Peoples Physics Book Version-3

5.2. Newton’s Laws Explained http://www.ck12.org

b) If Tom has a bigger mass than Mary, who goes farther?

c) If Tom and Mary have identical masses and Tom pushes twice as hard as Mary, who goes farther?

Solution

a) Neither. Both Tom and Mary will travel the same distance. The forced applied to each person is the same
(Newton’s Third Law). So

Ma =ma

which cancels to

a=a

Therefore both people will travel the same distance because the acceleration controls how far someone will travel
and Tom and Mary have equal acceleration.

b) Mary will go farther. Again, the same force is applied to both Mary and Tom so

Ma=ma

Since Tom has the larger mass, his acceleration must be smaller (acceleration and mass are inversely proportional).
Finally, because Mary’s acceleration is greater, she will travel farther.

c) Neither. Newton’s Third Law states that for every action there is an equal and opposite reaction. Therefore if Tom
pushes twice as hard as Mary, Mary will essentially be pushing back with the same strength. They will therefore
travel the same distance if they have the same mass.

Key Concepts

• An object will not change its state of motion (i.e., accelerate) unless an unbalanced force acts on it. Equal and
  oppositely directed forces do not produce acceleration.


• If no unbalanced force acts on an object the object remains at constant velocity or at rest.


• The force of gravity is called weight and equalsmg, wheregis the acceleration due to gravity of the planet
  (g= 9 .8 m/s^2 , downward, on Earth).


• Your mass does not change when you move to other planets, because mass is a measure of how muchmatter
  your body contains, and not how much gravitational force you feel.


• A free body diagram (FBD) is a diagram that shows all the forces acting on an object. It is essential to draw
  this diagram before attempting to answer a question or solve a problem. If the FBD is drawn correctly then
  solving the problem is made easy.