Physics Practice Test 2: Newton's Laws
- Newton's Laws
- 10 questions
- 15 minutes
This second set on Newton's laws focuses on systems: pulleys, blocks in contact and forces at angles. Use g = 9.8 m/s². Draw a free-body diagram for each object and choose the direction of motion as positive.
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Question 1
A 5.0 kg lamp hangs at rest from a single vertical cord. What is the tension in the cord?
Correct answer: C
The lamp is in equilibrium, so the tension balances its weight: T = mg = 5.0 × 9.8 = 49 N. 9.8 N is the weight of only 1 kg.
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Question 2
Masses of 3.0 kg and 5.0 kg hang on opposite sides of a light, frictionless pulley. What is the magnitude of their acceleration?
Correct answer: A
The net force is (5.0 − 3.0)(9.8) = 19.6 N and it moves 8.0 kg in total, so a = 19.6/8.0 = 2.45 m/s². 3.92 m/s² divides by only the 5.0 kg mass.
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Question 3
In the same pulley system (3.0 kg and 5.0 kg masses), what is the tension in the string?
Correct answer: D
For the 3.0 kg mass moving up: T − 3.0(9.8) = 3.0(2.45), so T = 3.0 × 12.25 ≈ 37 N. 29 N is only the weight of the lighter mass, which would mean no acceleration.
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Question 4
A person stands on a bathroom scale in an elevator that is moving downward at constant speed. How does the scale reading compare with the person's weight?
Correct answer: B
Constant speed means zero acceleration, so the normal force from the scale must exactly balance gravity. The reading drops below the weight only while the elevator accelerates downward.
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Question 5
Two forces act on a 2.0 kg object: 6.0 N east and 8.0 N north. What is the magnitude of the object's acceleration?
Correct answer: D
The forces are perpendicular, so the net force is √(6.0² + 8.0²) = 10 N and a = 10/2.0 = 5.0 m/s². 7.0 m/s² comes from adding the forces as if they pointed the same way.
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Question 6
A space probe far from any star or planet has its engines off. What will it do?
Correct answer: A
With no net force, Newton's first law says the velocity does not change. Motion does not need a force to continue; only a change in motion needs one.
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Question 7
A child pulls a 10 kg sled across level snow with a rope at 30° above the horizontal. The tension in the rope is 50 N. What is the normal force from the ground on the sled?
Correct answer: C
The rope's upward component is 50 sin 30° = 25 N, so N = mg − 25 = 98 − 25 = 73 N. 123 N adds the component instead of subtracting it, and 55 N uses cos 30° instead of sin 30°.
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Question 8
Which combination of units is equal to one newton?
Correct answer: B
From F = ma, a newton is a kilogram times a meter per second squared. kg·m/s is the unit of momentum and kg·m²/s² is the joule.
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Question 9
The same net force gives cart A an acceleration of 6.0 m/s². Cart B has three times the mass of cart A. What acceleration does the same force give cart B?
Correct answer: B
For a fixed force, acceleration is inversely proportional to mass, so a = 6.0/3 = 2.0 m/s². Multiplying by 3 (18 m/s²) treats mass as if it helped acceleration.
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Question 10
Blocks of 4.0 kg and 2.0 kg sit side by side on a frictionless floor. A horizontal 18 N force pushes on the 4.0 kg block, which pushes the 2.0 kg block. What force does the 4.0 kg block exert on the 2.0 kg block?
Correct answer: A
Both blocks accelerate at 18/6.0 = 3.0 m/s², and the 2.0 kg block needs 2.0 × 3.0 = 6.0 N. 12 N is the net force on the 4.0 kg block, not the contact force.
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