Unbalanced Forces Cause Motion
- 1. Laws of Motion—Force An object’s motion changes in response to a force . A force is a push or a pull. A force has a size and a direction—both are important in determining an object’s motion. Newton’s First Law The force you exert on the book when you push is called a contact force . A contact force is a force that is exerted when two objects are touching each other.
- 2. Long-Range Forces A force can be exerted if two objects are not in contact. If you bring a magnet close to a paper clip, the paper clip moves toward the magnet, so a force must be acting on the paper clip.
- 3. Long-Range Forces A dropped ball will fall downward, even though nothing appears to be touching it. The forces acting on the paper clip and the ball are long-range forces . In SI units the unit of force is the Newton , which is abbreviated N and named for Isaac Newton . One Newton is about the amount of force needed to lift a half cup of water.
- 4. Newton’s First Law of Motion The first law of motion —An object will remain at rest or move in a straight line with constant speed unless it is acted upon by a force.
- 5. Newton’s First Law of Motion It once seemed that a force had to be applied continually to keep an object moving. Newton and others theorized that if an object already is moving, it will continue to move in a straight line with constant speed. For the object to slow down, a force has to act on it.
- 6. Inertia and Mass The first law of motion is sometimes called the law of inertia . Inertia measures an object’s tendency to remain at rest or keep moving with constant velocity . Inertia depends on the mass of the object. The more mass an object has, the more inertia it has and the harder it is to change the motion of the object.
- 7. Adding Forces According to Newton’s first law , the motion of an object changes only if a force is acting on the object. Sometimes more than one force acts on an object, as when several people push a stalled car to the side of the road. Motion depends upon the size and direction of all the forces.
- 8. Adding Forces If two people push in opposite directions on a box with an equal amount of force, the box will not move. If the forces are equal but in opposite directions, they will cancel each other out and are called balanced forces . When forces on an object are balanced, no change will occur in the object’s motion because the total force on the object is zero.
- 9. Adding Forces If one force pushing on the box is greater than the other, the forces do not cancel. The box will move in the direction of the larger force. Forces acting on an object that do not cancel are unbalanced forces . The motion of an object changes only if the forces acting on it are unbalanced. The change in motion is in the direction of the unbalanced force.
- 10. Changes in Motion and Forces The motion of an object changes only when unbalanced forces act on the object. If the motion of an object changes, the object is accelerating. The object can speed up, slow down, or turn. In all cases, an object acted on by an unbalanced force changes velocity.
- 11. The Second Law of Motion The motion of an object can be explained by Newton’s second law of motion . The second law of motion , an object acted on by an unbalanced force will accelerate in the direction of the force with an acceleration.
- 12. The Second Law of Motion If more than one force acts on the object, the force in this formula is the combination of all the forces, or the total force that acts on the object.
- 14. Using the Second Law The second law of motion enables the acceleration of an object to be calculated. Knowing the acceleration helps determine the speed or velocity of an object at any time. For motion in a straight line an acceleration of 5 m/s 2 means that every second the speed is increasing by 5 m/s.
- 15. Using the Second Law If you know the mass and acceleration of an object, you can use Newton’s second law to find the force .
- 17. The Force of Gravity Every object exerts a gravitational force on every other object. That force depends on the mass of the objects and the distance between them. Because gravity depends on mass, less massive objects, like people, bicycles, and books, exert very small gravitational forces. These forces are so small you can’t feel them.
- 18. The Force of Gravity Objects as massive as planets, stars, and asteroids exert much greater gravitational forces. The gravitational forces between the Sun, planets, and other nearby space objects is the force that holds the solar system together.
- 19. The Force of Gravity Gravity also depends on distance. For objects on Earth’s surface, Earth is, by far, the most massive nearby object. It exerts the greatest gravitational force on objects on Earth’s surface. Near Earth’s surface, the force of Earth’s gravity causes all objects to fall toward Earth’s center with the same acceleration, 9.8 m/s 2 .
- 20. The Force of Gravity You are at rest because the ground exerts an upward force that balances the force of gravity . Even if you are standing on the ground, Earth still is pulling on you with this force. The force of gravity on you is
- 21. Mass and Weight The force of Earth’s gravity on an object near Earth is the weight of that object. Weight and mass are different. Weight is a force , just like a push of your hand is a force. Weight changes when the force of gravity changes .
- 22. Mass and Weight Mass is a measure of the amount of matter in an object. An astronaut would have the same mass on Earth as on the Moon .
- 23. Friction When you rub your hands together you can feel a force between your hands, slowing their motion. Friction is a force that resists sliding motion between surfaces that are touching. Friction always is present when two surfaces of two objects slide past each other.
- 24. Friction To keep the objects moving, a force has to be applied to overcome the force of friction. Friction sometimes can be reduced by adding oil or grease to the surfaces, but it always is present.
- 25. Friction There are several types of friction , and any form of motion will include one or more of them.
- 26. Static Friction Static friction keeps an object at rest from moving on a surface when a force is applied to the object. For example, when you stand on a slight incline, you don’t slide down because of the static friction between your shoes and the ground.
- 27. Sliding Friction Sliding friction is the force that occurs when two surfaces slide past each other. When you apply the brakes to a bike, a car, you use sliding friction to slow down.
- 28. Rolling Friction Rolling friction makes a wheel roll forward to backward. If the rolling friction is large enough, a wheel will roll without slipping. A car that is stuck doesn’t move because mud or snow makes the ground too slippery. There is not enough rolling friction to keep the wheels from slipping.
- 29. Rolling Friction Because rolling friction is the force that enables a wheel to roll on a surface, the force of rolling friction is in the same direction as the wheel is rolling. If the wheel is rolling forward, the rolling friction force also points forward.
- 30. Air Resistance Molecules in air collide with the forward-moving surface of an object, slowing its motion. This is called air resistance . Air resistance is less for a narrow, pointed object than for a large, flat object. Air resistance increases as the speed of an object increases .
- 31. Air Resistance Because air resistance is a type of friction , it acts in the direction opposite to an object’s motion. Before a sky diver opens his parachute, his air resistance is small.
- 32. Air Resistance The force of air resistance is upward, but it is not large enough to balance the downward force of gravity. As a result, the sky diver falls rapidly.
- 33. Air Resistance When he opens his parachute, the air resistance is much greater because the parachute has a large surface area. The force of air resistance is large enough to slow his fall and balance the force of gravity.
- 34. The Third Law of Motion Newton’s first two laws explain how forces acting on a single object affect its motion. The third law describes the connection between the object receiving a force and the object supplying that force. Newton’s Third Law The third law of motion forces always act in equal but opposite pairs or, for every action, there is an equal but opposite reaction.
- 35. Action and Reaction Forces Newton’s third law means that when you lift your book bag, your book bag pulls back. When you exert a force on the floor, the floor exerts an equal force on you in the opposite direction. The same is true for any two objects, regardless of whether the force between the objects is a contact force or a long-range force.
- 36. Action and Reaction Forces For example, if you place two bar magnets with opposite poles facing one another each magnet will apply a force on the other. No matter how one object exerts a force on another, the other object always exerts an equal force on the first object in the opposite direction.
- 37. Applying the Third Law Action and reaction forces are not the same as balanced forces . Balanced forces are forces that act on the same object and cancel each other. Action and reaction forces act on different objects.
- 38. Applying the Third Law When you kick a soccer ball, your force on the ball equals the ball’s force on you. The harder you kick, the greater the force the ball exerts on your foot. Unlike balanced forces, action and reaction forces can cause the motion of objects to change.
- 39. Using Friction When you push on a door, your feet are touching Earth, and static friction keeps you from sliding. The reaction force is exerted on you and Earth together. You don’t move because the door doesn’t exert a large enough reaction force to move both you and Earth.
- 40. Using Friction If you wear slippery shoes, or if the floor is very smooth, your feet might slide when you push on the door. The static friction force might not be large enough to keep you attached to Earth. The reaction force exerted by the door acts only on you, and not on you and Earth together.
- 41. Motion Caused by Force Pairs Although the action and reaction forces in a force pair are the same size, they can have different effects on the objects they act upon. Suppose a 50-kg student and a 20-kg box are in the middle of an ice skating rink. The student pushes on the box with a force of 10 N, and the box slides on the ice.
- 42. Motion Caused by Force Pairs The reaction force is the box pushing on the student with a force of 10 N, and the student slides in the opposite direction. These forces are exerted only while the student and the box are in contact. Although the same size force is acting on the student and the box, they will have different accelerations because their masses are different.
- 43. Motion Caused by Force Pairs The acceleration of each can be calculated using Newton’s second law .
- 44. Motion Caused by Force Pairs The acceleration of the student can be calculated by replacing 20 kg with 50 kg in the formula and is only 0.2 m/s 2 . The student and the box accelerate only while they are in contact. As a result, the student moves more slowly than the box moves.
- 45. Gravity and the Third Law According to the third law, you aren’t just pulled toward Earth. Earth also is pulled toward you. The force you exert on Earth is the same as the force Earth exerts on you. However, Earth is trillions of times more massive than you are. Because Earth has such a large mass, the force you exert on it doesn’t have a noticeable effect.
- 46. Gravity and the Third Law Newton’s laws of motion apply to all objects, even the distant galaxies. The Sun exerts a gravitational force on Earth, and Earth exerts an equal force on the Sun.
- 47. Gravity and the Third Law Astronomers look for variations in the motions of stars that might be caused by an orbiting planet. More than 100 planets have been detected around stars other than the Sun using this method.
- 48. Combining the Laws During a jump when you push on the ground, the ground pushes up on you with an equal and opposite force. The overall force is upward, so as the second law predicts, you accelerate upward as your foot pushes against the ground.
- 49. Combining the Laws When your feet leave the ground, gravity is the only force acting on you. According to the second law, you accelerate in the direction of this unbalanced force. This downward acceleration slows you until you stop at the top of your jump and then causes you to increase your speed downward until you reach the ground.
- 50. Combining the Laws When your feet hit the ground, the ground exerts an upward force on you. The force must be greater than the downward force of gravity to slow you down. When you stop moving, all of the forces on you are balanced. As the first law predicts, you remain at rest.
Editor's Notes
- #12: Cinda, read the last sentence. It doesn’t make any sense. They crossed out “According to” from the beginning, but I think we should leave it in.