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Free Body Diagrams & Net Force
• Today, you will learn:
• The four main forces working on a moving object:
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The force of gravity
The force of friction
The normal force
The applied force
• How to combine the above four forces to determine the
net force.
• How to draw the above four forces as a free body diagram.
• How to draw a net force diagram.
• How to solve mathematical problems involving the above
four forces.
• There are numerous forces working on moving objects.
The main forces are:
1) The force of gravity (Fg = 9.81 m/s2 [down])
2) The force of friction (Ff – air resistance is a type of friction
force)
3) The normal force (FN – not natural force)
4) The applied force (Fapp – the push or pull on an object)
• The above forces work in different directions and when
they are combined, this results in the net force (Fnet). The
net force determines in which direction an object will
accelerate based on its mass (Fnet = ma).
• In order to determine the net force on an object we will
need to be able to draw a free body diagram, which shows
all of the forces acting upon a moving object.
• Consider an object resting on a surface. The force of gravity (Fg)
acts to pull the object downwards.
• Obviously, if gravity were the only force acting upon this object,
then it would fall at a rate of 9.81 m/s2 [down]. However, the
object is at rest and not accelerating downwards. This occurs
because the surface is producing an equal and opposite force
(according to Newton’s Third Law of Motion).
• The opposite force being applied to the object (by the surface) is
known as the NORMAL FORCE. The normal force is always
perpendicular (at right angles) to the surface. If the surface is flat,
then the normal force is in the opposite direction of gravity.
• According to Newton’s Third Law of Motion, the normal force must
be the same strength as the force of gravity. These forces cancel
out and there is no net force, so the object does not accelerate.
• Now imagine that you pushed or pulled on an object, i.e. you applied a
force to the object. In this case, you would need to account for the
applied force, as well as the force of friction. These two forces oppose
each other.
FNET = Fapp – Ff
• If the applied force does not overcome the force of friction, then the
object will not move (i.e. Fapp < Ff, then FNET is a negative number).
• At this point, you need to be able to draw the four types of forces on an
object. Remember that gravity must always be assumed.
• PROBLEM: A block is pushed with 25 N of force. The force of friction is 10
N. Draw the free body diagram.
• Afterwards, you need to draw the net force diagram. The net force
diagram should only have one arrow, in the direction in which the
object will accelerate.
• A stationary object will have no net force and will not accelerate.
• PROBLEM: If the object has a mass of 5 kg, what will be its
acceleration?
• PROBLEM: A 264 N force is applied to a 39.5 kg object. If it
accelerates at a rate of 2.60 m/s2, what is the force of friction?
• PRACTICE: A block is pushed, experiencing a force of friction of
33.2 N. If the block is 14.3 kg and accelerates at a rate of 2.87 m/s2,
what is the applied force?
• H.W. Try the following problems. Draw the free body diagram as
well as the net force diagram.
1)
A 23 kg object is pushed with 54.2 N. If it experiences 23.1 N of
friction, at what rate will it accelerate?
2)
An object is pushed with 31.9 kN, experiencing 11.7 kN of friction.
If it accelerates at a rate of 19.4 m/s2, what is its mass?
3)
A 46.5 N force is applied to a 21.6 kg object. If it accelerates at a
rate of 4.6 m/s2, what is the force of friction?
4)
A block is pushed, experiencing a force of friction of 34.9 N. If the
block is 12.0 kg and accelerates at a rate of 4.23 m/s2, what is the
applied force?
ANSWERS: a = 1.4 m/s2, m = 1040 kg, Ff = 52.9 N, Fapp = 85.7 N