Click on each force value to see the data that John recorded. After reading the chapter on forces and Newton's laws, John wanted to investigate several new ideas for himself. Accordingly, he found a classmate who owned a small car, and determined that the car's mass was 1000 kg. He then procured a stop watch and a rope, which he tied to a spring scale that his physics teacher loaned him. He then attached the spring scale to the car's bumper one afternoon after school. Since the spring scale reading was in Newtons, John reasoned that he could put a known force on the car, as indicated by the spring scale reading, and he could measure the acceleration of the car from the kinematic equations that he had previously learned.
To begin his experiment, John had one of his friends watch the spring scale to ensure that he pulled with a constant force while the car rolled across the school parking lot in neutral. Another of John's friends used the stop watch to time the car in 1 meter intervals as John pulled it a 10m distance. John pulled with five different forces and measured the acceleration.
Click on each force value to see the data that John recorded.
Then click on the word "Graph" to see a graph of this data.
From this graph, John immediately saw that the acceleration of the car was directly proportional to the applied force.
John showed his results to his teacher, who encouraged John to do one more experiment in which he would hold the force constant and increase the mass of the auto by pulling several different and increasingly larger automobiles.
Accordingly, John reconvened his small group of helpers in the parking lot a few days later after school, and he pulled five different automobiles with a constant force of 1000 Newtons, with the following results:
Click on each force value to see the data that John recorded.
Then click on the word "Graph" to see a graph of this data.
From this graph, John immediately noted that when the mass doubled, the acceleration was cut in half, and when the mass tripled, the acceleration was decreased by a factor of 3. John concluded that the acceleration of an object is inversely proportional to the object's mass, which leads to Newton's 2nd law:
The acceleration of an object is directly proportional to the net external force acting on it and inversely proportional to the object's mass.
Newton's 2nd law is the only one of Newton's laws which is associated with an equation. This equation, in its most familiar form, is Fnet = ma. When John reworked Newton's 2nd law, and used algebra to solve for acceleration, he obtained a = F m . A brief inspection of this equation clearly indicates that acceleration is directly proportional to the applied force, and inversely proportional to the object's mass, which is exactly what John found out when he analyzed all of his data.
