To determine which objects are in mechanical equilibrium, we need to understand the conditions required for mechanical equilibrium. An object is in mechanical equilibrium if it is either at rest or moving with a constant velocity. This means there are no unbalanced forces acting on the object, and it is not accelerating.
Let's analyze each of the given scenarios:
1. A car parked at the Seven Eleven:
- A parked car is at rest, meaning its velocity is zero and constant. Since there are no unbalanced forces acting on the car, it is in mechanical equilibrium.
2. A car moving north at a constant speed of 60 mph:
- A car moving at a constant speed in a straight line has a constant velocity. Since there is no acceleration, there are no unbalanced forces acting on the car. Therefore, it is in mechanical equilibrium.
3. A car speeding up from 7 mph to 20 mph in 20 seconds:
- A car that is speeding up is accelerating. Accelerating means that there are unbalanced forces acting on the car, so it is not in mechanical equilibrium.
4. A car moving in a perfect circle at 60 mph:
- Even though the car's speed might be constant, its direction is continuously changing as it moves in a circle. This change in direction implies acceleration (centripetal acceleration), indicating unbalanced forces are acting on the car. Thus, it is not in mechanical equilibrium.
Now, based on the analysis:
- The first car (parked at Seven Eleven) is in mechanical equilibrium.
- The second car (moving north at a constant speed of 60 mph) is also in mechanical equilibrium.
- The other two scenarios involve acceleration and are therefore not in mechanical equilibrium.
Thus, the correct statement is: "Two of the four answers are correct."
