Physics: Newton's Laws of Motion

Terms in this set (15)

What do Newton's three laws describe?

How forces change motion.

Newton's first law, the law of ___, states that an object at rest stays at rest and an object in motion keeps moving at constant velocity unless acted on by a net external force.

inertia

Define inertia.

An object's resistance to changes in its motion.

What is the measure of inertia?

Mass

What condition means the net force on an object is zero?

Equilibrium

Newton's second law states that the net force on an object equals its mass times its acceleration. What is the formula?

F_net = ma

What is the unit for force, and what is its equivalent in base units?

Newtons (N), where 1 N = 1 kg·m/s².

What is the formula for weight?

W = mg

Compare mass and weight regarding their constancy.

Mass is the same everywhere, but weight depends on the local gravitational field.

Newton's third law states that for every action force there is an ___ and ___ reaction force.

equal and opposite

On what objects do the two forces in a Newton's third law pair act?

They act on different objects.

What do free-body diagrams show?

All forces acting on one object.

What is the formula for calculating friction?

f = μN

What is the difference between static and kinetic friction?

Static friction prevents an object from starting to slide, while kinetic friction acts on a sliding object and is usually smaller than the maximum static friction.

In an elevator accelerating upward, how does the normal force (your apparent weight) compare to mg?

It is greater than mg.

Practice questions (15)

1. 1.According to Newton's first law, what must be true for an object to be in equilibrium (i.e., to have a constant velocity)?

   • AThe object must be at rest.
   • BThe net force acting on the object must be zero.
   • CNo forces can be acting on the object at all.
   • DThe object's mass must be zero.
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   Answer: The net force acting on the object must be zero.

   This question checks the student's understanding of a key condition of Newton's first law. Equilibrium doesn't mean an absence of forces, but rather that all forces acting on the object are balanced, resulting in a net force of zero. This is true for objects at rest and for objects moving at a constant velocity.

2. 2.A student pushes a heavy box across the floor at a constant velocity. According to Newton's third law, the student's push on the box and the box's push on the student are an action-reaction pair. Do these two forces cancel each other out? Why or why not?

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   Answer: No, the forces do not cancel each other out because they act on different objects. The student's push acts on the box, and the box's push acts on the student. Forces can only cancel out if they act on the same object.

   This question challenges a common misconception about Newton's third law. Students often think that if forces are equal and opposite, they must cancel. This question forces them to apply the principle that action-reaction forces act on *different* objects and therefore cannot cancel each other out, which is a critical insight for analyzing how objects move.

3. 3.If you double the net force applied to an object, what happens to its acceleration, according to the relationship F_net = ma?

   • AThe acceleration is doubled.
   • BThe acceleration is quadrupled.
   • CThe acceleration is halved.
   • DThe acceleration remains the same.
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   Answer: The acceleration is doubled.

   This question directly tests the student's comprehension of the proportional relationship between net force and acceleration described in Newton's second law. It requires them to understand that acceleration is directly proportional to the net force when mass is constant.

4. 4.A passenger on a bus lurches forward when the bus suddenly brakes. Which of Newton's laws best explains this experience, and what is the underlying concept called?

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   Answer: Newton's first law explains this. The concept is called inertia, which is the tendency of the passenger's body to resist the change in motion and continue moving at the bus's original speed.

   This question asks students to connect a common, relatable experience to the formal physical principles in the text. By explaining the lurching effect using the specific concepts of Newton's first law and inertia, they demonstrate a deeper, more applicable understanding of the physics.

5. 5.A student states, 'My mass is 60 kg, so my weight is also 60 kg.' Is this statement correct? Explain the difference between mass and weight based on Newton's laws.

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   Answer: The statement is incorrect. Mass is the measure of an object's inertia (its resistance to changes in motion) and is measured in kilograms (kg). It is a scalar quantity and is the same everywhere. Weight is the force of gravity acting on an object, calculated as W = mg. It is a vector quantity, measured in newtons (N), and depends on the local gravitational field strength (g). So, a 60 kg mass has a weight of approximately 60 kg * 9.8 m/s² = 588 N on Earth.

   This question directly addresses the common confusion between mass and weight. By requiring an explanation, it forces the student to define both terms, state their units, and describe the relationship between them (W=mg), thus revealing the conceptual difference.

6. 6.An astronaut takes an object to the Moon, where the gravitational acceleration is about 1/6th of that on Earth. Which of the following properties of the object would be different on the Moon compared to Earth?

   • AIts mass
   • BIts inertia
   • CIts weight
   • DThe force required to accelerate it from rest
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   Answer: Its weight

   This question tests the student's ability to apply the definitions of mass and weight in a practical scenario. It highlights that mass is an intrinsic property of an object, while weight is an extrinsic property dependent on location. Understanding this distinction is crucial for applying Newton's second law correctly in different gravitational environments.

7. 7.A book is at rest on a horizontal table. Which of the following best describes the forces acting on the book as shown in a free-body diagram?

   • AOnly the force of weight acting downwards.
   • BThe normal force acting upwards and the force of the table acting downwards.
   • CThe force of weight acting downwards and the force of friction acting horizontally.
   • DThe force of weight acting downwards and an equal and opposite normal force acting upwards.
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   Answer: The force of weight acting downwards and an equal and opposite normal force acting upwards.

   This question assesses the ability to identify all relevant forces on a simple object in equilibrium. According to Newton's first law, an object at rest has a net force of zero. This means the downward force of weight (gravity) must be balanced by an equal and opposite upward force from the table, which is the normal force. A free-body diagram must include all forces.

8. 8.Imagine a box being pushed across a rough floor at a constant velocity. What forces would you draw on its free-body diagram?

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   Answer: The diagram should show four forces: weight (downwards), the normal force from the floor (upwards), the applied pushing force (in the direction of motion), and the kinetic friction force (opposite to the direction of motion). The upward normal force balances the downward weight, and the forward applied force balances the backward friction force.

   This question requires applying the concept of a free-body diagram to a scenario with motion and friction. Since the velocity is constant, Newton's first law says the net force is zero. This means the forces must be balanced in both the horizontal and vertical directions, requiring the student to identify the applied force, friction, weight, and the normal force and set them in opposition to each other.

9. 9.A block is sliding down a frictionless ramp tilted at an angle θ. In a free-body diagram for the block, the force of gravity (weight, W = mg) is drawn straight down. Why is it incorrect to also draw a force equal to 'mg sin θ' pointing down the slope as a separate force on the diagram?

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   Answer: It is incorrect because 'mg sin θ' is not a separate force acting on the block. It is the component of the gravitational force (weight) that acts parallel to the slope. A free-body diagram should only show the fundamental, external forces acting on the object, which in this case are the weight (mg) and the normal force from the ramp. The components are mathematical constructions used to analyze the effect of the weight along the incline.

   This question challenges the student to distinguish between actual forces and the components we use to analyze them. A common mistake is to draw both a force and its components on a free-body diagram. Understanding this distinction is crucial for correctly applying Newton's second law (F_net = ma) to solve problems on inclines. The net force is the vector sum of the *actual* forces (weight and normal force), not the sum of forces and their own components.

10. 10.A 10 kg box is pushed across a frictionless surface with a net force of 50 N. According to Newton's second law (F_net = ma), what is the acceleration of the box?

    • A0.2 m/s²
    • B5 m/s²
    • C50 m/s²
    • D500 m/s²
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    Answer: 5 m/s²

    This question assesses the ability to apply Newton's second law, F_net = ma, in a direct calculation. To find the acceleration (a), you rearrange the formula to a = F_net / m. Plugging in the values: a = 50 N / 10 kg = 5 m/s². This reinforces the direct relationship between net force, mass, and acceleration.

11. 11.A block is accelerating at 2 m/s². If the net force on the block is 10 N, what is its mass? Use the formula F_net = ma.

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    Answer: 5 kg

    This question requires rearranging Newton's second law (F_net = ma) to solve for mass (m = F_net / a). This tests a slightly different application of the formula, ensuring the student can manipulate it to find any of its three components. The calculation is 10 N / 2 m/s² = 5 kg.

12. 12.Imagine you are designing a new braking system for a 1500 kg car. To be safe, the car must be able to decelerate (have a negative acceleration) at a rate of 8 m/s². Based on Newton's second law (F_net = ma), what is the minimum net force the braking system must be able to apply to achieve this?

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    Answer: 12,000 N

    This question challenges the student to apply Newton's second law in a practical, real-world design scenario. It requires them to calculate the necessary force to produce a desired acceleration for a given mass (F_net = 1500 kg * 8 m/s² = 12,000 N). This moves beyond simple formula repetition to its application in engineering and problem-solving.

13. 13.An elevator is accelerating upwards. How does the normal force (the force the floor exerts on you, which you feel as your apparent weight) compare to your actual weight (mg)?

    • AThe normal force is less than your weight.
    • BThe normal force is equal to your weight.
    • CThe normal force is greater than your weight.
    • DThe normal force is zero.
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    Answer: The normal force is greater than your weight.

    According to Newton's second law (F_net = ma), for you to accelerate upward, the net force on you must be upward. The forces acting on you are the upward normal force and the downward force of gravity (your weight). For the net force to be upward, the normal force must be larger than your weight. This question requires applying the second law to a specific, non-equilibrium scenario.

14. 14.True or False: For a block resting on a frictionless inclined plane with an angle of θ, the normal force exerted by the plane on the block is equal to the block's weight (mg).

    • ATrue
    • BFalse
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    Answer: False

    This question challenges a common misconception. On a horizontal surface, the normal force equals the weight. However, on an inclined plane, the force of gravity (weight) must be resolved into components. The normal force is perpendicular to the surface and balances only the perpendicular component of the weight (mg cos θ), not the full weight.

15. 15.A box is sliding down a frictionless ramp tilted at an angle θ. What is the acceleration of the box along the ramp?

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    Answer: The acceleration is g sin θ.

    This question requires students to analyze the forces acting on an object on an inclined plane. The net force causing the acceleration down the ramp is the component of the gravitational force (weight) that is parallel to the ramp's surface, which is mg sin θ. Using Newton's second law, F_net = ma, we get mg sin θ = ma. The mass 'm' cancels out, leaving the acceleration a = g sin θ.