Find the Horsepower of a Toy Car

Abstract

Have you ever heard someone describe how much horsepower a car has? Why do we use horses to measure how powerful cars are? What exactly is horsepower? How is horsepower related to things like speed and torque? Find out in this fun science project as you use a simple setup to find the horsepower of a battery-operated toy car.

Summary

Areas of Science

Physics

Difficulty

Method

Scientific Method

Time Required

Very Short (≤ 1 day)

Prerequisites

None

Material Availability

Readily available

Cost

Low ($20 - $50)

Safety

No issues

Credits

Ben Finio, PhD, Science Buddies

Science Buddies is committed to creating content authored by scientists and educators. Learn more about our process and how we use AI.

https://www.youtube.com/watch?v=ZbdS3nATiiE

Objective

Calculate the horsepower of a battery-operated toy car at different speeds.

Introduction

In the United States, we usually describe how powerful cars are in terms of their horsepower (hp). You probably associate more horsepower with faster cars (Figure 1) or the ability to tow heavier loads (Figure 2). Modern cars can have hundreds of horsepower! But why do we describe the power of a car in terms of a unit related to horses?

Image Credit: Pixabay user oskaline / Pixabay License
Figure 1. Sports cars are designed to drive fast.

Image Credit: Wikimedia Commons user IFCAR / Public domain
Figure 2. Pickup trucks are designed to haul or tow heavy loads.

Horsepower is a unit that was developed back when steam engines were first invented and started replacing horses. People needed a way to compare the power output of a new steam engine to the power output of a horse, so the comparison made sense. While it may seem outdated, the convention has stuck around, and we still use horsepower to measure the power of cars today.

But what exactly does power mean? Power is a measurement of how fast you can exert energy or do work, such as lifting a heavy weight. In general, cars with more power can go faster or pull heavier loads, but they are generally not designed to do both. You probably would not drive a pickup truck on a racetrack or tow a trailer with a race car!

When you talk about cars, you might also use words like "speed" and "torque." Speed is how fast the car can go, usually measured in miles per hour (mph) in the U.S. or kilometers per hour (kph) in other countries. (Physicists use the word velocity, which includes both an object's speed and its direction). Torque is the "twisting force" generated by the engine. You exert torque with your hand whenever you twist a doorknob or a screwdriver. In a car, the torque from the engine is transferred to the wheels. You might wonder how speed, torque, and power are all related. In this project you will do an experiment to find out.

In this science project, you will measure the speed of a battery-powered toy car as it drives up a ramp (Figure 3). By measuring the speed, you can use some equations to calculate the car's power and torque. These equations are summarized in the Technical Note below, and you can read more about them using the references in the bibliography or a physics textbook. Do not worry if you have not taken a physics class yet. The procedure section of this project will show you how to do all the calculations.

How many horsepower do you think your toy car has? Do you think the car's power, speed, and torque will change if you change the angle of the ramp? Try this project and find out!

Image Credit: Ben Finio, Science Buddies / Science Buddies

A battery-powered toy car drives up a ramp made from a board propped up with blocks at one end.

Figure 3. Experimental setup for this project.

Technical Note

Figure 4 shows the setup for the physics problem in this experiment. A car of mass m drives a distance d up a ramp, with a vertical change in height h. The car's wheels have a radius r. By measuring these parameters, you can calculate the car's velocity, power, and torque using the following equations.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 4. Diagram for the physics problem in this experiment.

If the car takes t seconds to drive up the ramp, then the car's linear velocity v is

Equation 1: $\: v = \frac{d}{t}$

The car's change in potential energy PE is

Equation 2: $\: PE = mgh$

where g is the acceleration due to gravity.

Ignoring the car's change in kinetic energy (i.e. assuming it immediately accelerates to its top speed when it starts), the power P exerted by the car is equal to the change in potential energy divided by the time it takes to go up the ramp:

Equation 3: $\: P = \frac{PE}{t}$

The torque T exerted by the wheels is related to the car's power output by the following equation. (Note that this is the total torque exerted by all four wheels, not the torque of a single wheel.)

Equation 4: $\: P = T\omega$

where ω is the angular velocity of the wheels in radians per second.

The angular velocity of the wheels is related to the car's linear velocity by the equation

Equation 5: $\: v = r\omega$

Table 1 provides a summary of all of the variables in the equations above and their SI units.

Swipe left to see more

Variable	Units	Description
d	meters (m)	Distance the car travels up the ramp
h	meters (m)	Vertical change in height of the car
m	kilograms (kg)	Mass of the car
r	meters (m)	Radius of the car's wheels
t	seconds (s)	Amount of time it takes car to travel up the ramp
v	meters per second (m/s)	Car's linear velocity
PE	joules (J)	Car's change in potential energy
g	meters per second squared (m/s2)	Acceleration due to gravity (9.81 m/s2)
P	watts (W)	Power exerted by the car
T	newton meters (Nm)	Total torque exerted by all four wheels
ω	radians per second (rad/s)	Angular velocity of the wheels

Table 1. Summary of variables and their units.

You can convert power from watts to horsepower using the equation

Equation 6: $\: 1\: W = \frac{1}{745.7} \:hp$

You can convert angular velocity from radians per second to degrees per second using the equation

Equation 7: $\: 1\: \frac{rad}{s} = 57.3\: \frac{deg}{s}$

And you can convert degrees per second to revolutions per minute (RPM) using the equation

Equation 8: $\: 1\: \frac{deg}{s} = 60\: \frac{deg}{min} = \frac{1}{6} \:RPM$

Terms and Concepts

• Horsepower
• Power
• Speed
• Velocity
• Torque

Questions

• Why is power important for a car?
• What are the differences between power, speed, and torque?
• How do you think the car's speed will change if you make the ramp steeper?
• How do you think the car's torque will change if you make the ramp steeper?
• How do you think the car's power will change if you make the ramp steeper?

Materials and Equipment

• Battery-powered toy car
• Wooden board. The required size will depend on the size and speed of your car.
  • The board should be wide enough that the car can easily drive on it without falling off. If the board is too narrow, the car may fall off the edges easily.
  • The board should be long enough that you can time how long it takes the car to drive up the ramp. If the board is too short, the car may drive up it too quickly, making it difficult to time.
• Stack of objects (wooden blocks, thick books, cardboard boxes, etc.) to hold up the wooden board to make a ramp
• Stopwatch
• Tape measure
• Level, meterstick, or other straight edge that is about as long as the wooden board
• Volunteer to help with the experiment. (It is difficult to operate the car and the stopwatch at the same time by yourself.)
• Kitchen scale
• Pencil
• Lab notebook

This project follows the Scientific Method. Review the steps before you begin.

1. Draw a start line at one end of the board. Place the car so the rear wheels are almost touching one end of the board and draw the line at the front end of the car (Figure 5). You will start the car from the same place in each trial.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 5. Start line drawn on the board.

2. Make a data table like Table 2 in your lab notebook or in a spreadsheet. This is the data you will record initially, but you will add more columns for calculated values later.

Swipe left to see more

Trial	Mass (kg)	Wheel radius (m)	Distance (m)	Height (m)	Time (s)
1
2
3

Table 2. Example data table.

3. Measure your car's mass (using a kitchen scale), its wheel radius, and the distance from the start line to the end of the board and record them in your data table. These values will stay the same for each trial. (See Variations section for ideas about changing them.) Make sure you record mass in kilograms and the distances in meters, since you will need to use the correct units for your calculations later.
4. Prop up one end of the board a little bit — for example, using a single thick book, a wooden block, or a small box (Figure 6).

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 6. Ramp set up at a shallow angle.

5. Measure the vertical height from the start line to the top edge of the other end of the board (see Figure 4 in Introduction) and record it in your data table. It will be easiest to measure this height if you use another straight edge (like a meterstick, level, or another wooden board) as a horizontal reference for the start line. If you know trigonometry, you can also use a protractor to measure the angle of the board and calculate the height. This height will change as you increase the angle of the board.
6. Get your stopwatch and volunteer helper ready. One person should operate the stopwatch and one person should operate the car.
   1. Turn the car on and hold it just above the board at the start line.
   2. Gently place the car on the board and immediately start the stopwatch.
   3. Grab the car right when the front wheels reach the top of the board and immediately stop the stopwatch.
   4. Turn the car off.
   5. Record the time in your data table.
7. Repeat step 6 two more times, for a total of three trials at the same height.
8. Increase the angle of the ramp (for example, by propping it up with another wooden block) and repeat step 6 three times at the new height.
9. Keep increasing the angle of the ramp, performing three trials at each height, until the ramp is too steep for the car to go up. Depending on the car, the wheels might not spin at all because the car can no longer lift its own weight up the ramp, the wheels might start slipping because there is not enough friction, or the car might even flip over backwards if it is too top-heavy.
10. Add columns to your data table for velocity, power, and torque. You can calculate the values for each trial using these equations:

    Equation 9: $\: velocity\: (meters\: per\: second) = \frac{distance\: (meters)}{time\: (seconds)}$

    Equation 10: $\: power\: (watts) = \frac{mass\: (kilograms)\: \times \:9.81 (meters\: per\: second\: squared)\: \times \:height\: (meters)}{time\: (seconds)}$

    Equation 11: $\: torque\: (newton\: meters) = \frac{power\: (watts)\: \times \:radius\: (meters)}{velocity\: (meters\: per\: second)}$

11. Analyze your data.
    1. Make a graph with ramp height on the horizontal axis and velocity on the vertical axis. How does the car's velocity change as the ramp steepness increases?
    2. Make a graph with velocity on the horizontal axis and torque on the vertical axis. What is the relationship between torque and velocity?
    3. Make a graph with velocity on the horizontal axis and power on the vertical axis. What is the relationship between power and velocity?
12. Add a column to your data table and convert your power ratings in watts to power in horsepower using the following equation. What is the maximum horsepower that you found for your toy car? How does this compare to the horsepower of a real car?

    Equation 12: $\: power\: in\: horsepower\: = \frac{power\: in\: watts}{745.7}$

Ask an Expert

Do you have specific questions about your science project? Our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Post a Question

Global Goals

The United Nations Sustainable Development Goals (UNSDGs) are a blueprint to achieve a better and more sustainable future for all.

This project explores topics key to Industry, Innovation and Infrastructure: Build resilient infrastructure, promote sustainable industrialization and foster innovation.

Variations

Careers

If you like this project, you might enjoy exploring these related careers:

Automotive Engineer

Log in to add favorite More Menu

• Read More

Career Profile

Cars are an important part of our daily lives. We depend on them to perform everyday tasks—getting to and from school and work, sports practice, grocery shopping, and various errands—and also to keep us safe while doing so. Our cars can keep us cool or warm while we drive them, and they even help us find our way. The automobile is made up of complicated braking, steering, and electrical systems, in addition to the engine and drive train. All of these systems require a tremendous… Read more

Automotive Technician

Log in to add favorite More Menu

• Read More

Career Profile

There are so many different types of cars and trucks on the road: sports cars, sport utility vehicles (SUVs), pickup trucks, and even electric vehicles and hybrids. Have you ever wondered how they work? And, how to keep them running? How does the engine make a car's wheels turn? Why does pushing the brake pedal make a car stop? Automotive service technicians (also called mechanics) can pop open the hoods of these different types of cars and answer these questions. Automotive technicians know… Read more

Physics Teacher

Log in to add favorite More Menu

• Read More

Career Profile

Our universe is full of matter and energy, and how that matter and energy moves and interacts in space and time is the subject of physics. Physics teachers spend their days showing and explaining the marvels of physics, which underlies all the other science subjects, including biology, chemistry, Earth and space science. Their work serves to develop the next generation of scientists and engineers, including all healthcare professionals. They also help all students better understand their… Read more

News Feed on This Topic

 

, ,

Explore Our Science Videos

Measure Electrolytes in Sports Drinks - Science Project

How to Use a Force Sensor with an Arduino (Lesson #23)

DIY Adaptive Switch with Craft Materials | Science Project