Free body
diagrams,
acceleration, quiz
Friday, November 6, 2015
Friday, 11/6
Unit 4: Forces

Respond to the following questions for both pictures.
1. Using your knowledge of inertia, describe how the image demonstrates
Newton’s 1st Law of motion.
2. What could you do to change the motion of the bus or the race car?
Upcoming dates
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Friday, 11/6 – quiz
Monday, 11/16 – quiz
Monday, 11/16 – review due at when the late bell rings
Wednesday, 11/ 18 – Unit 4 test
W/O 11/23 – Thanksgiving break
Friday, 12/11 – 2NW extra credit due
Weight vs. Mass
 Mass
is matter (atoms)
 Weight is the force of gravity acting on
mass.
 Weight is represented by the symbol Fg
 Fg will always act on an object.
How to Calculate force of
gravity (weight)
Note: when calculating Fg
acceleration will always be 9.8 m/s2
Example – calculating weight (Fg)
A child pulls a wagon with a mass of 6 kg, ten meters
across the grass to the garage. The child applies a
10 N force to overcome 2Ns of friction. Calculate the
weight of the wagon.
G: m = 6kg
E: Fg = (m)(a)
U: Fg
S: Fg = (6)(9.8)
S: 58.8 N
Practice calculating weight (Fg)
Practice calculating weight (Fg)
245 N
14.7 N
1127 N
How to calculate force
Note: acceleration will not
Be 9.8 m/s2 unless the object
Is in free-fall.
How to Calculate net force
Net Force - answers
Newton’s 2nd Law – Practice problem 1
A car has a mass of 550 kg.
1. Find the weight of the car on Earth
2. Find the net force needed to accelerate the car
at 12 m/s2.
3. If a net force of 2200 N were to act on the car,
what would its acceleration be?
Newton’s 2nd Law – Practice problem 2
A sled weighs 2500 N on Earth
1. What is the mass of the sled?
2. What is the mass of the sled on the moon, where g = 1.6 m/s2?
3. What is the weight of the sled on the moon?
4. What net force is needed to accelerate the object at 4.0 m/s2 on the moon?
Free body diagram (FBD)
 Pictorial
representation of forces acting
on an object.
Free body diagram steps
Replace the object with a dot.
Identify the force pairs acting on the object
Identify contact forces acting on the object.
1.
2.
3.
•
•
4.
5.
Remember that only something that is physically
touching the object can apply a contact force
Possible types of contact forces – push, tension, normal,
and friction
Add arrows force and label the forces
Add any force values that are known
Example - FBD
A child pulls a wagon with a mass of 6 kg, ten
meters across the grass to the garage. The child
applies a 10 N force to overcome 2Ns of friction.
Calculate the weight of the wagon.