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**Newton’s Laws of Motion**

Activity 80 Follow-up Newton’s Laws of Motion

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Stopping to Think #1 Which has more inertia, a heavy ball or a light ball rolling at the same speed in the same direction? Think about which one is more resistant to a change in motion. The heavy ball has more inertia because it would take more force to change its motion than the light one.

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Stopping to Think #2 What would happen to a baseball if you could throw it in outer space? Explain in terms of inertia and friction. Without friction, a thrown baseball would continue in a straight line at a constant speed forever, or until it encountered another force that changes its motion.

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Stopping to Think #3 A car travels along a straight road at a steady 40 MPH. Are the forces on the car balanced or unbalanced? Explain. The forces are balanced. The frictional forces within the car and from the road are balanced by the equal but opposite force applied by the engine. The net force is zero, and the car travels with constant speed and direction.

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Stopping to Think #4 Can a light object that is hit with a small force accelerate as rapidly as a heavier object hit with a big force? Why or why not? Yes, because the ratio of force to mass could be the same for the two situations. For example, the acceleration of 400 newtons/100 kg is the same as 4 newtons/1 kg.

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Stopping to Think #5 If you hold a backpack in your hand, the force of gravity pulls it downward. What force keeps it from falling to the ground? Your hand applies an equal and opposite force on the backpack.

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Analysis Question #1 Spaceships that travel millions of miles into outer space use very little fuel. How can they go so far on so little fuel? Because there is no friction in outer space, an object in motion remains in motion. Once a space probe leaves behind the friction of Earth’s atmosphere, it needs very little fuel to get it to its final destination. (However, it uses fuel to slow down if it is going to land on a planet or to change direction.)

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Analysis Question #2 Use Newton’s Laws to explain why it is easier to turn a truck when it is empty than when it is carrying a heavy load. Newton said that heavier things have more inertia. This means that the truck with the heavy load has a greater tendency to keep moving at the same speed and in the same direction. The truck needs more force when it is full to make the same change in motion as when it is empty. So the inertia of the heavier truck keeps it from turning as easily as the empty truck.

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Analysis Question #3 An engine can exert a force of 1,000 newtons. How fast can this engine accelerate: a. a 1,000 kg car F = ma a = F/m a = 1,000 N/1,000 kg = 1 m/s2 b. a 2,000 kg car F = ma a = F/m a = 1,000 N/2,000 kg = 0.5 m/s2

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Analysis Question #4 Use Newton’s third law to explain why a blown up, but untied balloon flies around the room when you let it go. Newton’s third law states that forces come in equal and opposite pairs. For every action force there is an equal and opposite reaction force. The balloon exerts a force on the air (the action force), causing the air to rush out the opening, while at the same time the air exerts an equal and opposite reaction force on the balloon, causing the balloon to fly around the room.

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Analysis #5 Motor oil, axle grease, and other lubricants are slippery. Why do you think people spend the money to put these lubricants in their cars? Lubricants reduce friction, and less friction means that the engine and the wheels move more smoothly (and stay cooler). This means the engine needs to exert less force, which often results in in lasting longer, using less fuel, and requiring fewer repairs.

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**Activity 80 Major Concepts**

An object will stay in motion or at rest unless acted upon by a force. Friction is a force that causes changes in speed of an object’s motion. The magnitude of the change in motion can be calculated using the relationship F = ma, which is independent of the nature of the force. The bigger the force applied to an object, the greater the acceleration. Whenever an object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

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