Bennett Mechanical Force and Torque Questions and Answers 1 — Questions and Answers
Question 1: Two people are carrying a heavy log. If the log has a uniform weight and they are holding it at the ends, but one person's end is propped on their shoulder (higher up) while the other person holds it at waist level (lower down), who is supporting more of the log's weight?
- The person holding the log lower. (Correct answer)
- The person holding the log higher.
- They are both supporting equal weight.
- It depends on the length of the log.
Correct answer: The person holding the log lower.
Gravity acts vertically downwards. By holding the log at an angle, the lower person supports a greater component of the vertical force (weight) than the person holding it higher. The distribution of force is no longer equal as it would be on a horizontal plane.
Question 2: To balance the lever shown, where should a 20 lb weight be placed on the right side? (The lever is marked in equal distance units from the fulcrum).
- At mark 2
- At mark 4
- At mark 5 (Correct answer)
- At mark 10
Correct answer: At mark 5
The principle of torque states that for a lever to be in balance, the torque on both sides of the fulcrum must be equal (Torque = Force x Distance). The left side has a torque of 10 lbs * 10 units = 100 lb-units. To balance this with a 20 lb weight, the distance must be 100 lb-units / 20 lbs = 5 units. Therefore, the weight must be placed at mark 5.
Question 3: Which of the following actions generates the most torque to loosen a tight bolt with a wrench?
- Applying force close to the bolt.
- Applying force at the middle of the wrench handle.
- Applying force at the very end of the wrench handle. (Correct answer)
- Applying force at a 45-degree angle to the handle.
Correct answer: Applying force at the very end of the wrench handle.
Torque is the product of the force applied and the distance from the point of rotation (the bolt). To maximize torque with a given force, you must maximize the distance from the fulcrum. Applying force at the very end of the handle provides the longest lever arm, thus generating the most torque.
Question 4: A pulley system is used to lift a 100 kg crate. System A has one fixed pulley. System B has one fixed and one movable pulley. Which statement is true regarding the force required to lift the crate?
- System A requires less force.
- Both systems require the same force.
- System B requires less force. (Correct answer)
- The force depends on the rope's thickness.
Correct answer: System B requires less force.
A single fixed pulley only changes the direction of the force and has a mechanical advantage of 1. A system with one fixed and one movable pulley has a mechanical advantage of 2, meaning it effectively halves the force required to lift the weight. Therefore, System B requires less force.
Question 5: If a force of 30 Newtons is applied to a box, but the box does not move, what is the force of static friction?
- Less than 30 Newtons.
- Exactly 30 Newtons. (Correct answer)
- More than 30 Newtons.
- Zero.
Correct answer: Exactly 30 Newtons.
According to Newton's First Law, an object at rest stays at rest unless acted upon by a net force. Since the box is not moving, the net force on it is zero. This means the force of static friction is equal in magnitude and opposite in direction to the applied force. Therefore, the static friction is exactly 30 Newtons.
Question 6: Two gears are in mesh. Gear A has 15 teeth and is driving Gear B, which has 45 teeth. If Gear A makes 3 complete rotations, how many rotations does Gear B make?
- 3 rotations
- 9 rotations
- 1 rotation (Correct answer)
- 1/3 of a rotation
Correct answer: 1 rotation
The gear ratio is the ratio of the number of teeth on the driven gear (B) to the driver gear (A), which is 45:15 or 3:1. This means for every 3 rotations of the smaller gear (A), the larger gear (B) will complete only 1 rotation. Therefore, if Gear A rotates 3 times, Gear B will rotate once.
Bennett Mechanical Force and Torque Questions and Answers 1
Two people are carrying a heavy log.
If the log has a uniform weight and they are holding it at the ends, but one person's end is propped on their shoulder (higher up) while the other person holds it at waist level (lower down), who is supporting more of the log's weight?