from which an object’s motion may be observed and

described.

Observe objects around you in the room. Choose your point of reference then tell whether the objects are moving or not.

SPEED AND VELOCITYLesson 2

SPEED AND VELOCITY

When you ride a car or bus, you see an instrument on the dashboard at the front side of the vehicle. This is called a speedometer. It measures the speed of the vehicle. It has a pointer which moves up when the vehicles goes fast and goes down when the vehicle slows down. Speed is indicated in terms of kilometers per hour or kph in the speedometer. What does speed mean? Is speed the same as velocity? How do you measure the speed or velocity of a moving object?

Maybe an hour more if we maintain the same speed and velocity!

How many hours before we reach the beach?

Motion is described as a change in position. The simplest kind of motion is motion in a straight line and at a constant rate. Speed describes the rate of motion of an object. It is measured with a speedometer. Speed and velocity are both used to describe motion. But, velocity is the rate of motion or speed in a certain direction. So, velocity is a more complete description of motion.

If the speedometer of a car indicates 70kph, has the car moved for an hour or has the car traveled 70 kilometers? Speed is indicated on the speedometer but neither the time nor the distance traveled is indicted. When the terms speed and velocity are used, the total amount of time and the distance traveled are not specified. However, the relationship between distance, speed and time are shown below. If any of the three factors are known, you can calculate the third factor.

DISTANCE TRAVELED = SPEED X TIME

Example 1:

A car travels at a constant speed of twenty meters per second. How far will the car travel in ten seconds? In forty seconds?

Solution:

Distance= 20 meters x 10 seconds

= 200 meters

TIME= DISTANCE TRAVELED SPEED

Example 2:

How long would it take the car to travel two hundred meters if it was moving at a speed of twenty meters per second?

Solution:

Time= 200 meters

20 meters

= 10 seconds

SPEED= DISTANCE TRAVELEDTIME

Example 3:

If a car travels at 200 meters in ten seconds, what is the speed of the train?

Solution:

Speed= 200 meters

10 seconds

= 20 m/sec

But you know it is sometimes difficult to travel from one place to another at a constant speed. Sometimes, you need to stop or rest. Your speed may also change as you go uphill or downhill. You may start out moving rapidly; slow down then, speed up again. Speed is usually not constant.

An increase in speed or velocity is called acceleration.

A decrease in speed or velocity is called deceleration.

What is the direction of motion in the activity you did? No matter which route you take, at the end of your trip, your change in position or displacement will be the same. Displacement can be described by telling the distance traveled and the direction of travel. Displacement can be shown by an arrow called a vector. A vector may be used to show the magnitude of velocity which is speed and its direction.

REMEMBERING SCIENCE FACTS

• Speed is the rate of motion or the rate of change in position.

• Velocity is the rate of motion or speed in a certain direction.

• Acceleration is an increase in speed or velocity.

• Deceleration is a decrease in speed or velocity.

REACHING OUT

A. Using a frame of reference, describe the star’s movement.

B. Two buses, each traveling ninety kilometers per hour, pass each other. One train is westbound while the other is eastbound. Do the buses have the same speed? Why?

C. Do the buses have the same velocity? Why?

ACCELERATION AND VELOCITYLesson 3

ACCELERATION AND VELOCITY

When you ride a car or any vehicle, you experience acceleration and deceleration. When the driver steps on the gas pedal, the car accelerates or increases in speed. When the driver steps on the brake pedal, the car decelerates or decreased its speed and even puts into a stop. Have you ridden a roller coaster? When does the roller coaster accelerate? Does it decelerate when it is moving downward? What happens when it is moving upward? When does it move the fastest?

What is acceleration? What can cause an object to accelerate? Is acceleration affected by the mass of an object?

The motion of a falling object seems to take place by itself. But, scientists believe that changes in the motion of an object can be explained as a result of forces acting on the object. Forces must be unbalanced in order to cause a change in motion or its velocity. Falling objects must therefore be affected by some force. What causes objects to fall and accelerate?

The law of gravitation developed by Sir Isaac Newton states that every particle and object in the universe attracts every other particle and object. This force of attraction between particles and objects is called gravitational attraction. The amount of gravitational attraction depends on how far the objects are and how much mass the objects have. It is greater when the objects are close together and have greater mass.

What happens when objects of different weights are dropped from the same height at the same time? Of course, you would assume that the heavier object would hit the ground first. But, Galileo’s famous experiment proved it otherwise – both objects struck the ground at the same time. Why?

Provided air resistance is not considered, gravity causes objects to constantly accelerate at 9.8 meters per second for each second of fall. It is the same no matter what the mass of the object may be.

But air resistance balances the gravitational attraction of the earth on any falling object. The faster an object falls, the greater the air resistance. Air resistance is greater as the speed is increased. Have you ever held your hand out of a car traveling at about thirty kilometers per hour? Do you feel your hand being pushed in the opposite direction from the direction at which the car is traveling? As the car increases its speed, it would be difficult for you to keep your hand in one direction. Air resistance makes it so. It also makes objects such as feathers, ping pong balls and sheets of paper fall more slowly from high places than objects such as marbles, ball bearings and baseballs.

REMEMBERING SCIENCE FACTS

Acceleration is a change in velocity. It is caused by a force such as gravity.

The amount of gravitational attraction depends on how far the objects are and how much mass the objects have.

The faster an object falls, the greater the air resistance. Air resistance becomes greater as speed is increased.

REACHING OUT• A. Crumple a sheet of paper. From the height of a meter, drop the

crumpled paper at the same time as a fresh paper. Which paper hits the floor first? Why?

• B. At what point is a skydiver falling fastest? Falling slowly? Why?

WHY OBJECTS MOVELesson 4

WHY OBJECTS MOVE

Can an object move by itself? Why do objects move? It is a very simple yet seemingly foolish question. But, motion is a very important part of your life. Without motion, no work would have been done. Suppose you want to move a pencil lying on the table. Name some ways by which you could move it. Which of these ways would you have done?

You can move a pencil in a number of ways. But you need a force which can either be a push or pull to move it. This force must be greater than the resistance to motion which exists between the pencil and the table. The force applied should be greater than the force holding it back.

In order to move an object, a force must be applied to the object. This force must be greater than the forces holding it back. The forces acting on the object must be unbalanced. Unbalanced forces cause the object to move or change its motion. The object moves in the direction in which the greater force is acting. For example, if you hold a magnet over a nail on a table top, what happens to the nail? Yes, the nail “jumps” to the magnet. Gravity tends to hold the nail to the tabletop but magnetic force is greater. So, the nail moves upward.

But what about moving objects? Do unbalanced forces affect them also? What forces are there on an object which is moving at a constant speed? How are moving objects accelerated or speeded up and stopped?

Sir Isaac Newton formulated the three laws of motion.

1. AN OBJECT IN MOTION TENDS TO STAY IN MOTION IN A STRAIGHT LINE; AND AN OBJECT AT REST TENDS TO STAY AT REST, UNLESS THE FORCES

ACTING ON THE OBJECT BECOME UNBALANCED.

The first law deals with inertia. Inertia is the tendency of an object to resist any change in its condition of motion. Suppose you are riding in a car and it stops suddenly. Your body keeps moving forward until the force of the windshield or dashboard stops you if you are seated in the front seat. This could be painful and may even be fatal. That is why you should always fasten your seatbelt when you are riding in a car.

What will happen if the car accelerates quickly?

2. FORCE IS EQUAL TO THE PRODUCT OF THE MASS TIMES THE ACCELERATION

When you shop at the supermarket, you use a cart to carry the groceries. When is it easier to push the cart- when it is empty or when it is loaded with groceries? You need more force to accelerate a cart full of groceries because it has more mass. A greater force causes a greater acceleration or deceleration. F=ma (Force is equal to mass times acceleration). If the force is small and the object is large, there is only little acceleration. If the force is large and the mass of the object is small, the acceleration will be fast or rapid. Furthermore, if the object is not moving and the forces are unbalanced, the object will move in the direction of the greater force. If the object is moving and the forces become unbalanced, the speed of the object will be changed. That is, if the unbalanced force is in the direction of motion, the object will accelerate. If it is in the opposite direction, the object will decelerate. This explains why friction slows down a moving object.

FORCE = MASS X ACCELERATION

3. FOR EVERY ACTION, THERE IS AN EQUAL AND OPPOSITE REACTION

When one object exerts a force on another object, the second object also exerts a force equal to the first force as the first object in an opposite direction.

Examples:• When two people run into each other, who is pushed away? Do both people

move away from each other?

• When a man fires a shotgun, the action forces the bullet out. He feels the shotgun’s kick as it pushes against his shoulder.

• Gases explode from a rocket’s engine- this is called the action. The reaction is that the rocket rises from the ground and travels into space.

• When you blow up a balloon and release it, the balloon flutters around until all air is used up.

REMEMBERING SCIENCE FACTS

• Forces acting on the object must be unbalanced to cause or change motion in any way.

• Any change in the velocity or direction of an object is called acceleration. The amount of acceleration depends on the strength of the applied force. The change in direction of a moving object depends on the applied force.

IDENTIFY THE WORD BEING DESCRIBED

1. Any change in the speed or direction of an object.

2. The rate of motion of an object in a linear motion.

3. An instrument used to measure speed.

4. The tendency of an object to resist any change in its condition of motion.

5. Forces acting on an object which may cause it to move or change its motion in any way.

6. The famous scientist who formulated the laws of motion.

7. The force of attraction between all the particles and objects in the universe.

8. It balances the gravitational attraction of the earth on any falling object.

9. A decrease in the speed or velocity of a moving object.

10. The scientist who proved that gravity causes falling objects to accelerate constantly no matter what the mass of the object may be.

CIRCULAR MOTIONLesson 5

CIRCULAR MOTION

You have learned that there are generally two kinds of motion. These are linear motion (up and down and side to side) and circular motion.

Many objects ordinarily move in straight lines or in linear motion. The movement of objects in circular motion is a special kind of motion. How is circular motion done? What forces make circular motion?

Circular motion is caused by exerting a force on a moving object. However, the force must be exerted at right angles to the direction of motion. When you hold the wire of a toy airplane on a string and keep the plane in a circular path, you supply the force. As the plane circles around you, you also turn so that you will not be encircled by the wire. The force is directed inward toward the center of the circular path. This force is called centripetal force. Centripetal means “seeking the center.”

However, as you hold the wire, the plane seems to tug outward on you. A force pulls on you, not the plane. This is called centrifugal force. Centrifugal force is a reaction force to the centripetal force.

The circular motion of the object is really caused by the centripetal force which you exert in holding the wire. If you were to let go of the wire, the plane would tend to follow a straight or curvy path but not circular anymore.

The centripetal force pulls the object toward the center.

The centrifugal force pulls the object away from the circle.

REMEMBERING SCIENCE FACTS

• Centripetal force is directed inward toward the center of a circular path.

• Centrifugal force is directed outward or away from the center of a circular path.

REACHING OUT

A. Play “tug of war” with your classmates. Have an equal number of players on each team. Pull the rope with the same amount of force in each direction. What happens to the position of the rope? Is it possible to have balanced forces acting on a moving object? Why or why not?

B. Have you seen a tornado? What kind of force does it have which keeps the wind funnel-shaped?

C. Have you seen a car race? Why are the curves on a car racetrack slanted upward?

PROJECTILE MOTIONLesson 6

PROJECTILE MOTIONHave you played basketball or baseball?

What happens when the ball is hit with a bat by a player? How does the basketball move on its way to the net? Does it travel in a straight line? Does it travel in a circular path? What is this special kind of motion called?

A thrown basketball, baseball or darts is an example of a projectile. Other examples are footballs thrown or kicked into the air, a stone thrown out into the water and a snowball thrown at somebody. A projectile is any object that can be thrown, hurled or shot. Spears, knives, darts, bullets and artillery shells may also be projectiles.

During the medieval wars, scientists became interested and studied about projectile motion. They wanted to improve the accuracy of cannons which they used to fight their enemies. They discovered that it is possible to predict the position if you know the velocity of the object, the elevation of the final position. The projectile accelerates downward at a rate of 9.8 meters per second for each second of fall. The projectile also curves on its downward path.

A long range projectile such as a rocket ship or satellite shot into the air at high speed is very complex. The motion of the earth has to be taken into account. Why doesn’t the satellite fall to the earth? The satellite really falls toward the earth. But since the satellite moves forward very fast, it remains in orbit. Thus, the motion of a satellite is like the path of a projectile except that its velocity is much greater.

FOR YOU TO REMEMBER

• Motion is a change in position.

• A frame is a position from which an object’s motion may be observed and described.

• Speed is the rate of motion or the rate of change in position.

• Velocity is the rate of motion or speed in a certain direction.

• Acceleration is an increase in speed or velocity.

• Deceleration is a decrease in speed or velocity.

• Acceleration is a change in velocity. It is caused by a force such as gravity.

• The amount of gravitational attraction depends on how far the objects are and how much mass the objects have.

• The faster an object falls, the greater the air resistance. Air resistance becomes greater as speed is increased.

• Forces acting on the object must be unbalanced to cause or change motion in any way.

• Any change in the velocity or direction of an object is called acceleration. The amount of acceleration depends on the strength of the applied force. The change in direction of a moving object depends on the applied force.

• Centripetal force is directed inward toward the center of a circular path.

• Centrifugal force is directed outward or away from the center of a circular path.