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Test Review Answers Dynamics
1. A force is a push or a pull. It can also be the force of gravity. A force causes an
object/mass to accelerate or to stretch/bend/or squeeze.
2. Contact forces are forces that result when two objects are physically in contact
with one another. Example: push/pull, normal force, friction, spring force,
tension, air resistance. Non-contact Forces: forces that result when two objects
are not in physical contact. Example: gravitational force, nuclear force, magnetic
force, electrostatic force (electric charge)
3. Inertia is the natural tendency of an object to remain in its current state of motion
(either moving or at rest).
4. Weight and mass are NOT THE SAME. Weight = the force of gravity acting on
a mass. Weight can change. It is measured in Newtons (force). Weight = mass x
gravitational force (Fg = mg). Mass = the quantity of matter an object contains.
Mass for the same object is constant. It is measured in kg. Mass is the same no
matter where you are on Earth, on another planet, etc. Weight changes according
to the gravitational acceleration.
5. Friction is a contact force. It is more specifically an electromagnetic force
(between surface atoms of objects touching). There are 2 types of friction: static
friction and kinetic friction.
 Static Frictional Force
 When you start to move an object from rest
 Larger than Kinetic Frictional Force due to Inertia
 ųs
 Kinetic Frictional Force
 Exists when the object is moving
 ųK
 The strength of friction depends on…
 Surface materials
 Magnitude of forces pressing surfaces together
 The strength of friction DOES NOT depend on…
 Surface area
 Velocity of object moving
 The coefficient of friction is Stickiness value”
 ų (symbol mu)
 ų has no units
6. Normal force is a force that acts in a direction perpendicular to the common
contact surface between two objects. You must have a common surface area to have
normal force. It is usually the same value as Fg but only on a horizontal surface! If
the surface is sloped, they are not equal (and Fn is not vertical, but perpendicular to
the surface). It is measured in Newtons and is called Fn.
7. The net force is a vector sum which means that both the magnitude and direction
of the forces must be considered. Fnet = ma.
8. FBDs or free body diagrams are diagrams that show the forces acting on an
object, the direction of the forces and the magnitude/size of the forces. Use
mathematical symbols to show equality of forces if they balance/cancel.
9. Newton’s First Law of Motion (Newton’s Law of Inertia): An object at rest or in
uniform motion (ie, constant velocity) will remain at rest or in uniform motion
unless acted on by an external force. Ex: If a car is driving along a straight road
at 100km/h, it will continue to do so (given the car still has gas!) until the brakes
are applied (applied force), there is a turn or the road surface changes (more or
less friction).
Newton’s Second Law: Newton’s first law states that an object does not accelerate
unless a net force is applied to the object. But how much will an object accelerate
when there is a net force? The larger the force the larger the acceleration. Therefore
acceleration is directly proportional to mass. Acceleration also depends on mass.
The larger the mass, the smaller the acceleration. Therefore acceleration is inversely
proportional to mass. We say that a massive body has more INERTIA than a less
massive body. Fnet = ma, where the acceleration is in the same direction as the
force.
Newton’s Third Law: When one object exerts a force on a second object, the second
object exerts a force on the first that is equal in magnitude but opposite in direction.
These forces are called action-reaction forces. Ex: If you push against a wall, you
don’t go through it as the wall “pushes back”. Only the forces on an object
determine its acceleration. This means that only you pushing on the wall affects the
wall (not the wall pushing back). If two objects have equal and opposite forces, one
can still move due to the mass of the object, the friction, etc.
10. Remember to draw the system in a linear format (all parallel forces so you can use
+ and -). Also remember that you can use the entire system, or either of the two
parts separately to find acceleration.
When two or more masses are attached by a string or rope and hang over a pulley
system, there is a system of masses.
Some assumptions that must be made:
- Strings only exert pulling forces.
- The tension in the string is the same throughout its length.
- A frictionless pulley changes the direction of a string without diminishing its
tension.
- Strings do not stretch.
- The strings’ mass is negligible.
11. Tension is the magnitude of the pulling force exerted by a string, cable, chain,
or similar object on another object. It is measured in Newtons. It is measured
parallel to the string on which it applies.
Word Problems:
7. Fapp = -9000N = Fnet = ma
 -9000 = (1500)a
A = 6m/s2 (negative!) The force is
braking so negative!
Vi = 20.0m/s
Vf = 0m/s
A = -6 m/s2
Vf2 = vi2 + 2ad
0 = 400 + 2(-6)d
0 = 400 – 12d
-400 = -12d
D = 33 m [forward]
8. F = ma (apple)
F = 0.40 (-9.8) = -3.92N
F = ma (earth)
F = + 3.92N (equal and opposite direction)
3.92 = (5.98 x10 24)a
a = 6.6 x 10 -25 m/s (up)
Obviously this is hardly an acceleration at all since the Earth is so much larger!!!
Page 2, Non-mathematical questions
1. a) Newton’s First Law states that you will want to continue moving in the direction
and at the same speed as you were before. Therefore your body wants to keep moving
forward at the speed the car was going. However, your body is held back with the
seatbelt. Your head therefore continues to go forward before it gets “snapped” or
whiplashed back.
b) The headrest helps as it means when your head does come back to meet up the
car/body, it has a softer place to land and doesn’t go as far.
2. No, the automobile may not be moving if you are not pushing enough to overcome
friction and gravitational force. Unless you are moving, you don’t have a positive
velocity.
3. The horse is correct that there are equal and opposite forces but they act on different
things. For example, the horse pulls the cart so the force is on the cart. The opposite and
equal force is not pulling on the cart but on the horse (F(cart on horse). The horse has a
larger mass so it should accelerate. Also the cart is on wheels so there is less friction.
Both of these reasons mean the Fnet on the cart will be large enough to accelerate it
forward. Once moving the cart and horse will be attached and the internal forces (cart on
horse and horse on cart) will balance.
4. The black block is larger as it requires the same force to accelerate it less.
Fnet (black) = m1a1
Fnet (brown) = m2a2
Fnet (black) = Fnet (brown)
And 3a1 = a2
Fill in: Fnet (black) = Fnet (brown)
m1a1
= m2a2
m1a1
= m2(3a1)
m1
= 3m2
Therefore mass 1 is 3 times as large as mass 2!
5. a) Forces: Fg, Fn (these are equal and opposite)
Ff (opposite direction to Fapp but smaller)
There is no force applied to the dishes, only the cloth.
b) The cloth is pulled away but according to Newton’s First Law, the dishes stay on the
table due to inertia (as long as the force of friction does not get in the way!). The force of
friction is minimal if the cloth is slippery (works better).
6. Inertial: Watching a car drive by when you are standing still on the curb. You are
in a non-accelerating space.
7. Non-inertial: Watching a car drive by when you are driving in another car. You
are accelerating.
8. Inertial
Question 1 (System of Masses/Newton’s Third Law)