These boys are racing around the track at Newton’s Skate Park. The boy who can increase his speed the most will win the race. Tony, who is closest to the camera in this picture, is bigger and stronger than the other two boys, so he can apply greater force to his skates.
Q: Does this mean that Tony will win the race?
A: Not necessarily, because force isn’t the only factor that affects acceleration.
Whenever an object speeds up, slows down, or changes direction, it accelerates. Acceleration occurs whenever an unbalanced force acts on an object. Two factors affect the acceleration of an object: the net force acting on the object and the object’s mass. Newton’s second law of motion describes how force and mass affect acceleration. The law states that the acceleration of an object equals the net force acting on the object divided by the object’s mass. This can be represented by the equation:
Q: While Tony races along on his rollerblades, what net force is acting on the skates?
A: Tony exerts a backward force against the ground, as you can see in the Figure below, first with one skate and then with the other. This force pushes him forward. Although friction partly counters the forward motion of the skates, it is weaker than the force Tony exerts. Therefore, there is a net forward force on the skates.
Direct and Inverse Relationships
Newton’s second law shows that there is a direct relationship between force and acceleration. The greater the force that is applied to an object of a given mass, the more the object will accelerate. For example, doubling the force on the object doubles its acceleration.
The relationship between mass and acceleration is different. It is an inverse relationship. In an inverse relationship, when one variable increases, the other variable decreases. The greater the mass of an object, the less it will accelerate when a given force is applied. For example, doubling the mass of an object results in only half as much acceleration for the same amount of force.
Q: Tony has greater mass than the other two boys he is racing (pictured in the opening image). How will this affect his acceleration around the track?
A: Tony’s greater mass will result in less acceleration for the same amount of force.
- Newton’s second law of motion states that the acceleration of an object equals the net force acting on the object divided by the object’s mass.
- According to the second law, there is a direct relationship between force and acceleration and an inverse relationship between mass and acceleration.
At the following URL, use the simulator to experiment with force, mass, and acceleration. Click the "graph acceleration" and "graph applied force" buttons to display graphs for these values against time. Turn off friction in the toolbar to the right. Start with the textbook, set the slider next to the graph of applied force to its highest setting and then press go. Take a screenshot of your graphs to save them for later. Repeat this with several objects of different weights. Now compare the graphs of acceleration and applied force for each object with those of the other objects. Describe in words the relationship you see between mass and acceleration when the applied force is constant.
- State Newton’s second law of motion.
- How can Newton’s second law of motion be represented with an equation?
- If the net force acting on an object doubles, how will the object’s acceleration be affected?
- Tony has a mass of 50 kg, and his friend Sam has a mass of 45 kg. Assume that both friends push off on their rollerblades with the same force. Explain which boy will have greater acceleration.