1. Unbalanced Forces

2. Topic Overview A force is a push or a pull applied to an object. A net Force (F net ) is the sum of all the forces on an object (direction determines whether a force is + or -) F net = 6N to the right

3. 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”

4. Topic Overview Remember that Fnet is not just a single force Check out this example: F net = ma 40-25 = (7.5)a 15 = 7.5 a 2 = a The object accelerates Upward at 2m/s 2 7.5kg

5. Sample problem The object shown below accelerates at 5m/s 2 to the right. If the cart experiences a 2N frictional force, what is the force pulling on the handle? Answer: 27N Ff =2N

6. Circular Motion

7. 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

8. Topic Overview The velocity of the object is always “tangent” to the path of the object. The circular force (F c ) is always directed toward the center The acceleration is always toward the center of the circle Force Velocity a

9. Equations r is the radius of the circle

10. 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

11. Momentum/ Collisions

12. 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.

13. 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 p 1i +p 2i + p 3i = p 1f +p 2f + p 3f

14. 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

15. 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

16. 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

17. Impulse

18. 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! F net  t =  p = m  v Units = Ns

19. 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

20. Sample Problem Answer: 50Ns