Unbalanced
Forces
Topic Overview
•A force is a push or a pull applied to an
object.
• A net Force (Fnet) is the sum of all the
forces on an object (direction determines
whether a force is + or -)
Fnet = 6N to the right
Topic Overview
•When an object has unbalanced forces
acting on it, the object will accelerate in
the direction of that excess force:
• Fnet = ma
• This is called “Newton's Second Law”
Topic Overview
•Remember that Fnet is not just a single force
•Check out this example:
Fnet =
40-25 =
15 =
2 =
ma
(7.5)a
7.5 a
a
7.5kg
The object accelerates
Upward at 2m/s2
Sample problem
•The object shown below accelerates at 5m/s2
to the right. If the cart experiences a 2N
frictional force, what is the force pulling on the
handle?
Ff =2N
Answer: 27N
Circular
Motion
Topic Overview
•An object in circular motion has a changing
velocity but constant speed.
•This is possible because the objects speed
does not change (same m/s) but the
direction of its motion does change
Topic Overview
•The velocity of the object
is always “tangent” to the
path of the object.
•The circular force (Fc ) is
always directed toward
the center
• The acceleration is
always toward the center
of the circle
Velocity
a
Force
Equations
• r is the radius of the circle
Example Problem
The picture below show a student seated on
a rotating circular platform holding a 2.0kg
block with a spring scale. The block is 1.2m
from the girl. If the block ha s a constant
speed of 8m/s, what is the force reading on
the spring scale?
Answer: 106.7 N
Momentum/
Collisions
Momentum Recap
• Momentum: The product of the mass and
velocity of an object
• Equation:
p = mv
• Units:
p = kilograms meters per second
(kgm/s)
• Momentum is a vector: When describing the
momentum of an object, the direction matters.
Momentum Recap
• Collision: When 2 or more objects interact
they can transfer momentum to each other.
• Conservation of Momentum: The sum of the
total momentum BEFORE a collision, is the
same as the sum of the total momentum AFTER
a collision
Momentum Before = Momentum After
p1i +p2i + p3i = p1f+p2f + p3f
Momentum Recap
•
To Solve Collision Problems:
Step 1: Find the total momentum of each object before they
interact
Step 2: Set it equal to the total momentum after they collide
Remember momentum is a vector, so you have to consider if the
momentum is (+) or(-) when finding the total!!!
Initial
0
=
Final
= -1.2 (v) + (1.8)(2)
Initial
(1)(6)+ 0
=
=
Final
(1 + 3.0) v
•
Sample Problem 1
Step 1: Find the total momentum of each object before they
interact (In this case both objects start at REST)
Step 2: Set it equal to the total momentum after they collide
Answer: 5m/s left
•
Sample Problem 2
Step 1: Find the total momentum of each object before they
interact
Step 2: Set it equal to the total momentum after they collide
Answer: 4m/s west
Impulse
Impulse
• An outside force will cause a change in the
momentum of an object. This is called an
impulse.
• IMPULSE: A change in momentum
You can use ANY of the following equations!
Fnett = p = mv
Units = Ns
Impulse
To find the impulse under a force vs. time graph, you
would find the area under the line.
I = Ft
In this case the area is that of a triangle
A =1/2bh = ½ (1.0 x 10) = 5Ns
Sample Problem
Answer: 50Ns