Motion and Forces: Newton's Second Law of Motion

By Accessible Science on Nov 20, 2014

The first part of the activity described below can also be used to illustrate inertia. In the second part of the activity, mass is added to the toy car to illustrate Newton’s Second Law. Give the student with a visual impairment an opportunity to feel the weight of the car before and after the clay is added.
 
Of course, friction is another force that affects motion, but this is disregarded as a factor in the activity.
 
Vocabulary
Newton - SI unit of force 
 
Effects of Unbalanced Forces
Unbalanced forces cause acceleration. When an unbalanced force acts on an object, the motion of the object is changed. If the object is at rest, the force makes it move. If the object is in motion, the force changes its velocity. Any change in velocity is acceleration.
 
Force, Mass and Acceleration
The amount by which an object accelerates depends on three things. They are the size of the force, the direction in which the force acts, and the mass of the object. If two forces act on the same object, the greater force will produce more acceleration than the smaller force.
 
Newton’s Second Law
Newton’s second law describes the relationship among force, mass, and acceleration. Newton’s second law states that the unbalanced force acting on an object is equal to the mass of the object times its acceleration. Newton’s second law can be describe by this equation 
 
F=mxa
 
In this equation, F is the force, m is the mass, and a is the acceleration. When the mass is measured in kilograms and the acceleration is measured in meters per second per second, the forced is measured in newtons (N). A newton is the SI unit of force. An unbalanced force of 1 N will accelerate a mass of 1 kg at 1 m/s2. One Newton of force is equal to one kilogram-meter per second per second (1kg-m/s2).
 
Using Newton’s Second Law
If no friction is involved, how much force would you have to apply to 10 kg object to make it accelerate at a rate of 45 m/s2? This may seem like a difficult problem at first. However, if you use the equation for Newton’s second law, it becomes easy.
 
F=mxa
F = 10 kg x 45 m/s2
F = 450 kg m/s2
 
You would have to apply a force of 450 kg m/s2 or 450 N.
 
Sir Isaac Newton (1642-1727)
Isaac Newton was born in England on December 25, 1642. He was a physicist, an astronomer, and a mathematician. At the age of 45, Newton published his theories of motion and gravity. Newton’s great book is usually called the Principia. It is considered one of the most important works in the history of science.
In the Principia, Newton explained his three laws of motion and his theory of gravitation. Newton also invented a branch of mathematics called calculus to help predict motion using his three laws. Newton also made many important discoveries about light and color.
 
Newton was a professor of mathematics at Cambridge University and a member of the Royal Society. He was knighted by Queen Anne in 1705. Newton once said about himself, “If I have further seen than others, it is because I have stood on the shoulders of giants.”
 

Materials

  • A board (about 2 m long)model car
  • Toy car
  • Modeling clay
  • Measuring tape or meter stick 
  • A place to record your data

 

Procedure

  1. Lay a board about 2 meters long on the floor.
  2. Place a toy car at one end of the board.
  3. Slowly lift the end of the board with the car on it until the car begins to move.
  4. Hold the end of the board at that level and have a partner measure the height to which the end of the board was raised.
  5. Record your data.
  6. Press a piece of modeling clay on the top of the car to increase its mass and
  7. repeat steps 2-5.
  8. Predict how adding a second piece of clay to the car will affect the height you will have to raise the board before the car moves and record your prediction.
  9. Test your prediction and record your data.
  Questions and Conclusions
  1. What will happen to an object at rest when an unbalanced force acts on it? 2. What will happen to a moving object when an unbalanced force acts on it? 3. What is a change in velocity called?
  2. What is 1 N of force equal to?
  3. Which will case a bigger acceleration, a small force or a big force?
  4. What does Newton’s second law describe?
  5. What is a Newton?
  6. How much force is needed to give a 5 kg mass an acceleration of 20 m/s2?

 

Variations

An adapted meter stick.

Article and activity adapted from Concepts and Challenges: Physical Science, Fourth Edition. Parsippany, NJ: Globe Fearon Inc., Pearson Learning Group, 2009, pages 280 to 281.     Purchase the full book here or download the FREE PDF for this activity.

 

NGSS Standards:

PS2.A
Forces and motion: Newton’s 2nd law (F=ma) and the conservation of momentum can be used to predict changes in the motion of macroscopic objects (grade 9 -12)

 


 

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