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