< backnext >previewmain chapter 13 forces and motion preview section 1 gravity: a force of...

< Back Next > Preview Main

Chapter 13 Forces and Motion

Preview

Section 1 Gravity: A Force of Attraction

Section 2 Gravity and Motion

Section 3 Newton's Laws of Motion

Concept Map


Section 1 Gravity: A Force of AttractionChapter 13

Bellringer

Recall pictures or videos you have seen showing astronauts under “weightless” conditions in space. Try to imagine what it would be like to live your daily life in weightless conditions.

In your Science Journal, write one or two paragraphs explaining how you would carry out daily activities while weightless. Describe eating, sleeping, going to school, working in class, and any other activity you would like to include.



What You Will Learn

• Gravity affects all matter, including the parts of the solar system.

• Because gravity is a force, it can change the velocity of objects.

• The law of universal gravitation explains how distance, mass, and gravitational force are related.

• The weight of an object depends on gravity, but the mass of the object does not.



The Effects of Gravity on Matter

• Gravity is a force of attraction between objects that is due to their masses.

• All matter has mass. Gravity is a result of mass. Therefore, all matter is affected by gravity.



The Effects of Gravity on Matter, continued

• Gravity between the objects of the solar system holds the solar system together.

• Compared with all objects around you, Earth has a huge mass. The gravitational attraction of Earth is thus an important force that you experience all the time.



Newton and the Study of Gravity

• Newton concluded that the same unbalanced force that affects the motion of small objects, such as an apple, also affects the motion of larger objects, such as the moon.

• Newton called this unbalanced force gravity.



Newton and the Study of Gravity, continued

• Newton summarized his ideas about gravity in the law of universal gravitation.

• This law is called universal because it is thought to apply to all objects in the universe.



The Law of Universal Gravitation

• The law of universal gravitation states that all objects in the universe attract each other through gravitational force.

• The gravitational force between objects depends on the product of the masses of the objects. So, the gravity between objects increases as the masses of the objects increases.


Forces and MotionChapter 13



The Law of Universal Gravitation, continued

• The force of gravity depends on the distance between two objects.

• As the distance between two objects gets larger, the force of gravity gets much smaller.



The Law of Universal Gravitation, continued

• The gravitational force of the sun affects the movement of all the planets. This force helps them stay in orbit around the sun.

• So, the force of gravity has an important role in maintaining the shape of the solar system.



Weight and Gravitational Force

• Weight is related to mass, but they are not the same. Weight is a measure of the gravitational force on an object.

• Weight is expressed in the SI unit of force, the newton (N).

• The value of an object’s weight can change with the location of the object in the universe.



Weight and Gravitational Force, continued

• Mass is a measure of the amount of matter in an object. Mass is usually expressed in kilograms (kg) or grams (g).

• An object’s mass does not change when gravitational force changes.



Gravity and Static Objects

• Gravity also acts on nonmoving, or static, objects.

• Gravity is often balanced by elastic forces due to tension or compression.


Section 2 Gravity and MotionChapter 13

Bellringer

Imagine holding your arm straight out from your shoulder. A solid ball is in your hand. Then, you drop the ball and watch it as it falls. What force causes the ball to fall to the ground?

In your Science Journal, write one or two sentences describing the motion of the ball as it falls. Describe the direction of motion and tell whether the ball falls at a constant velocity or whether its velocity changes. Remember that the ball is not moving until you let go.



What You Will Learn

• The acceleration due to gravity is the same for all objects near Earth’s surface.

• Air resistance is a force that opposes the motion of objects through air.

• Projectile motion has two components—horizontal motion and vertical motion.



Gravity and Falling Objects

• All objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects near Earth’s surface.

• Acceleration is the rate at which velocity changes over time.



Gravity and Falling Objects, continued

• An object accelerates when the forces on it are unbalanced.

• Gravity exerts a downward, unbalanced force on falling objects. So, falling objects accelerate.

• Falling objects accelerate toward Earth at a rate of 9.8 m/s2.



Air Resistance and Falling Objects

• Air resistance is the force that opposes the motion of objects through air. Air resistance slows the acceleration of falling objects.

• The amount of air resistance acting on a falling object depends on the size, shape, and speed of the object.



Air Resistance and Falling Objects, continued

• An object falls at its terminal velocity when the upward force of air resistance equals the downward force of gravity.

• An object is in free fall if gravity is the only force acting on it.



Air Resistance and Falling Objects, continued

• Because air resistance is a force, free fall can happen only where there is no air.

• The term vacuum is used to describe a place in which there is no matter. Vacuum chambers are special containers from which air can be removed to make a vacuum.



Free Fall



Projectile Motion and Gravity

• Projectile motion is the curved path that an object follows when thrown, launched, or otherwise projected near the surface of Earth.

• Projectile motion is made of two different motions, or movements: horizontal movement and vertical movement. When these two movements are put together, they form a curved path.



Projectile Motion and Gravity, continued

• Horizontal movement is movement parallel to the ground.

• Gravity does not affect the horizontal movement of projectile motion.



Projectile Motion and Gravity, continued

• Vertical movement is movement perpendicular to the ground.

• Gravity affects the vertical movement of an object in projectile motion by pulling the object down at an acceleration of 9.8 m/s2 (if air resistance is ignored).



Orbiting and Gravity

• An object is orbiting when it is moving around another object in space.

• The two movements that come together to form an orbit are similar to the horizontal and vertical movements in projectile motion.



Orbiting and Gravity, continued

• The path of an orbiting object is not quite a circle. Instead, the path is an ellipse.

• Centripetal force is the unbalanced force that makes objects move in an elliptical path.

• Gravity provides the centripetal force that keeps objects in orbit.



Gravity and Orbit



Orbiting and Gravity, continued

• Gravity helps maintain the shape of the solar system by keeping large objects such as the planets in their orbit around the sun.

• Gravity also affects the movement of very small objects in the solar system, such as the tiny particles that make up the rings of Saturn.


Section 3 Newton's Laws of MotionChapter 13

Bellringer

Think of what forces you would feel in each of these two scenes. Then, in your Science Journal, for each scene, answer the two questions that follow each scene.

Scene 1: You are sitting still in your seat on a bus traveling straight ahead at a constant rate of speed. Are the forces acting on your body balanced or unbalanced? How do you know?

Scene 2: The driver suddenly applies the brakes, and the bus slows down. Are the forces acting on your body balanced or unbalanced? How do you know?



What You Will Learn

• Newton’s first law of motion states that the motion of an object will change only if unbalanced forces act on the object.

• Newton’s second law of motion states that the acceleration of an object depends on the object’s mass and on the force exerted on the object.

• Newton’s third law of motion states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.



Newton’s First Law

• Newton’s first law of motion states that the motion of an object will not change if the forces on it are balanced.

• Newton’s first law of motion describes the motion of an object that has a net force of 0 N acting on it.



Newton’s First Law, continued

• An object that is not moving is said to be at rest. Objects at rest will not move unless acted upon by an unbalanced force.

• Objects in motion will continue to move at a constant speed and in a straight line unless acted upon by an unbalanced force.




• Friction is an unbalanced force that changes the motion of objects.

• Because of friction, observing Newton’s first law is often difficult.

• Newton’s first law of motion is sometimes called the law of inertia.




• Inertia is the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts upon the object.

• Mass is a measure of inertia. An object that has a small mass has less inertia than an object that has a large mass.



Newton’s First Law



Newton’s Second Law of Motion

• Newton’s second law of motion states that the acceleration of an object depends on the mass of the object and the amount of force applied.

• Newton’s second law describes the motion of an object when an unbalanced force acts on the object.



Newton’s Second Law of Motion, continued

• The greater the mass of an object is, the greater the force needed to achieve the same acceleration.

• The acceleration of an object is always in the same direction as the net force applied.

• An object’s acceleration increases as the force on the object increases.



Newton’s Third Law of Motion

• Newton’s third law of motion states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

• All forces act in pairs. When a force is exerted, there is always a reaction force.



Newton’s Third Law of Motion, continued

• Action and reaction force pairs are present even when there is no movement.

• A force is always applied by one object on another object. However, action and reaction forces in a pair do not act on the same object.



Action and Reaction Forces


Chapter 13 Forces and Motion

Use the terms below to complete the concept map on the next slide.

force

free fall

terminal velocity

gravity

air resistance

projectile motion

Concept Map



Concept Map

< backnext >previewmain chapter 13 forces and motion preview section 1 gravity: a force of...

Documents