Difficulty: Medium

Average Score: 73%

Engines on a spacecraft are briefly fired, which causes the spacecraft to accelerate to a speed of 20,000 km/h.

What happens after the engines are shut off, assuming no other forces are acting on the spacecraft?

Rationale

The spacecraft continues to travel at a velocity of 20,000 km/h.

Once the engines are shut off, the spacecraft will maintain its speed due to the principle of inertia, as stated in Newton's first law of motion. In the absence of external forces, such as friction or gravitational pull, the spacecraft will not decelerate or change its velocity.

A (The spacecraft continues to travel at a velocity of 20,000 km/h.) CORRECT

This statement is correct because, according to Newton's first law, an object in motion will remain in motion with the same speed and in the same direction unless acted upon by an external force. Since the engines are off and no other forces are acting, the spacecraft maintains its velocity.

B (The spacecraft reverses direction.) YOUR ANSWER

This choice is incorrect because the spacecraft does not have any forces acting on it to cause a change in direction. The engines would need to fire in the opposite direction for the spacecraft to reverse its trajectory, which is not the case once the engines are shut off.

C (The spacecraft gradually slows to a stop.) YOUR ANSWER

This option is misleading as it implies that there are forces acting on the spacecraft to decelerate it, such as air resistance or friction. In the vacuum of space, these forces are negligible, and thus the spacecraft will not gradually slow down.

D (The spacecraft accelerates to a velocity greater than 20,000 km/h.) YOUR ANSWER

This statement is incorrect because the spacecraft cannot accelerate without the engines firing. Once the engines are shut off, it will not gain additional speed unless acted upon by another force.

Conclusion

In summary, when the engines of a spacecraft are turned off and no forces are exerted on it, the spacecraft will continue on its path at a constant speed of 20,000 km/h. This behavior exemplifies Newton's first law of motion, which underscores the principle of inertia. All other options inaccurately suggest changes in speed or direction that cannot occur without external influences.

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