A model rocket with a mass of 500 grams is launched straight up. It achieves a speed of 20.0 m/s in 2.00 seconds. Ignoring any change in mass of the rocket due to expelled fuel, and neglecting air resistance, the average thrust force of the engines on the rocket is
A) 7.50 N.
B) 9.90 N.
C) 12.0 N.
D) 13.5 N.
E) 20.0 N

Question

Quizplus · Accepted Answer

The average thrust force can be calculated using Newton's second law, $F = ma$, where $m$ is the mass and $a$ is the acceleration. First, find the acceleration: $a = \frac{\Delta v}{\Delta t} = \frac{20.0 \, \text{m/s}}{2.00 \, \text{s}} = 10.0 \, \text{m/s}^2$. The mass of the rocket is 500 grams, which is 0.5 kg in SI units. Thus, $F = 0.5 \, \text{kg} \times 10.0 \, \text{m/s}^2 = 5.0 \, \text{N}$. However, this is just the force to achieve the acceleration. The total force (thrust) must also overcome gravity, which is $mg = 0.5 \, \text{kg} \times 9.8 \, \text{m/s}^2 = 4.9 \, \text{N}$. Therefore, the total average thrust force is $5.0 \, \text{N} + 4.9 \, \text{N} = 9.9 \, \text{N}$.

A Model Rocket with a Mass of 500 Grams Is