Net Force

1. 2.1Force Net Force A force is a push or a pull. A force of some kind is always required to change the state of motion of an object. The combination of all forces acting on an object is called the net force. The net force on an object changes its motion. The scientific unit of force is the newton, abbreviated N.

2. 2.1Force Net Force The net force depends on the magnitudes and directions of the applied forces.

3. 2.1Force Tension and Weight A stretched spring is under a “stretching force” called tension. Pounds and newtons are units of weight, which are units of force.

4. 2.3Support Force What forces act on a book lying at rest on a table? • One is the force due to gravity—the weight of the book. • There must be another force acting on it to produce a net force of zero—an upward force opposite to the force of gravity. The upward force that balances the weight of an object on a surface is called the support force. A support force is often called the normal force.

5. 2.4Equilibrium for Moving Objects The state of rest is only one form of equilibrium. An object moving at constant speed in a straight-line path is also in a state of equilibrium. Once in motion, if there is no net force to change the state of motion, it is in equilibrium.

6. Assessment Questions • When you hold a rock in your hand at rest, the forces on the rock • are mainly due to gravity. • are mainly due to the upward push of your hand. • cancel to zero. • don’t act unless the rock is dropped.

7. Assessment Questions • When you hold a rock in your hand at rest, the forces on the rock • are mainly due to gravity. • are mainly due to the upward push of your hand. • cancel to zero. • don’t act unless the rock is dropped. Answer: C

8. Assessment Questions • When a desk is horizontally pushed across a floor at a steady speed in a straight-line direction, the amount of friction acting on the desk is • less than the pushing force. • equal to the pushing force. • greater than the pushing force. • dependent on the speed of the sliding crate.

9. Assessment Questions • When a desk is horizontally pushed across a floor at a steady speed in a straight-line direction, the amount of friction acting on the desk is • less than the pushing force. • equal to the pushing force. • greater than the pushing force. • dependent on the speed of the sliding crate. Answer: B

10. Assessment Questions • When Nellie hangs at rest by a pair of ropes, the tensions in the ropes • always equal her weight. • always equal half her weight. • depend on the angle of the ropes to the vertical. • are twice her weight.

11. Assessment Questions • When Nellie hangs at rest by a pair of ropes, the tensions in the ropes • always equal her weight. • always equal half her weight. • depend on the angle of the ropes to the vertical. • are twice her weight. Answer: C

12. 6.1Force Causes Acceleration Recall from the previous chapter that the combination of forces acting on an object is the net force. • Acceleration depends on the net force. • To increase the acceleration of an object, you must increase the net force acting on it. • An object’s acceleration is directly proportional to the net force acting on it: acceleration ~ net force (The symbol ~ stands for “is directly proportional to.”)

13. 6.3Newton’s Second Law Newton’s second law describes the relationship among an object's mass, an object's acceleration, and the net force on an object.

14. 6.4Friction Friction is a force and affects motion: • Friction acts on materials that are in contact with each other. • It always acts in a direction to oppose relative motion. • When two solid objects come into contact, the friction is mainly due to irregularities in the two surfaces.

15. 6.5Applying Force-Pressure The amount of force per unit of area is called pressure. When the force is perpendicular to the surface area, P is the pressure and A is the area over which the force acts. Pressure is measured in newtons per square meter, or pascals (Pa). One newton per square meter is equal to one pascal.

16. 6.5Applying Force-Pressure The driving force per nail is not enough to puncture the skin. CAUTION: Do not attempt this on your own!

17. 6.7Falling and Air Resistance A feather and a coin fall with equal accelerations in a vacuum, but very unequally in the presence of air. When falling in air, the coin falls quickly while the feather flutters to the ground. The force due to air resistance diminishes the net force acting on the falling objects.

18. 6.7Falling and Air Resistance Speed and Area You experience the force due to air resistance when you stick your hand out of the window of a moving car. • If the car moves faster, the force on your hand increases. • If instead of just your hand, you hold your physics book out the window with the large side facing forward, the air resistance force is much larger than on your hand at the same speed.

19. Assessment Questions • When a net force acts on an object, its acceleration depends on the object’s • initial speed. • mass. • volume. • weight.

20. Assessment Questions • When a net force acts on an object, its acceleration depends on the object’s • initial speed. • mass. • volume. • weight. Answer: B

21. Assessment Questions • A cart is pushed and undergoes a certain acceleration. Consider how the acceleration would compare if it were pushed with twice the net force while its mass increased by four. Then its acceleration would be • one quarter. • half. • twice. • the same.

22. Assessment Questions • Friction is a force like any other force and affects motion. Friction occurs in • solids sliding over one another. • fluids. • air. • all of these Answer: D

23. Assessment Questions • When you stand on one foot instead of two, the pressure you exert on the ground is • half. • the same. • twice. • quadruple.

24. Assessment Questions • When you stand on one foot instead of two, the pressure you exert on the ground is • half. • the same. • twice. • quadruple. Answer: C

25. 7.1Forces and Interactions A hammer exerts a force on the nail and drives it into a board. • There must also be a force exerted on the hammer to halt it in the process. • Newton reasoned that while the hammer exerts a force on the nail, the nail exerts a force on the hammer. • In the interaction, there are a pair of forces, one acting on the nail and the other acting on the hammer.

26. 7.2Newton’s Third Law Newton’s third law describes the relationship between two forces in an interaction. • One force is called the action force. • The other force is called the reaction force. • Neither force exists without the other. • They are equal in strength and opposite in direction. • They occur at the same time (simultaneously).

27. 7.4Action and Reaction on Different Masses In the interaction between the boulder and Earth, the boulder pulls up on Earth with as much force as Earth pulls down on the boulder. • The forces are equal in strength and opposite in direction. • The boulder falls to Earth and Earth falls to the boulder, but the distance Earth falls is much less.

28. 7.4Action and Reaction on Different Masses F represents both the action and reaction forces; m (large), the mass of the cannon; and m (small), the mass of the cannonball. Do you see why the change in the velocity of the cannonball is greater compared with the change in velocity of the cannon?

29. 7.4Action and Reaction on Different Masses think! A tug of war occurs between boys and girls on a polished floor that’s somewhat slippery. If the boys are wearing socks and the girls are wearing rubber-soled shoes, who will surely win, and why?

30. 7.7Action Equals Reaction If a sheet of paper is held in midair, the heavyweight champion of the world could not strike the paper with a force of 200 N (45 pounds). The paper is not capable of exerting a reaction force of 200 N, and you cannot have an action force without a reaction force. If the paper is against the wall, then the wall will easily assist the paper in providing 200 N of reaction force, and more if needed!

31. Assessment Questions • A force interaction requires at least a(n) • single force. • pair of forces. • action force. • reaction force.

32. Assessment Questions • A force interaction requires at least a(n) • single force. • pair of forces. • action force. • reaction force. Answer: B

33. Assessment Questions • Whenever one object exerts a force on a second object, the second object exerts a force on the first that is • opposite in direction and equal in magnitude at the same time. • in the same direction and equal in magnitude a moment later. • opposite in direction and greater in magnitude at the same time. • in the same direction and weaker in magnitude a moment later.

34. Assessment Questions • Whenever one object exerts a force on a second object, the second object exerts a force on the first that is • opposite in direction and equal in magnitude at the same time. • in the same direction and equal in magnitude a moment later. • opposite in direction and greater in magnitude at the same time. • in the same direction and weaker in magnitude a moment later. Answer: A

35. Assessment Questions • When you jump vertically upward, strictly speaking, you cause Earth to • move downward. • also move upward with you. • remain stationary. • move sideways a bit.

36. Assessment Questions • When you jump vertically upward, strictly speaking, you cause Earth to • move downward. • also move upward with you. • remain stationary. • move sideways a bit. Answer: A

37. Assessment Questions • A system undergoes acceleration only when acted on by a(n) • net force. • pair of forces. • action and reaction forces. • internal interactions.

38. Assessment Questions • A system undergoes acceleration only when acted on by a(n) • net force. • pair of forces. • action and reaction forces. • internal interactions. Answer: A

39. True or False? 1. The amount of matter in an object is called its weight. 2. The combination of all the forces on an object is called the net force. 3. Inertia is the reluctance any material object has to change in its state of motion. 4. Objects move only when a force is exerted. 5. The acceleration of an object is inversely proportional to the net force acting on it. 6. Air resistance is caused by friction between the air and an object moving through air.

40. True or False? 1. The amount of matter in an object is called its weight. 2. The combination of all the forces on an object is called the net force. 3. Inertia is the reluctance any material object has to change in its state of motion. 4. Objects move only when a force is exerted. 5. The acceleration of an object is inversely proportional to the net force acting on it. 6. Air resistance is caused by friction between the air and an object moving through air.

41. True or False? 7. The speed of an object dropped in air will continue to increase without limit until it strikes the ground. 8. When one object exerts a force on a second object, the second object always exerts the same amount of force back on the first object. 9. A rocketship is pushed forward by gases that are forced out the back of the ship. 10. In order to make a cart move forward, a horse must pull harder on the cart than the cart pulls on the horse. 11. If a bicycle and a parked car have a head-on collision, the force of impact is greater on the bicycle. 12. The earth and moon pull on each other with equal amounts of force.