Take out E81 Write up and open your books to page E-37 HW: None Do Now for 5/10/13 Take out E81 Write up and open your books to page E-37 HW: None

E81 The Net Force Challenge Today’s Target: I will be able to tell if the forces on an object are balanced or unbalanced. Review the concept of Net Force Introduce activity Gather Data Clean up Begin discussing data Analysis?

E81 The Net Force Challenge In many situations, more than one force is acting on an object. Ex. Driving a car Engine Power Wind resistance The combination of all the forces acting on an object is called the Net Force or total force. F=ma represents the Net Force. Friction

E81 The Net Force Challenge A force diagram shows an object or a system as a simple square and is labeled only with the forces applied to it. Shows all the forces applied to the object. Always need to show both the size AND the direction of the forces. The same situation, but only shows the NET FORCE.

E81 The Net Force Challenge Read the Background and Challenge on page E-37. If there is a situation with unbalanced forces, there is a net force. Balanced force means there is a net force of zero.

E81 The Net Force Challenge Can you describe the motion of the following:

E81 The Net Force Challenge Use of the Force Meters Gently pull Each mark on the scale is equal to 0.1 Newtons Complete parts A,B, and C. Motion – Any change in position, which could mean an object moves at a constant speed and direction. Change in Motion – A change in speed or direction due to an acceleration or deceleration.

E81 The Net Force Challenge 1. Describe an example and draw a force diagram of a situation with: a. balanced forces B. Unbalanced forces This block will have NO change in its motion – If it was at rest, it will stay at rest. If it was in motion, it will stay in motion at a constant speed. This block will accelerate towards the right.

E81 The Net Force Challenge 2. Imagine that a parked car is hit from the left with 30,000 N of force at the exact same time it is hit from the right with 40,000 N of force. a. Draw a force diagram showing the forces acting on the parked car.

E81 The Net Force Challenge b. Draw another force diagram showing only the net force on the parked car.

E81 The Net Force Challenge 3. The force diagram below shows an object with zero net force, but there is one force missing. What is the missing force? Draw the diagram in your notebook, and complete it by drawing and labeling the missing force. The missing force is friction, and it must be 10 N to the right in order to balance the forces.

E81 The Net Force Challenge 4. Look back at your work from Part A. Could the block in Part A have been moving? Explain. Yes, the block could have been moving and still have had balanced forces, as was seen in Part C. Although the block in this case would be moving, it has no change in motion to cause an acceleration or net force.

E81 The Net Force Challenge 5. For each situation a-e below, explain why there is or is not a zero net force acting on the car. a. A car is parked on a level parking space. The car is stopped, which means it has a constant speed (0) and direction, so there must be zero net force acting on it.

E81 The Net Force Challenge b. A traffic light turns green, and a car starts to move. The car is accelerating, and so there is a nonzero net force acting on it. c. A car drives steadily at 25 MPH. The car is traveling at a constant speed and so there is zero net force acting on it.

E81 The Net Force Challenge d. A car is slowing down from 30 MPH to 10 MPH. The car is decelerating (from 30 MPH to 10 MPH) so there is a nonzero net force acting on it. e. A car goes around a corner at 10 MPH. The car is changing direction, a form of acceleration, so there is a net force acting on it.

E81 The Net Force Challenge – Key Points. EXAMPLES NEEDED!!! 1. When an object experiences a nonzero net force (unbalanced forces), some aspect of its acceleration— speed and/or direction of motion—will always change. 2. Any object that is not traveling at a constant speed and direction must be in a situation where there is a nonzero net force (unbalanced forces) acting on it. 3. When an object experiences zero net force (balanced forces), its speed and direction of motion are constant. 4. Any object traveling at a constant speed and direction, which could be either an object at rest or a moving object, has a zero net force (balanced forces).