Newton’s second law of motion states that the acceleration of an object is in the same direction as the net force on the object, and that the acceleration can be calculated from the following equation:
Suppose you give a skateboard a push with your hand.
Friction 3.1 Newton’s Second Law
According to Newton’s first law of motion, if the net force acting on a moving object is zero, it will continue to move in a straight line with constant speed.
Does the skateboard keep moving with constant speed after it leaves your hand?
The larger the force pushing the two surfaces together is, the stronger these microwelds will be, because more of the surface bumps will come into contact.
Sticking Together 3.1 Newton’s Second Law
To move one surface over the other, a force must be applied to break the microwelds.
Pushing together, the box moves. Together you and your friend have exerted enough force to break the microwelds between the floor and the bottom of the box.
The terminal velocity is the highest speed a falling object will reach.
Terminal Velocity 3.1 Newton’s Second Law
The terminal velocity depends on the size, shape, and mass of a falling object.
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3.1 Section Check Question 1 A. acceleration B. momentum C. speed D. velocity Newton’s second law of motion states that _________ of an object is in the same direction as the net force on the object.
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3.1 Section Check Answer The answer is A. Acceleration can be calculated by dividing the net force in newtons by the mass in kilograms.
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3.1 Section Check Question 2 A. joule B. lux C. newton D. watt The unit of force is __________.
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3.1 Section Check Answer The answer is C. One newton = 1 kg · m/s 2
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3.1 Section Check Answer Friction results from the sticking together of two surfaces that are in contact. Question 3 What causes friction?
In this equation G is a constant called the universal gravitational constant, and d is the distance between the two masses, m 1 and m 2.
The law of universal gravitation enables the force of gravity to be calculated between any two objects if their masses and the distance between them is known.
According to the law of universal gravitation, the gravitational force between two masses decreases rapidly as the distance between the masses increases.
However, for a typical mission, the shuttle orbits Earth at an altitude of about 400 km.
3.2 Gravity Weightlessness and Free Fall
According to the law of universal gravitation, at 400-km altitude the force of Earth’s gravity is about 90 percent as strong as it is at Earth’s surface.
So an astronaut with a mass of 80 kg still would weigh about 700 N in orbit, compared with a weight of about 780 N at Earth’s surface.
If you were to throw a ball as hard as you could from shoulder height in a perfectly horizontal direction, would it take longer to reach the ground than if you dropped a ball from the same height?
3.2 Gravity Horizontal and Vertical Distance Click image to view movie
According to the second law of motion, when a ball has centripetal acceleration, the direction of the net force on the ball also must be toward the center of the curved path.
3.2 Gravity Centripetal Force
The net force exerted toward the center of a curved path is called a centripetal force .
Imagine whirling an object tied to a string above your head.
3.2 Gravity Gravity Can Be a Centripetal Force
The string exerts a centripetal force on the object that keeps it moving in a circular path.
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3.2 Gravity Gravity Can Be a Centripetal Force
In the same way, Earth’s gravity exerts a centripetal force on the Moon that keeps it moving in a nearly circular orbit.
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3.2 Section Check Question 1 Gravity is an attractive force between any two objects and depends on the masses of the objects and the distance between them. Gravity is an attractive force between any two objects and depends on __________. Answer
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3.2 Section Check Question 2 A. gravity B. net C. strong nuclear D. weak nuclear Which is NOT one of the four basic forces?
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3.2 Section Check Answer The answer is B. The fourth basic force is the electromagnetic force, which causes electricity, magnetism, and chemical interactions between atoms and molecules.
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3.2 Section Check Question 3 A. F = G(m 1 m 2 /d 2 ) B. G = F(m 1 m 2 /d 2 ) C. F = G(m 1 - m 2 /d 2 ) D. F = G(d 2 /m 1 m 2 ) Which of the following equations represents the law of universal gravitation?
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3.2 Section Check Answer The answer is A. In the equation, G is the universal gravitational constant and d is the distance between the two masses, m 1 and m 2 .
Newton’s third law of motion describes action-reaction pairs this way. When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction.
In a rocket engine, burning fuel produces hot gases. The rocket engine exerts a force on these gases and causes them to escape out the back of the rocket.
By Newton’s third law, the gases exert a force on the rocket and push it forward.
The results of a collision depend on the momentum of each object.
3.3 The Third Law of Motion
When the first puck hits the second puck from behind, it gives the second puck momentum in the same direction.
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When Objects Collide 3.3 The Third Law of Motion
If the pucks are speeding toward each other with the same speed, the total momentum is zero.
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3.3 Section Check Question 1 According to Newton’s law, the second object exerts a force on the first that is equal in strength and opposite in direction. According to Newton’s third law of motion, what happens when one object exerts a force on a second object? Answer
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3.3 Section Check Question 2 A. mass, acceleration B. mass, velocity C. mass, weight D. net force, velocity The momentum of an object is the product of its __________ and __________.
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3.3 Section Check Answer The correct answer is B. An object’s momentum is the product of its mass and velocity, and is given the symbol p .
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3.3 Section Check Question 3 When two objects collide, what happens to their momentum?
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3.3 Section Check Answer According to the law of conservation of momentum, if the objects in a collision exert forces only on each other, their total momentum doesn’t change, even when momentum is transferred from one object to another.
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