Change in Motion

Motion When something moves, its in motion. Motion is change in an object’s position. A motionless object is at rest or stationary. In order to describe an object as moving or stationary, you have to compare it to another object. Think about people standing on a corner watching a bus go by. The people on the bus are stationary in their frame of reference. Observers on the street see a bus moving in their frame of reference.

One of history’s greatest scientists and mathematicians was an Englishman, Sir Isaac Newton. Newton developed three laws of motion. The laws were the first to describe the movement of objects in terms of forces. For example, Newton didn’t say that an apple falls from a tree because it is made of earth. He said that a force (gravitation) causes Earth and the apple to attract each other. The laws of motion describe the movement of objects on Earth. They also describe the movement of planets and moons. In 1687, he presented his laws of motion.

Newton’s First Law Newton’s first law of motion explains what happens to objects that are at rest or moving. Newton realized that a moving object would continue to move in a straight line until a force interfered with it. He also decided that an object that was not moving would remain motionless unless a force acted on it. The first law states that an object at rest will remain at rest unless a force acts on it. It will remain in motion unless a force acts on it. Can you give an example?

Principle of Inertia Newton’s first law also states that an object in motion will continue to move at a constant speed in the same direction if no forces act on it. This tendency of an object to remain at rest or an object in motion to remain in motion is called inertia. So Newton’s first law of motion is also called the principle of inertia.

First Law of Motion According to this law, a ball rolling down a sidewalk stops because a force (friction) acts on it. In space, there is very little friction. A spacecraft will continue moving at the same speed and in the same direction practically forever. Its motion will change only if forces from a nearby star or planet act on it.

Speed, Velocity, and Acceleration Newton’s first law of motion means that, without the action of a force, an object will move at a constant speed at the constant direction. The speed of an object is a measure of the distance in moves in a given amount of time. Velocity is the measure of both an object’s speed and direction. Think of two joggers running at the same speed (8 km) but in opposite directions. The joggers may have the same speed, but their velocities are different. This is because their direction is different.

A force can change the speed of an object, its direction, or both. A force causes the velocity to change. A change in velocity is known as acceleration. Since velocity describes both speed and direction, acceleration can be a change in speed, a change of direction, or both. Any change in motion is acceleration.

Balanced and Unbalanced Forces Newton’s first law applies to situations in which no force acts on an object. What happens when force acts upon an object? Suppose you are having a tug-of-war with your dog. You are pulling on one end, and your dog is pulling on the other end of a rope in the opposite direction. If each of you pull with equal force, the rope will not move. The two forces are equal in strength, but opposite in direction- the two forces are balanced.

Now, suppose a friend helps you pull your end of the rope. Together, you pull with a greater force than the dog does. The forces are unbalanced. When forces are balanced, they cancel each other out, and there is no movement. An object will accelerate or change its motion, only when an unbalanced force acts on it. This is Newton’s second law of motion.

Newton’s Second of Law An object’s acceleration depends on the size and direction of the force acting on it and on the mass of the object. 1.Size of the force As the ball approaches the bat, the ball and the bat exert forces in different directions. 2.The direction of the force When the ball and the bat meet, the bat exerts a greater force. 3.The mass of an object The force exerted by the bat accelerates the ball-its velocity changes. The more force the bat exerts, the faster the ball accelerates.

Second Law of Motion Newton’s second law of motion describes the relationship between force, mass, and acceleration. You push a wagon causing it to accelerate. Imagine an adult pushing the same wagon. Because the adult pushes the same mass with a greater force, the acceleration is greater. The greater the force applied to an object; the greater the acceleration of the object.

Force is measured in a unit called Newton, in honor of Sir Isaac Newton. One newton (N) is the force required to accelerate a mass of 1 kg at 1 m/s per second. Imagine a child sitting in the wagon and the adult pushes the wagon. An adult pushes the wagon with a greater force causing a greater acceleration. If the child pushes the wagon with the adult in it, would the acceleration be less or greater? It would be greater because of the adult’s greater mass. F=ma

Gravity and Friction Gravity is a force that causes objects with a mass to be attracted or pulled down toward one another. Gravity is a non-contact force, meaning it acts on a force without touching it. Newton’s Law of Universal Gravitation describes the attractive force between 2 masses. The force increases with the masses of the two objects. However, the farther apart the two masses are, the weaker the force. Earth’s mass is much greater than the mass of anything on its surface. That’s why you experience gravity as a downward force.

Friction is the force that resists motion of one surface across another surface. Friction is a contact force, which means two surfaces must touch. Friction is usually greater between rough surfaces than smooth surfaces. Skater’s brakes against concrete will slow and eventually stop the skater. Air resistance will also slow down the skater. Air resistance, or drag, is a type of friction that opposes motion through air. Sky divers use a parachute to increase air resistance and slow down their rate of falling.

As you’ve seen, gravity and friction affect motion on Earth. In outer space, however, no air slows down a moving object. And far away from stars and planets, there is little gravity. Objects moving through space continue in a straight line at constant velocity.

Inertia would make the moon fly off in a straight line unless a force was acting on it. What is that force? In the moon’s orbit, the force is gravitation between Earth and the moon. The moon’s orbit is the path that results when its inertia and the force of gravitation act together. Inertia: The property of matter that keeps it moving in a straight line or keeps it a rest. Gravitation: The force that pulls all objects in the universe toward one another.

The law of universal gravitation states that all objects in the universe are attracted to all other objects. Why does a dropped book fall to the ground rather than Earth being pulled up to the book? Earth’s mass is greater. Size and distance affect orbits. The size of a planet’s orbit is related to the planet’s speed and its mass. Earth moves around the sun at an average speed of about 30km/sec. The outer planets move in much longer, slower orbits.

Newton’s Third Law of Motion For every action, there is an equal and opposite reaction. Think about an in-line skater holding a basketball. If the skater throws the ball really hard, the ball goes forward and the skater rolls backward. The third law is also stated this way: The first force is the action force. The force that pushes or pulls back is the reaction force. The skater exerts an action force on the basketball by throwing it. The thrown ball exerts an equal reaction force on the skater, but in the opposite direction. The result is the basketball moves in one direction and the skater moves in the other direction. These movements are a sure sign that forces have acted on both of them.

Summary

Review 1. A parachute jumper jumps from an airplane and falls very fast until his parachute opens. As the parachute opens, what causes the jumper to slow down as he falls? AnswerAnswer 2. What is the force that prevents objects on or close to earth from getting “lost in space”, such as the Space station, space shuttles, or airplanes? AnswerAnswer 3. If you travel to the moon, you would weigh (less than, the same, or more than) you weigh on Earth because of the gravitational effects on the moon. AnswerAnswer 4. Jupiter’s inner moons will be most affected by (size, distance or both)? AnswerAnswer 5. The theory that “all objects in the universe are attracted to each other” was created by what scientist? AnswerAnswer 6. What is the force that would cause any object to fall? AnswerAnswer 7. True or False Air resistance is a type of friction that opposes motion of an object through air. AnswerAnswer 8. True or False Friction is a contact force, which means that two objects or surfaces MUST touch one another. AnswerAnswer

Review 1. What is the scientific tool used to measure weight and force? AnswerAnswer 2. A falling object with more surface area will experience (more, same, or less) air resistance than a falling object with less surface area? AnswerAnswer 3. What is force? AnswerAnswer 4. Explain the Law of Universal Gravitation. Answer Answer

Air resistance

Gravity

less than

Size and Distance

Sir Isaac Newton

Gravity

True

Spring Scale

More

A push or pull that acts on an object.

The greater an object’s mass, the greater the gravitational force between it and other objects. The farther apart the objects are, the weaker the force.