SIFT Physics and Simple Machines 4 — Questions and Answers
Question 1: Power is defined as:
- The rate at which work is done (Correct answer)
- The total work performed
- Force multiplied by mass
- Energy stored in a system
Correct answer: The rate at which work is done
Power = Work / Time, measuring how quickly energy is transferred or work is done, expressed in watts (W).
Question 2: An object at rest remains at rest unless acted upon by an unbalanced force. This describes:
- Newton's First Law (inertia) (Correct answer)
- Newton's Second Law
- Newton's Third Law
- The law of conservation of momentum
Correct answer: Newton's First Law (inertia)
Newton's First Law, also called the law of inertia, states that objects resist changes in their state of motion.
Question 3: A block-and-tackle system uses 5 rope segments to support a load. What is its ideal mechanical advantage?
- 5 (Correct answer)
- 2.5
- 10
- 1
Correct answer: 5
In a block-and-tackle system, the ideal MA equals the number of rope segments supporting the load, which is 5.
Question 4: When a ball is thrown upward, its kinetic energy at the peak of its trajectory is:
- Zero (Correct answer)
- Maximum
- Equal to its initial kinetic energy
- Equal to its weight
Correct answer: Zero
At the peak, the ball momentarily stops (velocity = 0), so kinetic energy (½mv²) equals zero; all energy has converted to potential energy.
Question 5: The efficiency of a machine is defined as:
- (Useful work output / Work input) × 100% (Correct answer)
- (Work input / Useful work output) × 100%
- Mechanical advantage × 100%
- Load / Effort × Friction
Correct answer: (Useful work output / Work input) × 100%
Efficiency measures what fraction of input energy does useful work, always less than 100% due to friction and other losses.
Question 6: A hydraulic jack multiplies force by having a small piston with area 2 cm² and a large piston with area 20 cm². What force does the large piston exert if 100 N is applied to the small piston?
- 1000 N (Correct answer)
- 10 N
- 100 N
- 200 N
Correct answer: 1000 N
Pascal's principle: F_large / A_large = F_small / A_small, so F_large = 100 N × (20/2) = 1000 N.
Question 7: Momentum is conserved in a collision between two objects when:
- No external net force acts on the system (Correct answer)
- Kinetic energy is also conserved
- The objects stick together
- The collision is elastic only
Correct answer: No external net force acts on the system
Conservation of momentum applies whenever the net external force on a system is zero, regardless of collision type.
