Newtons 2nd Law of Motion
If a net (resultant)force acts on an object, the object will accelerate in the direction of the net force. The acceleration
𝟏
is directly proportional to the force (𝒂 ∝ 𝑭𝒏𝒆𝒕) and inversely proportional to the mass of the object (𝒂 ∝ ).
𝒎
HOW TO SOLVE NEWTON’S 2ND LAW OF MOTION PROBLEMS:
11. Draw a Free Body or a Force Diagram.
Draw the one the questions states, otherwise choose the one you are more comfortable with. If you have
more than one object, just do one at a time. Label all forces clearly!
Hints:
• Tension in a rope is always in the direction of the rope (ropes can only pull).
• The normal force (n) is always perpendicular to the surface (it is not always up). There can be more
than one normal force on an object.
• Friction is always parallel to the surface and in the opposite direction as the motion (or for static
friction, it is in whatever direction will prevent motion).
22. Choose directions for x and y axes
Choose your positive direction in the direction of motion to make it easier to interpret the answer.
+
33. Set up the two Newton's 2nd law equations
∑ 𝑭𝒙 = 𝒎𝒂𝒙
∑ 𝑭𝒚 = 𝒎𝒂𝒚
Hints:
• Be careful of the signs (+/-) of the components.
• Make sure you break up any forces acting at an angle into x and y components.
• You will have 2 sets of equations for each object with a mass in your system. (Remember that in these
problems the mass of the rope is negligible.)
• Determine which direction the object is moving in to determine whether acceleration equals zero and
therefore which equation equals zero.
44. Solve!!
You will now use your simultaneous equation skills to solve these problems.
If a net (resultant)force acts on an object, the object will accelerate in the direction of the net force. The acceleration
𝟏
is directly proportional to the force (𝒂 ∝ 𝑭𝒏𝒆𝒕) and inversely proportional to the mass of the object (𝒂 ∝ ).
𝒎
HOW TO SOLVE NEWTON’S 2ND LAW OF MOTION PROBLEMS:
11. Draw a Free Body or a Force Diagram.
Draw the one the questions states, otherwise choose the one you are more comfortable with. If you have
more than one object, just do one at a time. Label all forces clearly!
Hints:
• Tension in a rope is always in the direction of the rope (ropes can only pull).
• The normal force (n) is always perpendicular to the surface (it is not always up). There can be more
than one normal force on an object.
• Friction is always parallel to the surface and in the opposite direction as the motion (or for static
friction, it is in whatever direction will prevent motion).
22. Choose directions for x and y axes
Choose your positive direction in the direction of motion to make it easier to interpret the answer.
+
33. Set up the two Newton's 2nd law equations
∑ 𝑭𝒙 = 𝒎𝒂𝒙
∑ 𝑭𝒚 = 𝒎𝒂𝒚
Hints:
• Be careful of the signs (+/-) of the components.
• Make sure you break up any forces acting at an angle into x and y components.
• You will have 2 sets of equations for each object with a mass in your system. (Remember that in these
problems the mass of the rope is negligible.)
• Determine which direction the object is moving in to determine whether acceleration equals zero and
therefore which equation equals zero.
44. Solve!!
You will now use your simultaneous equation skills to solve these problems.
