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Net Force Particle Model Worksheet 2:
Newton's 2nd Law
1. A 4600 kg helicopter accelerates upward at 2.0 m/s2. Determine the lift force exerted on the
propellers by the air. Make a quantitative force diagram. Write a net force equation for
the axis along which forces are not balanced.
Fl
Fnet = ma
Fl + Fg= ma
Fl = ma - Fg
Fl = 4600kg(2.0m/s^2)-(4600kg x -10 n/kg =55,200N
Fg
2. The maximum force that a grocery bag can withstand without ripping is 250 N. Suppose that
the bag is filled with 20 kg of groceries and lifted with an acceleration of 5.0 m/s2. Do the
groceries stay in the bag? Make a quantitative force diagram. Write a net force equation
for the axis along which forces are not balanced.
Fn g,b
Fnet = ma
Fn + Fg = ma
Fn - 200N = 20kg (5.0m/s^2)
Fn = 300N
The bag would have to provide a 300N
force on the groceries to produce this
acceleration. The 300N force provided by
the groceries on the bag are greater than
the 250 N limit. The bag rips.
©Modeling Instruction 2013
1
Fg g,E
U5 Net Force – ws2 v3.1
3. A student, standing on a scale in an elevator at rest, sees that his weight is 840 N. As the
elevator rises, the scale reading increases to 1050 N, then returns to normal. When the
elevator slows to a stop at the 10th floor, the scale reading drops to 588 N, then returns to
normal. Draw a motion map for the student during his elevator ride. Determine the
acceleration at the beginning and end of the trip. Make quantitative force diagrams. Write
a net force equation for the axis along which forces are not balanced.
Fn
The scale reads the normal force acting on
the student. W = Fg= –840N
Fn
Fnet
Fnet
Slowing to a stop
Starting up
Fnet/m = a
Fn + Fg/m = a
588-840N/84Kg = a = -3.0m/s/s
Fnet/M = a
Fn + Fg /M = a
1050N - 840N/84Kg = a
=2.5m/s/s
Fg
Fg
When the scale reads 840 N, the net force on the student = 0, so velocity is constant.
4. A sign in an elevator states that the maximum occupancy is 20 persons. Suppose that the
safety engineers assume the mass of the average rider is 75 kg. The elevator itself has a mass
of 500 kg. The cable supporting the elevator can tolerate a maximum force of 30, 000 N.
What is the greatest acceleration that the elevator's motor can produce without snapping the
cable? Make a quantitative force diagram. Write a net force equation for the axis along
which forces are not balanced.
Ft
Fg=(20 x 75Kg + 500kg)(-10N/Kg) = -20,000N
Fnet/m = a
Ft + Fg/m = a
30,000N - 20,000N/2000Kg = a = 5.0 m/s/s
Fg
©Modeling Instruction 2013
2
U5 Net Force – ws2 v3.1