Posts: 25
Joined: Sun Oct 07, 2018 12:20 pm
Occupation: Parent

Rocket Car Quaetion

Postby HopePressler1 » Tue Feb 02, 2021 1:39 pm

My son who is in the 5th grade used a model rocket engine powered car for his science fair project. He added weight to the car and asked the question "Will changing the weight of the car change the speed of the car?"

He found that adding more weight decreased the speed. I want to make sure he understands why this happened, and I need a little clarification. Should he ask "Will changing the mass of the car change the speed of the car?" Is the slower speed because of Newton's second law? Since there was a change in the mass and the force stayed the same then there was an inverse change in the acceleration. I understand that acceleration and speed are not the same, and I want to make sure that I am explaining this correctly and using the terminology correctly. Since the car is moving in a straight line is it correct to state that the added mass decreases the acceleration which decreases the speed?

He also launched the car going up a slope and asked the question "Will changing the slope of the ground change the speed of the car?" Would it be better to say "Will the rocket car's speed decrease when going up an inclined plane?" The car went slower going up the slope. Was this because of gravity pulling the car down the slope requiring more force? Is this also Newton's second law? Is it correct to say the same force was used (model rocket engine), but the gravitational force worked against the force of the model rocket engine decreasing the overall force? All of the research he found was about going down a slope not up. I couldn't find an easy answer either. Apparently most people test rolling things down slopes not launching them up slopes.

Thank You,

Student Expert
Posts: 28
Joined: Tue Dec 17, 2019 5:21 pm
Occupation: Student

Re: Rocket Car Quaetion

Postby cvionis » Sun Feb 07, 2021 9:45 am


The slower speed is indeed a result of Newton's second law, as it states that, assuming the force remains the same, mass and acceleration are inversely related (as mass increases, acceleration decreases, and vice versa). When mass is added to the car, its acceleration decreases; in other words, the car stops speeding up; or, depending on the magnitude of the mass, it slows down. This can then result in a loss of velocity/speed.

It would probably be best in this case for your child to ask, "Will changing the mass of the car affect the car's acceleration?" rather than to ask whether or not a change in the car's mass affects its speed. This will allow him to more directly and clearly integrate Newton's second law into his experiment, and back his experiment with the second law's equation, F=m*a. However, if your son wishes to discuss the relationship between mass and speed rather than acceleration, then it is advisable that he uses velocity instead of speed (velocity is the speed of an object in a certain direction, whereas speed does not take direction into account). This is due to the fact that acceleration (which is used in the Newton's second law equation you referred to) is defined as a change in velocity rather than speed. Also, using velocity instead of speed will make it much easier to find the relationships between how fast the car is, the car's acceleration, and the car's mass.

For his second experiment dealing with the slope/inclined plane, it would be better to use the second question you suggested ("Will the rocket car's speed decrease when going up an inclined plane?") rather than the first, as it is more specific. The reason why the car was slower going up the inclined plane was because the force of gravity was working against the force of the car, therefore causing the car to slow down (decelerate).

I hope these suggestions will prove helpful to you; if you have any more questions, please feel free to ask.


Posts: 351
Joined: Mon Oct 03, 2011 12:26 pm
Occupation: US Air Force Space & Missile Operations

Re: Rocket Car Quaetion

Postby theborg » Thu Feb 25, 2021 3:01 pm

Adding to the inclined experiment question.

Keep in mind that gravity is present in both experiments. It always acts toward the center of the Earth (aka nater). when the car is on a level surface, the force of gravity is acting perpendicular to the thrust of the engine, therefor not adding or subtracting anything. With the car, the thrust vector is along the path of the car. In the case of experiment 2, that means also on an incline, effectively reducing the force pushing the car. So, in experiment 1 F=m*a you are changing m. In experiment 2 you are changing F. Both having an impact on acceleration.

Return to “Grades K-5: Physical Science”