Scientist: _____________________
Per: ________
Date: ___________________________
UNIT 3: NEWTON’S
LAWS
#5
NET FORCE AND ACCELERATION
Define the following terms:
1) acceleration
2) net force
3) weight
4) friction
5) normal force
6) equilibrium
Please write True or False for each statement
7) ___________ An object at rest has balanced forces on it.
8) ___________ An object at a constant speed has balanced forces on it.
9) ___________ An object accelerating has balanced force on it.
10) ___________ An object decelerating or slowing down has balanced forces on it.
11) ___________ An object in equilibrium must be at rest
Free Body Diagrams
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


The object is represented by a box
All the forces are represented by arrows. The length of the arrow indicates the size of the force
Weight is the force of gravity and is always shown with a down arrow.
o Weight is equal to mass times gravitational acceleration. Fw = mg
o It is sometimes labeled as Fw (force of weight), Fgrav (force of gravity), or simply “mg”
o On Earth, gravitational acceleration = 9.8 m/s2
The net force (Fnet) is the sum of all forces.
Equations and Constants
Fnet = ma
Fw = mg
Net force = mass x acceleration
weight = mass x gravitational acceleration, where g = 9.8 m/s2
For each of the following problems, give the net force on the block, and the acceleration, including units.
1)
12 N
26 N
7 kg
2)
60 N
40 kg
300 N
Net Force = ________
a = _______
Net Force = ________
a = _______
Hint: determine the net force. Then use the net
force to find acceleration using Fnet = ma
180 N
3)
40 kg
4)
70 N
20 N
Net Force = ________
a = _______
Net Force = ________
20 kg
a = __________
5)
30 N
100 kg
Net Force = ________
200 N
190 N
a = _______
For problems 6-9, using the formula net Force = Mass • Acceleration, calculate the net force on the object.
6)
9 kg
a = 3 m/s2
7)
5 kg
Fnet = ma = _____________
8)
12 kg
a = 4 m/s2
Fnet = ma = _____________
a = 40 m/s2
F = ma = _____________
9)
200 kg
a = 6 m/s2
Fnet = ma = _____________
10) Challenge: A student is pushing a 50 kg cart, with a force of 600 N. Another student measures the speed
of the cart, and finds that the cart is only accelerating at 3 m/s2. How much friction must be acting on the
cart? Hint: Draw a diagram showing the cart, and the two forces acting on it.
Directions: Draw a free body diagram. Determine the Net Force (Fnet) and use Newton’s Second Law
(F = ma) to calculate your answer
Section I: Complete 4 of these.
1) A force of 20 N acts upon a 5 kg block. Calculate the acceleration of the object.
2) An object of mass 300 kg is observed to accelerate to the right at the rate of 4 m/s 2. Calculate the force
required to produce this acceleration.
3) A 5 kg block is pulled across a table by a horizontal force of 40 N with a frictional force of 8 N opposing the
motion. Calculate the acceleration of the object.
4) An object of mass 30 kg is in free fall in a vacuum where there is no air resistance. Determine the
acceleration of the object.
5) An object of mass 30 kg is falling in air and experiences a force due to air resistance of 50 newtons.
a. Determine the net force acting on the object and
b. Calculate the acceleration of the object.
6) Just before opening her parachute a skydiver of mass 50 kg reaches terminal velocity. Calculate the force
of air resistance.
Section II: Challenge. Complete 1 of these
7) A force of 200 N is exerted on an object of mass 40 kg that is located on a sheet of perfectly smooth ice.
Neglect friction.
a. Calculate the acceleration of the object.
b. If a second object identical to the first object is placed on top of the first object, what acceleration
would the 200 N force produce?
8) For a person who has a mass 60 kg, calculate the weight in newtons and in pounds.
newtons.
An object of mass 10 kg is accelerated upward at 2 m/s2. What force is required? Hint. This object is
being accelerated against gravity
Section 1: 1) 4 m/s2 2) 1200 N
Section 2: see Topham
3) 6.4 m/s2
4) 9.8 m/s2
9)
1 pound equals 4.4
5a) 250 N 5b) 8.33 m/s2 6) 600N up