Quiz: Applications of Newton's Laws
Test your understanding of applications of newton's laws with these 10 questions.
1. What is kinetic friction?
- The friction that prevents motion from starting
- The friction acting on a moving object
- The maximum possible friction force
- The friction that changes with time
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The correct answer is B. Kinetic friction is the friction force acting on an object that is already in motion, sliding across a surface. It opposes the direction of motion and is typically less than maximum static friction. Static friction prevents motion from starting, while kinetic friction opposes ongoing motion.
Concept Tested: Kinetic Friction
2. How does the coefficient of kinetic friction compare to the coefficient of static friction?
- They are always equal
- Kinetic friction coefficient is typically larger
- Kinetic friction coefficient is typically smaller
- The relationship varies unpredictably
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The correct answer is C. The coefficient of kinetic friction (μₖ) is typically smaller than the coefficient of static friction (μₛ). This is why objects often accelerate slightly when they first start moving—the kinetic friction is less than the maximum static friction that was holding them in place.
Concept Tested: Coefficient of Friction
3. A 10 kg box is being pulled across a horizontal floor with a rope. The tension in the rope is 50 N. Which statement is correct?
- The tension force is balanced by the weight of the box
- The tension force pulls the box and is opposed by friction
- The tension force has a vertical component equal to the weight
- Tension cannot act on boxes, only on rope
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The correct answer is B. Tension is the pulling force transmitted through the rope. When the rope pulls the box, tension pulls the box forward. Friction opposes this motion by pulling backward. If tension exceeds friction, the box accelerates forward.
Concept Tested: Tension
4. A block of mass m is placed on an inclined plane at angle θ to the horizontal. What is the component of weight parallel to the incline?
- mg
- mg cos(θ)
- mg sin(θ)
- mg tan(θ)
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The correct answer is C. Weight points straight down (mg). On an incline, weight can be decomposed into two components: parallel to the incline (mg sin(θ)) and perpendicular to the incline (mg cos(θ)). The parallel component pulls the block down the slope.
Concept Tested: Inclined Plane
5. In an Atwood machine with two masses (M = 5 kg and m = 3 kg) connected by a rope over a pulley, which statement is correct?
- Both masses experience the same acceleration
- The heavier mass always remains stationary
- The rope tension is different on each side of the pulley
- The system has no acceleration because the pulley is frictionless
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The correct answer is A. In an Atwood machine, the two masses are connected by an inextensible rope, so they must move with the same magnitude of acceleration. However, they move in opposite directions—one accelerates downward while the other accelerates upward. The lighter mass moves up, the heavier mass moves down.
Concept Tested: Atwood Machine
6. What is centripetal force?
- A force that always points toward the center of circular motion
- A force that exists only in space without gravity
- A new type of fundamental force discovered by Newton
- The friction force that keeps a car from skidding
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The correct answer is A. Centripetal force is any force (or combination of forces) that acts toward the center of a circular path, providing the acceleration needed to keep an object moving in a circle. It's not a new type of force—gravity, tension, friction, or normal force can all provide centripetal force.
Concept Tested: Centripetal Force
7. A car travels around a horizontal curve of radius 50 m at constant speed. For the car to maintain circular motion, what must be true?
- The car's velocity must be zero
- The car needs a centripetal force directed toward the center of the curve
- The car's acceleration is zero because speed is constant
- The normal force must equal the car's weight
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The correct answer is B. Even though the car's speed is constant, it is accelerating because its direction is changing. Centripetal acceleration is directed toward the center of the curve, and centripetal force must be applied to maintain this acceleration. Friction between tires and road typically provides this force.
Concept Tested: Centripetal Acceleration
8. Why are highway curves banked (tilted) in many places?
- To increase the speed limit safely
- To use the normal force component to provide part of the centripetal force, reducing reliance on friction
- To prevent the road from wearing unevenly
- To make the curves more visually interesting
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The correct answer is B. Banking tilts the road surface so that the normal force from the road has a component pointing toward the center of the curve. This component contributes to centripetal force, so friction alone doesn't need to provide all of it. This allows safer driving at higher speeds.
Concept Tested: Banked Curves
9. A box slides down a 30° incline with kinetic friction coefficient μₖ = 0.2. What is the net force along the incline for a 10 kg box? (Use g = 10 m/s²)
- 20 N down the incline
- 40 N down the incline
- 34 N down the incline
- 6 N down the incline
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The correct answer is C. Component of weight down the incline: mg sin(30°) = 10 × 10 × 0.5 = 50 N. Normal force: N = mg cos(30°) = 10 × 10 × 0.866 ≈ 86.6 N. Friction force: f = μₖN = 0.2 × 86.6 ≈ 17.3 N (up the incline). Net force down = 50 - 17.3 ≈ 33 N (approximately 34 N).
Concept Tested: Friction
10. In a pulley system with a movable pulley, what advantage does it provide?
- It changes the direction of the force but provides no mechanical advantage
- It requires less force to lift an object, trading effort for distance
- It eliminates friction completely
- It reduces the weight of the object being lifted
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The correct answer is B. A movable pulley provides mechanical advantage by distributing the load over multiple rope segments. If a movable pulley has two supporting rope segments, you only need to apply half the load force, but you must pull the rope twice the distance. This is the essence of simple machines—they trade force for distance.
Concept Tested: Pulley Systems