Upload
giles-atkinson
View
217
Download
0
Embed Size (px)
DESCRIPTION
Topic Overview When an object has unbalanced forces acting on it, the object will accelerate in the direction of that excess force: F net = ma This is called “Newton's Second Law”
Citation preview
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 = ma40-25 = (7.5)a 15 = 7.5 a 2 = a
The object accelerates Upward at 2m/s2
7.5kg
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?
Answer: 27N
Ff =2N
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
Force
Velocity
a
Equations
• r is the radius of the circle
Example ProblemThe 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 = Final 0 = -1.2 (v) + (1.8)(2)
Initial = Final (1)(6)+ 0 = (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!
Fnett = p = mvUnits = 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