Balanced and unbalanced forces Transitional Reasoning Download PDF

Questions

1. 1.

   A cable elevator carrying passengers travels upward inside a tall office building. During Segment 1, the elevator slows down as it approaches its destination, decreasing its upward speed from 6 m/s to 2 m/s. During Segment 2, the elevator continues rising toward the landing doors at a steady speed of 2 m/s. Which statement correctly evaluates the vertical forces acting on the elevator during these two segments?

   • A) In Segment 1, the forces are unbalanced with the upward force exceeding the downward force, whereas in Segment 2, the forces are balanced.
   • B) In Segment 1, the forces are unbalanced with the downward force exceeding the upward force, whereas in Segment 2, the forces are balanced.
   • C) In Segment 1, the forces are balanced because the elevator is coming to rest, whereas in Segment 2, the forces are unbalanced with the upward force exceeding the downward force.
   • D) In both Segment 1 and Segment 2, the forces are unbalanced with the upward force exceeding the downward force to keep the elevator moving against gravity.
2. 2.

   A student investigates horizontal forces by pushing on a stationary 30 kg crate resting on a rough classroom floor. In Stage 1, the student pushes horizontally with 50 N to the east, but the crate does not move; in Stage 2, the push is increased to 80 N to the east, and the crate still remains motionless. In Stage 3, the student applies a 120 N eastward push, causing the crate to accelerate across the floor. Which statement correctly analyzes the balance of horizontal forces acting on the crate across these stages?

   • A) In Stages 1 and 2, the horizontal forces are unbalanced because the student's push is active, but the crate does not move because its inertia is greater than the applied force.
   • B) In Stage 1, the horizontal forces are balanced, but in Stage 2, the forces are unbalanced because an 80 N push exceeds the original 50 N opposing friction force.
   • C) In Stages 1 and 2, the horizontal forces are balanced because static friction equals the applied push in each case, while in Stage 3, the forces are unbalanced because the push exceeds the maximum static friction.
   • D) In all three stages, the horizontal forces are unbalanced because any external push applied to an object produces a net force in the direction of the push.
3. 3.

   A student observes two scenarios involving an identical tennis ball: in Scenario 1, the ball rests motionless on a flat table; in Scenario 2, the ball is tossed vertically upward and reaches its highest peak, where its velocity is momentarily zero. The student investigates whether the forces acting on the ball are balanced or unbalanced at these exact moments. Neglecting air resistance, which statement correctly evaluates the state of the forces in each scenario?

   • A) In both scenarios, the forces are balanced because the ball has a speed of zero in each case.
   • B) In Scenario 1, the forces are unbalanced because normal force and gravity pull in opposite directions, while in Scenario 2, the forces are balanced because upward motion has ceased.
   • C) In Scenario 1, the forces are unbalanced because gravity continuously pulls downward, while in Scenario 2, the forces are balanced because upward momentum counters gravity.
   • D) In Scenario 1, the forces are balanced because the upward normal force equals the downward gravitational force, whereas in Scenario 2, the forces are unbalanced because only the downward force of gravity acts on the ball.
4. 4.

   A dynamics cart on a frictionless horizontal track is pulled by two opposing horizontal cords, where Cord 1 continuously exerts a constant force of 18 N to the left. In Trial 1, the cart moves to the right at a constant velocity of 1.5 m/s. In Trial 2, the tension in Cord 2 is adjusted so that the cart speeds up toward the right. Which statement correctly evaluates the balance of forces and the magnitude of the force exerted by Cord 2 in each trial?

   • A) In Trial 1, the forces are unbalanced with Cord 2 exerting more than 18 N because the cart is moving rightward, whereas in Trial 2, the forces are balanced because the speed is increasing continuously.
   • B) In Trial 1, the forces are balanced with Cord 2 exerting 0 N because maintaining motion requires no forward pull, whereas in Trial 2, Cord 2 exerts exactly 18 N to produce acceleration.
   • C) In Trial 1, the forces are unbalanced with Cord 2 exerting less than 18 N to allow forward coasting, whereas in Trial 2, Cord 2 exerts exactly 18 N to cause the cart to speed up.
   • D) In Trial 1, the forces are balanced with Cord 2 exerting exactly 18 N, whereas in Trial 2, the forces are unbalanced with Cord 2 exerting more than 18 N.
5. 5.

   A stationary 15 N scientific monitor rests on a flat, horizontal workbench, experiencing balanced vertical forces. A student then presses straight down on top of the monitor with a steady force of 5 N, and the monitor remains completely motionless on the bench. Which statement correctly explains why the monitor remains at rest in terms of balanced and unbalanced forces?

   • A) The downward push temporarily cancels the upward normal force, allowing gravity alone to keep the monitor balanced at rest.
   • B) The forces acting on the monitor become unbalanced with a net downward force of 5 N, but the rigid bench mechanically blocks movement.
   • C) The monitor remains at rest because external contact forces can only produce an unbalanced net force if they act in a horizontal direction.
   • D) The upward normal force from the workbench increases to 20 N, balancing the total downward force and keeping the net force equal to zero.
6. 6.

   A research sled glides across a flat ice sheet at a constant velocity of 5 m/s directed north. Student X argues that a continuous net northward force is required to sustain this forward motion, while Student Y argues that the horizontal forces acting on the sled are balanced. While still sliding northward, the sled deploys a rear drag brake that exerts an opposing horizontal force of 30 N directed south with no forward force applied. Which statement correctly identifies which student is right and predicts the effect of the deployed brake on the sled's motion?

   • A) Student X is correct; once the brake engages, the horizontal forces balance each other, causing the sled to stop instantly.
   • B) Student Y is correct; once the brake engages, the horizontal forces become unbalanced, causing the sled to slow down while moving north.
   • C) Student Y is correct; once the brake engages, the horizontal forces remain balanced, enabling the sled to maintain its speed of 5 m/s.
   • D) Student X is correct; once the brake engages, the horizontal forces become unbalanced, causing the sled to speed up toward the north.
7. 7.

   A supply package attached to a parachute falls vertically toward the ground at a steady, constant speed because the downward force of gravity is equal in magnitude to the upward air resistance. As it approaches the landing zone, an upward gust of wind increases the air resistance so that it exceeds the package's weight. Based on the principles of balanced and unbalanced forces, how do the forces and the package's motion change during this gust?

   • A) The forces become unbalanced with a net upward force, causing the package to instantly reverse direction and move upward.
   • B) The forces become unbalanced with a net upward force, causing the downward falling speed of the package to decrease.
   • C) The forces remain balanced because gravity still pulls downward, allowing the package to continue falling at its constant speed.
   • D) The forces become unbalanced with a net downward force, causing the package to accelerate faster toward the ground.
8. 8.

   A science team analyzes the motion of four automated rovers operating on a flat surface: Rover 1 travels in a straight line at a constant speed, Rover 2 remains stationary at a charging dock, Rover 3 increases its speed along a straight path, and Rover 4 steers around a curved track at a constant speed. Which statement correctly evaluates the balance of forces acting on these rovers?

   • A) Rover 1 experiences an unbalanced forward force because continuous motion requires a non-zero net force.
   • B) Rover 4 experiences balanced forces because its speed does not increase or decrease along the curve.
   • C) Rover 3 and Rover 4 both experience unbalanced forces because their velocities are changing.
   • D) Rover 2 is the only rover experiencing balanced forces because stationary objects are the only ones with zero net force.
9. 9.

   An automated transport cart is moving east along a flat test track. In the first phase of testing, the cart's motor provides a forward thrust of 50 N while friction and air resistance together provide an opposing force of 50 N. In the second phase, the motor increases its forward thrust to 75 N while the total opposing force remains 50 N. Which statement accurately describes the forces and the resulting motion of the cart across both phases?

   • A) In the first phase, the cart immediately comes to a stop due to equal opposing forces; in the second phase, it moves at a steady higher speed.
   • B) In the first phase, the balanced forces allow the cart to maintain a constant speed, while in the second phase, the unbalanced net force causes it to speed up.
   • C) In the first phase, the cart accelerates forward because of its forward thrust, while in the second phase, the forces become balanced and maintain its speed.
   • D) In both phases, the cart must experience an unbalanced net force forward because an object cannot remain in forward motion without a net driving force.