Unit 4 Forces (Newton s Laws)

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1 Name: Pd: Date: Unit Forces (Newton s Laws) The Nature of Forces force A push or pull exerted on an object. newton A unit of measure that equals the force required to accelerate kilogram of mass at meter per second per second. net force The overall force on an object when all the individual forces acting on it are added together. unbalanced forces Forces that produce a nonzero net force, which changes an object s motion. balanced forces Equal forces acting on an object in opposite directions. Friction and Gravity friction The force that one surface exerts on another when the two surfaces rub against each other. static friction Friction that acts on objects that are not moving. sliding friction Friction that occurs when one solid surface slides over another. rolling friction Friction that occurs when an object rolls over a surface. fluid friction Friction that occurs as an object moves through a fluid. gravity The force that pulls objects toward each other. mass The amount of matter in an object. Newton s First and Second Laws inertia The tendency of an object to resist any change in its motion. Newton s Third Law momentum The product of an object s mass and velocity. law of conservation of momentum The rule that in the absence of outside forces the total momentum of objects that interact does not change weight The force of gravity on an object at the surface of a planet. free fall The motion of a falling object when the only force acting on it is gravity. air resistance The fluid friction experienced by objects falling through the air. terminal velocity The greatest velocity a falling object can achieve. projectile An object that is thrown.

2 NEWTON S FIRST AND SECOND LAWS Guide for Reading. What is Newton s first law of motion?. What is Newton s second law of motion? The English mathematician Sir Isaac Newton restated Galileo s ideas about motion in the first of his three laws of motion. Newton s first law of motion states that an object at rest will remain at rest, and an object moving at a constant velocity will continue moving at constant velocity, unless it is acted upon by an unbalanced force. Newton s first law is also called the law of inertia. An object s resistance to a change in motion is called inertia. The amount of inertia an object has depends on its mass. Objects with greater mass have more inertia, and require a greater force to cause a change in motion. Newton s second law of motion states that acceleration depends on the object s mass and the net force acting on the object. This relationship can be written in an equation. Acceleation = Net Force Mass When acceleration is measured in meters per second per second (m/s ) and mass is measured in kilograms, force is measured in kilograms meters per second per second (kg m/s ). This unit is called the newton (N). One newton equals the force required to accelerate one kilogram of mass at meter per second per second. N = kg m/s The acceleration of an object will increase if the force increases. According to the equation, acceleration and force change in the same way both get larger. The equation also shows that the acceleration will increase if the mass decreases. Acceleration and mass change in opposite ways.

3 NEWTON S FIRST AND SECOND LAWS Guide for Reading and Study The First Law of Motion force. For an unmoving object to start moving, a(n) has to act on it.. What is Newton s first law of motion? Newton s first law states that an object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity unless it is acted upon by an unbalanced force. 3. What is inertia? Inertia is an object s resistance to change in its motion.. What is another name for Newton s first law? The other name for Newton s first law is the law of inertia. 5. The amount of inertia an object has depends on its mass.

4 The Second Law of Motion 6. What is Newton s second law of motion? Acceleration depends on the object s mass and on the net force acting on the object. 3. What is the equation that describes the relationship among the quantities of force, mass, and acceleration? Acceleration = Net force / Mass 8. Circle the letters of the two answers below that are different names for the same unit of measure. a. m/s b. N c. kg m/s d. kg 9 How can you use Newton s second law to find force? Force = Mass x Acceleration 5 0. What are two ways to increase the acceleration of an object? You can increase the acceleration of an object by increasing the force acting on an object or by decreasing the mass of the object.

5 Review and Reinforce Newton s First and Second Laws Understanding Main Ideas. Newton s second law of motion describes the relationship of force, mass, and acceleration. Write the equation. Acceleration = Net Force/Mass. If you increase the force on an object, its acceleration a. decreases. b. stays the same. c. also increases. d. stops. 3. If you increase the mass on an object, its acceleration a. decreases. b. stays the same. c. also increases. d. stops.. How much force is needed to accelerate a 3 kg skateboard at 5 m/s? a. 8 N b. 0.6 N c..6 N d. 5 N 5. A resistance to a change in motion is a. acceleration b. inertia c. gravity d. velocity 6. The amount of inertia an object has depends on its a. speed b. volume c. mass d. length Building Vocabulary Skills. Define the term inertia. Inertia is an object s resistance to change in its motion.

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7 NEWTON S THIRD LAW Guide for Reading. What is Newton s third law of motion?. How can you determine the momentum of an object? 3. What is the law of conservation of momentum? Forces are not one-sided. Whenever one object exerts a force on a second object, the second object exerts a force back on the first object. The force exerted by the second object is equal in strength and opposite in direction to the first force. The first force is called the action and the other force is the reaction. Newton s third law of motion describes the relationship between these two forces. Newton s third law of motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. Newton s third law refers to forces on two different objects. The action and reaction forces described by this law cannot be added (combined) together because they are each acting on a different object. Forces can be added together only if they are acting on the same object. All moving objects have momentum. Momentum is a characteristic of a moving object that is related to the mass and the velocity of the object. The momentum of an object can by determined by multiplying the object s mass and its velocity. Momentum = Mass Velocity The unit for momentum is kilogram-meters per second (kg m/s), since mass is measured in kilograms and velocity in meters per second. Like velocity and acceleration, momentum is described by its direction in addition to its quantity. The momentum of an object is in the same direction as the velocity of the object. The more momentum a moving object has, the harder it is to stop. When two objects collide in the absence of friction, momentum is not lost. The law of conservation of momentum states that, in the absence of outside forces, the total momentum of the objects that interact does not change. It is the same before and after the interaction. The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects. Friction would be an example of an outside force that might act on the objects. A quantity that is conserved is the same after an event as it was before the event. Momentum is conserved when two objects, such as trains, collide. If one train traveling fast collides with a slower-moving train on the same track, the faster train slows down, and the slower train speeds up. If a moving train collides with a train at rest, the first train stops moving and the second train begins to move. If a moving train collides and locks with a train at rest, both cars will then move, but they will move more slowly than the first car did. In each of these examples, momentum is conserved.

8 Newton s Third Law Guide for Reading and Study Newton s Third Law of Motion. What is Newton s third law of motion? If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.. What is the name given to the force exerted by the first object on a second object? action force 3. What is the name given to the force exerted by the second object back on the first object? reaction force equal. The action and reaction forces in any situation will always be and. opposite 5. Explain why the equal action and reaction forces do not cancel each other when one person hits a ball. Forces can be combined only if they are acting on the same object. When one person hits a ball, the action force is exerted on the ball, while the equal reaction force is exerted back on the person. Therefore, the equal forces in this situation do not cancel each other.

9 6. Explain why the equal action and reaction forces cancel each other when two people hit volleyball in opposite directions. Forces can be combined together only if they are acting on the same object. When two people hit a volleyball, the action forces is exerted on the ball, while the equal reaction forces is exerted back on the two people. Therefore, the equal forces in this situation do cancel each other. Momentum. The product of an object s mass and velocity is its momentum 8. What is the equation you use to determine the momentum of an object? Momentum = Mass x Velocity p = m v 3 9. What is the unit of measurement for momentum? kilograms meters per second Conservation of Momentum kg m/s 5 0. What does the law of conservation of momentum state? The total momentum of the objects that interact does not change. 6

10 . Suppose a train car moving down a track at 0 m/s hits another train car that is not moving. Explain how momentum is conserved after the collision. The result of the collision is that the first train car stops and the second train car moves forward 0 m/s. In that case, the momentum of the first train car is transferred to the second train car so momentum is conserved. Review and Reinforce Newton s Third Law Understanding Main Ideas. What does it mean to say that momentum is conserved? It stays the same before and after an event How does the diagram illustrate Newton s third law of motion? In your answer, compare the force of the foot kicking the soccer ball with the force of the soccer ball on the foot. Newton s third law of motion says that if an object, such as a foot, exerts a force on another object, such as a soccer ball, the second object (the soccer ball) exerts a force of equal strength in the opposite direction on the first object (the foot).

11 . Could an elephant have the same momentum as a golf ball? Explain. Since momentum equals mass times velocity, a very slowmoving elephant could have the same momentum as a very fast-moving golf ball What is the momentum of a 0-kg dog running at a speed of 8 m/s? 0 kg x 8 m/s = 60 kg m/s 6. Suppose you have two toy cars. Each has a mass of 0.0 kg. The cars have tape on their bumpers that will cause them to couple (connect) together. One car is stopped on the track. The other car, traveling at a velocity of m/s, hits the first car. What is the momentum of the coupled cars? The momentum of moving car = 0.0 kg x m/s = 0.6 kg m/s The momentum of the nonmoving car was zero. After the cars couple (connect), the total momentum is 0.6 kg m/s. Momentum is conserved. Understanding Main Ideas. What is momentum? Momentum is mass times velocity. 3 or 8. Explain the law of conservation of momentum. The total momentum of a group of objects remains the same unless outside forces act on the objects. 6

12 Key Terms Forces Use the clues to help you find Key Terms hidden in the puzzle below. The terms may occur vertically, horizontally, or diagonally. force. A is a push or pull.. The overall force on an object after all forces are added together is called the force. 3. The tendency of an object to resist change in its motion is called inertia.. The amount of matter in an object is called. mass newton net 5. One equals the force required to accelerate kilogram of mass at meter per second per second. 6. The force that one surface exerts on another when the two rub against each other is called friction.. When solid surfaces slide over each other, the kind of friction that occurs is friction. sliding 8. The friction that occurs when an object moves through a fluid is called friction. 9. The force that pulls objects toward Earth is. gravity 0. When the only force acting on a falling object is gravity, the object is said to be in fall.. Objects falling through air experience a type of fluid friction called resistance. air. The force of gravity on a person or object at the surface of a planet is known as. momentum friction free weight 3. The of an object is the product of its mass and velocity.. Any object that travels around another object in space is a(n). satellite