Chapter 4 Notes: Newton’s Laws of Motion
FORCE
Newton stated that the change in velocity of an object is caused by FORCES.
When the velocity of an object is constant, or if the object is at rest, it is said to be in equilibrium.
Contact forces: forces that result from physical contact between two objects.
Examples: ______________________________________________________________
Field forces: forces that can act at a distance.
Examples: ______________________________________________________________
FORCE DIAGRAMS
Force is a vector.
Force diagrams – show forces vectors as arrows
Ex:
Free body diagrams – shows only the forces acting on a single object
Ex:
Newon’s First Law (____________________________):
A body at rest will remain at rest, a body in motion will remain in motion, traveling with a constant velocity
in a straight line, unless an unbalanced force acts on it.
INERTIA = a measure of a body’s ability to resist changes in velocity.
(the greater the mass of a body, the less it will accelerate under the action of an applied force)
Why is it easier to push a Volkswagen then a Mack truck?
Newton’s Second and Third Laws
Newton’s Second Law: ______________
The acceleration of an object is directly proportional to the resultant force acting on it and inversely
proportional to its mass.
The direction of the acceleration is in the direction of the resultant force.
The SI unit of force is the NEWTON.
1N = 1 kg•m/s2
Weight: w = ______
Any acceleration requires a force. An acceleration can be ___________________________________.
Example: An elevator accelerates upwards. If Bart steps on the scale, what will it read?
Example: Now the elevator travels upward with a constant velocity. If Bart steps on the scale, what will
it read?
Newton’s Third Law
If two bodies interact, the force exerted on a body 1 by body 2 is equal in magnitude, but opposite in
direction to the force exerted on body 2 by 1.
OR…
Implicit in these laws are the following ideas:
 If ___________________________ acts on a body, its acceleration must be __________.
o An unbalanced force is one whose vector sum does not equal zero.
 If an unbalanced force acts on a body, it must _______________. It will continue to accelerate
for as long as the force(s) are unbalanced.
 If a body has no acceleration, the vector sum of all the forces acting on it must be zero.
Multi-Object Force Diagrams:
 Draw all objects as boxes.
 Create a table of interactions within the system. (+ means interaction, - means no interaction 0
means irrelevant)
 Using the table of interactions, draw the force pairs acting on all objects. (dashed arrows for
field forces)
Example 1: Car hits a wall -
Example 2: Elephant vs. Man –
Example 3 – Hanging Block
Everyday Forces
The ___________ of an object is the force of ____________ acting of the _________ of that object.
USA: Pounds; Everywhere else (and in this class): ______________
Weight =
or
(mass) x (9.8m/s2)
force: the upward force acting on an object at rest on a surface; the normal force is
perpendicular to the surface on which the object is sitting.
Picture:
__________________ (T) is a force acting on a rope. When a rope is taught there is tension in the rope.
________________ (Ff) is a resistive force that opposes motion.
How to solve problems with Newton’s Laws:
 Draw the picture or Free Body Diagram
 Label ALL forces (in x and y direction)
 Sum the forces in each direction
 Solve
How to draw a free body diagram (FBD):
 All the forces on one object are draw as vectors
(arrows)
 The tail of the arrow starts on the object and
point off in the direction of the force.
 If the force is at an angle, include the angle.
Example: A traffic light weighing 100. N hangs from a cable tied to two other cables fastened to a
support. The upper cables make angles of 37° and 53° with the horizontal. Find the tension in each of
the three cables.
Example: A child holds a sled at rest on a frictionless snow-covered hill. If the sled weighs 100. N, find
the force the child must exert on the rope and the force the hill exerts on the sled.
Example: What is the tension in the cable of an elevator with a weight of 8800 N that ascends with an
acceleration of 1.30 m/s2?
Example: A 40.0 kg wagon is towed up a hill which is inclined at 18.5° with respect to the horizontal. The
tow rope is parallel to the incline and has a tension of 140. N in it. Assume that the wagon starts from
rest at the bottom of the hill and neglect friction. How fast is the wagon going after moving 80.0 m up
the hill?
Forces of Friction
The term friction refers to the resistive forces that arise to oppose the motion of a body past another
with which it is in contact.
____________friction (kinetic friction) is the frictional resistance a body in __________ experiences.
___________ friction is the frictional resistance a _________________ body must overcome in
order to be set in motion.
Where  = coefficient of friction
N = normal force
The magnitude of  depends on the nature of the surfaces in contact
NOTE: friction does NOT depend on the area of contact!
Example: You need to move a box of books into your dormitory room. To do so, you attach a rope to the
box and pull on it with a force of 90.0 N at an angle of 30.0°. The box of books has a mass of 20.0 kg, and
the coefficient of friction between the bottom of the box and the hallway surface is 0.5. Find the
acceleration of the box.
Example: A hockey puck is given an initial speed of 20.0 m/s on a frozen pond. The puck remains on the
ice and slides 120. m before coming to rest. Determine the coefficient of friction between the puck and
the ice.
Example: A 4.00-kg object is connected to a 7.00-kg object by a light string that passes over a
frictionless pulley. The coefficient of sliding friction between the 4.00-kg object and the surface is
0.300. Find the acceleration of the two objects and the tension in the string.
Air Resistance and Terminal Velocity:
 Air resistance is similar to friction in that it ____________________ the direction of motion.
 Air resistance is the resistance of the air that the object is falling through.
o When the object is _____________________, it is not moving through much air so the
air resistance is ______________.
o The ________________ the object moves, the more air it travels through and the
______________ the air resistance is.
 When the forces are balanced, acceleration stops.
Common Misconceptions:
 When a ball has been thrown, the force of the hand that threw it remains on it for awhile.
o NO! The force of the hand is a ____________ force; therefore, once contact is broken,
the ____________________________ exerted.
 Even if no force acts on a moving object, it will eventually stop
 _____________________ are two names for the same thing
 Air does not exert a force.
o NO. _________________________, but because it is balanced on all sides, it usually
exerts __________________ unless an object is moving.