Net Force Problems

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Net Force Problems
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1. An airplane has 4 forces acting on it
weight, drag (air friction), the thrust of the
engines and the lift force caused by the flow of
air over the wings.
55,000 N

• 5000 N up
• If the pilot wishes the airplane to move forward,
  then the ______ must be greater than the _______.

Flift
20,000 N
20,000 N
Fthrust
Fdrag
Airplane
Fweight
50,000 N
thrust
drag
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2. A 200 N table has a 30 N book resting upon it.

Ftable
Book
Fweight of book
30 N equal and opposite force of the table
pushing up on the book (called the normal
force)
Book
30 N mass of book x acceleration (9.81 m/s2)
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3. A child weighing 150 N is sitting on a swing.
The swing is equally supported by 2 ropes, one on
each side. What is the force exerted by each
rope?
Each rope must exert a force of 75 N. 150 N 75
N 75 N

Frope 75 N, which is the tension force
Frope
Swing seat
Fchild 150 N, which is the force of weight
due to gravity
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4. Two chains are used to support a small boat
weighing 1500 N. One chain has a tension of 600
N. What is the force exerted by the other chain?

• Fchain 2 Fweight - Fchain 1
• 1500 N 600 N
• 900 N

Fchain 2
Fchain 1 600 N
Boat
Fweight 1500 N
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5. A heavy box weighing 1000 N sits on the floor.
You press down on the box with a force of 450 N.
What is the normal force on the box? (Same method
as 4)
Fn 1450 N
Box
Fp 450 N
Fw 1000 N
A normal force is perpendicular to the plane of
action.
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6. A 40 N cat stands on a chair. If the normal
force on each of the cats back feet is 12 N,
what is the normal force on each front foot?
(Same method as 4)

• The normal force must equal the force of the
  cats weight
• 40 N 2(12 N) 2x
• 40 N 24 N 2x
• 16 N 2x
• 8 N x
• Therefore, the normal force on each front foot is
  8 N

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7. Two forces are acting on a pencil, pushing it
to the right. One force is 9 N and the other is 6
N. There is a 6 N force pushing the pencil to the
left. What is the net force acting on the pencil?
(Dont forget the direction!)
Fp 9 N
Fp 6 N
Pencil
Fp 6 N
The net force acting on the pencil is 9 Newtons
to the right.
Fp 9 N
Pencil
9
8. A train is climbing a gradual hill. The weight
of the train creates a downhill force of 150,000
N. Friction creates an additional force of 25,000
N acting in the same direction. How much force
does the trains engine need to produce so that
the train is in equilibrium (not sliding back
down the hill)?
Ft ? N
Train
Ff 25,000 N
Fw 150,000 N

• Ft Ff Fw
• Ft 25,000 N 150,000 N
• 175,000 N