Saved Tension forces always act upward. True False Question 4 (1 point) Newton’s third law states that forces always act in pairs but the two forces of any pair act on different objects. True False Question 7 (1 point) A rocket accelerates upward and the thrust of the engines overcome the frictional forces and the gravity acting against the rocket causing it to accelerate upwards. Which of Newton’s laws of motion best explains this situation? a) None of the laws explain this situation b) Newton’s Third Law c) Newton’s Second Law Od) Newton’s First Law Question 11 (1 point) Consider the following FBDs. B ? D Fr Fy Fr+FM FA+Fx E F F È An elevator is suspended by a cable and moves uniformly upward. Which of the above free body diagrams best represents the forces acting on the elevator? O a) B b) c Ode d) A O e D Question 13 (1 point) Which of the following would Newton’s first law alone not account for? A passenger leans forward in her seat when the car’s brakes are applied. During launch, astronauts feel that they are being pushed back into their seats. No matter how hard you push against it, the boulder would not move. O A cyclist pedals harder and increases the speed of her bike.
The Correct Answer and Explanation is:
Let’s go through each question, provide the correct answer, and explain the reasoning using at least 300 words for the full set.
Question 4:
Saved Tension forces always act upward.
Answer: False
Explanation:
Tension is a pulling force transmitted through a string, rope, cable, or similar object. It acts along the direction of the rope or cable, not necessarily upward. The direction of the tension force depends on the configuration of the system. For example:
- In a hanging object, tension acts upward because the rope pulls upward against the downward force of gravity.
- If two people pull on a rope in opposite horizontal directions, the tension acts horizontally.
- If a rope is inclined, the tension force has both horizontal and vertical components.
So, it’s incorrect to say tension always acts upward. It acts along the rope and away from the object, aligning with the rope’s direction.
Question 7:
Which of Newton’s laws of motion best explains the rocket accelerating upward?
Correct Answer: c) Newton’s Second Law
Explanation:
Newton’s Second Law states:
F = ma, where F is the net force, m is mass, and a is acceleration.
In this rocket scenario:
- The rocket engines produce thrust.
- Gravity and air resistance oppose this motion.
- When thrust > gravity + resistance, the net force becomes positive upward, and thus the rocket accelerates upward.
This is a perfect application of Newton’s Second Law. It explains how acceleration results from a net force acting on the rocket. Newton’s Third Law (action-reaction pairs) does explain the reaction thrust from expelling gas, but the question specifically focuses on why the rocket accelerates upward—making the Second Law most relevant.
Question 11:
An elevator is suspended by a cable and moves uniformly upward. Which FBD best represents this?
Correct Answer: b) C
Explanation:
Uniform upward motion means constant velocity—there is no acceleration. According to Newton’s First Law and Second Law:
- The net force is zero.
- Tension force (upward) is equal in magnitude to the gravitational force (downward).
So, the FBD should show:
- Two opposing forces of equal length: tension upward and gravity downward.
This corresponds to diagram C (assuming C shows balanced vertical forces), where Fy = weight and they cancel out, meaning zero net force and uniform motion.
Question 13:
Which of the following would Newton’s First Law alone not account for?
Correct Answer: A cyclist pedals harder and increases the speed of her bike.
Explanation:
Newton’s First Law explains motion in the absence of a net force:
- An object at rest stays at rest.
- An object in motion continues in a straight line at constant speed unless acted upon by a net external force.
Let’s analyze the choices:
- Passenger leans forward: Inertia (1st law)
- Astronauts feel pushed back: Inertia
- Boulder doesn’t move despite pushing: No net force
- Cyclist pedals harder and accelerates: This involves a net force causing acceleration, which is explained by Newton’s Second Law, not the first.
So, the last option is not explained by Newton’s First Law alone, making it the correct answer.