Why does a snowflake fall out of the sky instead of stopping in midair? Why do rocks not roll down mountains on their own? Questions such as these about force and motion fascinated a British scientist named Sir Isaac Newton. In the 1680s, Newton published a book called Mathematical Principles of Natural Philosophy, in which he tried to answer those types of questions. He described three laws of motion that are still used and studied today.
Newton’s first law of motion states that an object at rest will stay at rest unless it is acted upon by a nonzero net force. An object in motion will stay in motion with the same direction and speed unless it is acted upon by a nonzero net force.
Newton’s first law describes something called inertia. Inertia is the tendency of objects to resist a change in motion. In other words, an object that is still will not move unless a force makes it move, and an object that is moving will not stop unless a force makes it stop. For example, inertia explains why a person riding in a car keeps moving forward in his seat if the car brakes suddenly. The person only stops moving forward because the force of the seat belt stops him.
Inertia is affected by mass. Objects with more mass have more inertia, which means more force is required to move them or bring them to a stop. It takes more force to move a bowling ball than it does to move a tennis ball because the bowling ball has more mass, and therefore it has more inertia.
Question 1
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Question 2
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Question 3
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If a vehicle’s __________increases, its inertia also increases. If all the vehicles on this road are traveling at the same speed, a __________will take the longest amount of time to come to a complete stop.
Check for Understanding:
1. Newton’s first law of motion states that an object at rest will stay at rest unless it is acted upon by a nonzero net force. An object in motion will continue moving with the same direction and speed unless it is acted upon by a nonzero net force.
2. Inertia is the tendency of objects to resist a change in motion.
3. Objects with more mass have more inertia.
Newton’s first law tells scientists that if an unbalanced force acts on an object at rest, that object will accelerate. Newton’s second law describes that acceleration. Newton’s second law states that an object’s acceleration depends on its mass and the net force acting on it. This law can be written in the following way:
Newton’s second law can also be rewritten to show how much force is needed to move an object at a certain acceleration. That relationship is written in the following way:
Acceleration is measured in meters per second per second (m⁄s⁄s), which equals m/s2. Scientists measure mass in kilograms (kg). Newton’s second law says that force = mass × acceleration. That means that force = kg × m/s2. That unit of measurement, kg × m/s2, is called the newton (N), which is how scientists measure force. One newton is the force required to give an object with a mass of 1 kg an acceleration of 1 m/s2.
Sarah wants to throw a 2 kg ball with an acceleration of 15 m/s2. How much force should she use? Click the buttons below to learn how to use Newton’s second law to calculate the amount of force that she needs to throw the ball with.
It turns out that Sarah is much stronger than she thought. She throws the 2 kg ball with a force of 50 N. What is the acceleration of the ball? Click the buttons to see how to use Newton’s second law to calculate the ball’s acceleration.
Question 4
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Move the text boxes in order to complete the formula for Newton’s second law.
Other Answer Choices:
Speed
Acceleration
Velocity
Volume
Net Force
Question 5
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Question 6
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Use the formulas above to answer the following questions.
If a 10 N net force acts on a ball with a mass of 2 kg, the ball's acceleration will be
__________m/s2.
A net force of__________N would be needed to accelerate the 2 kg ball at 10 m/s2.
Check for Understanding:
1. Newton’s second law states that an object’s acceleration depends on its mass, and on the net force acting on it.
2. An object’s acceleration can be increased by decreasing its mass, or increasing the net force acting on it.
When a student sits in a chair, what prevents her from falling through the chair? Newton’s third law answered this question by stating that if one object exerts a force on a second object, then the second object exerts a force of equal strength in the opposite direction on the first object. In other words, for every action, there is an equal and opposite reaction. This explains why the student does not fall through the chair. When she sits and exerts a force downward on the chair, the chair exerts an equal force back up toward her.
Newton’s third law shows that forces come in action and reaction pairs. For example, when a swimmer moves her hand through the water, she exerts a force backward on the water. This is the action force. At the same time, the water exerts an equal reaction force back toward her hand.
But if the action and reaction forces are equal and act in opposite directions, why do they not cancel each other out in a nonzero net force? The forces do not cancel each other out because they act on different objects. The swimmer’s hand pushes a force on the water, but the water pushes a force on the swimmer’s hand. The water is sent backward, while the swimmer is accelerated forward.
Question 7
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Newton’s third law states that if one object exerts a force on a second object, then the second object exerts a force of__________strength in the__________
direction on the first object.
Question 8
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Other Answer Choices:
When two people push on a box with equal but opposite force, the equal forces cancel each other out in a nonzero net force. This means that the box does not accelerate in any direction.
When a tennis player hits a tennis ball with his racket, the ball pushes back on his racket with an equal and opposite force, but the ball accelerates away from the racket. The force of the racket and the force of the tennis ball do not cancel each other out because they are acting on different objects. The force of the racket is acting on the ball, and the force of the ball is acting on the racket. However, when the two people push on the box, the forces are acting on the same object: the box. That makes the net force on the box equal to zero.
Even though action and reaction forces have equal strength, their effects on the objects involved can be unequal. For example, when an apple falls from a tree, gravity pulls it toward the Earth. Newton’s third law states that the apple exerts an equal and opposite reaction on the Earth, so the apple also pulls the Earth toward it with the force of gravity. The Earth’s effect on the apple is easy to see, but it does not seem like the apple is pulling the Earth. Why is one effect easy to see, while the other is not?
Newton’s second law answers this question. The force on the apple is the same size as the force on the Earth. However, the mass of Earth is much greater than the mass of the apple, so Earth’s acceleration is much smaller than the apple’s acceleration.
An object can have many force pairs acting on it at once. Click the buttons in order to see how multiple force pairs work when a hockey stick hits a puck.
Question 9
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Question 10
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The player’s foot is exerting an action force on the ball. The ball is exerting a reaction force on the player’s foot. Furthermore, the player’s foot is exerting an action force on muscles and bones in the player’s leg. The muscles and bones in the player’s leg are exerting a reaction force on the player’s foot to keep it from moving backward.
Checking for Understanding:
1. Newton’s third law states that for every action, there is an equal and opposite reaction.
2. Action and reaction forces do not cancel each other out with a nonzero net force because they act on different objects.
3. Objects often have multiple forces working on them at once.
Question 11
11.
1. Newton’s first law states that an object at rest will remain at rest, and an object in motion will remain in motion, unless acted upon by a nonzero net force. This is sometimes called the law of inertia.
2. Newton’s second law states that an object’s acceleration depends on its mass and on the net force acting on it.
3. Newton’s third law states that if one object exerts a force on a second object, then the second object exerts a force of equal strength in the opposite direction on the first object.
Question 12
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Question 13
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A 10 kg dog runs forward with a net force of 80 N. The dog's acceleration is__________m/s2.
Remember, acceleration = net force/mass.
Question 14
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Question 15
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Question 16
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Question 17
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Question 18
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Question 19
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The speed of an object determines the amount of inertia it has.
True
False
Which of the following statements correctly describes Newton’s first law?
Nonzero net forces keep objects at rest from moving, and they keep objects that are moving in motion.
An object at rest will stay are rest, and an object in motion will stay in motion, unless it has inertia.
An object in motion stays in motion, and an object at rest stays at rest, unless acted upon by a zero net force.
An object at rest stays at rest, and an object in motion stays in motion, unless it is acted upon by an unbalanced force.
How could the acceleration of this airplane be increased (choose 2)?
by increasing its mass
by decreasing the force exerted by its engines
by decreasing its mass
by increasing the force exerted by its engines
When this runner exerts an action force on the ground, the ground exerts a reaction force. Move the arrow that shows the correct reaction force onto the diagram.
When Nina jumps into the air, the Earth pulls her back down through the force of gravity. What else occurs?
Nina pulls the Earth toward her, but the Earth’s acceleration is too small to notice.
Nina exerts a gravitational pull on the Earth, but the Earth does not accelerate at all.
The force of gravity pulling Nina back down to Earth is the only force occurring.
Nina’s gravitational force pushes away from the Earth.
Objects can have more than two forces acting on them at a time.
True
False
Inertia is __________.
the tendency of objects to resist a change in motion
a force that causes stationary objects to move
the law that says every action has an opposite reaction
a force that pushes objects away from each other
A rocket has been launched into outer space, and it no longer experiences significant gravitational attraction from Earth. According to Newton’s first law, what will happen to this rocket?
It will continue moving in the same direction forever, until it uses up all of its inertia.
It will return to the Earth once its force is less than its acceleration.
Its mass will decrease as its acceleration increases.
It will continue moving in the same direction forever, unless it is acted on by a nonzero net force.
When a tennis racket strikes a tennis ball with a force of 120 N, what else occurs?
The tennis ball pushes back on the racket with a force of 120 N.
The mass of the tennis ball decreases as its acceleration increases.
The tennis ball pushes away from the racket with a force of 120 N.
An asteroid is flying through space with a velocity of 1,500 m/s. If no forces act on the asteroid, what will happen to it?
The asteroid will continue moving forever with the same speed and direction.
The asteroid will continue moving until its acceleration is greater than its velocity.
The asteroid will eventually come to a stop when its velocity reaches 0 m/s.
The asteroid’s inertia will decrease and its acceleration will slowly increase.
How does an object’s mass affect its inertia?
Increasing an object’s mass will increase its inertia.
Changing an object’s mass has no effect on its inertia.
Increasing an object’s mass will decrease its inertia.
Decreasing an object’s mass will increase its inertia.
Abdul wants to increase the acceleration of his bike when he rides it. What can he do to accomplish that goal?
Check all that are true (choose 2)
He can add more mass to the bike so that it has greater inertia.
He can remove the water bottle holder and the bell so that the bike has less mass.
He can pedal with more force.
He can pedal slower so that he does not become tired.
If Abdul pedals his 20 kg bike with a force of 100 N, what will the bike’s acceleration be? The formulas above may be helpful in finding the bike's acceleration.
200 m/s2
5 m/s2
20,000 m/s2
10 m/s2
The picture above shows the force that this woman exerts on the wall as she leans against it. Which arrow below shows the force exerted by the wall?
