DAY 45

#Goals: SWBAT...

1. Use the concept "the stopping distance is proportional to the square of the velocity" to solve problems

2. Understand how internal vs. external forces affect the TME.

3. Use the Kinetic Energy (KE) equation to solve velocity problems.

4. Use the Work-Energy Equation (TME_i +W_ext = TME_f) to solve medium difficulty energy problems with conservative and non-conservative forces

Not checking last nights HW assignment for credit

Warm-Up (4min):

1. Copy the diagram into your warm-up, then answer questions 1-3.
   1. Use the following diagram to answer questions #1 - #3. Neglect the effect of resistance forces.

2. 1. As the object moves from point A to point D across the surface, the sum of its gravitational potential and kinetic energies ____.

• The only force doing work is gravity (The Normal Force doesn't do work, as it's always perpendicular to the direction of motion). Since it is an internal or conservative force, the total mechanical energy is conserved. Thus, the 100 J of original mechanical energy is present at each position. So the KE for A is 50 J.
• The PE at the same stairstep is 50 J (C) and thus the KE is also 50 J (D).
• The PE at zero height is 0 J (F and I). And so the kinetic energy at the bottom of the hill is 100 J (G and J).
• Using the equation KE = 0.5*m*v2, the velocity can be determined to be 7.07 m/s for B and E and 10 m/s for H and K.

2. #045B: PRACTICE PROBLEMS

4. Many drivers' education books provide tables that relate a car's braking distance to the speed of the car (see table below). Utilize what you have learned about the stopping distance-velocity relationship to complete the table.

5. Some driver's license exams have the following question.

A car moving 50 km/hr skids 15 meters with locked brakes. How far will the car skid with locked brakes if it is moving at 150 km/hr?

6. Two baseballs are fired into a pile of hay. If one has twice the speed of the other, how much farther does the faster baseball penetrate? (Assume that the force of the haystack on the baseballs is constant).

7. Use the law of conservation of energy (assume no friction) to fill in the blanks at the various marked positions for a 1000-kg roller coaster car.

8. If the angle of the initial drop in the roller coaster diagram above were 60 degrees (and all other factors were kept constant), would the speed at the bottom of the hill be any different? Explain.

9. Determine American ski jumper Lee Ben Fardest's (a mass of approximately 50 kg) speed at locations B, C, D and E.

10. An object which weighs 10 N is dropped from rest from a height of 4 meters above the ground. When it has free-fallen 1 meter its total mechanical energy with respect to the ground is ____.

11. During a certain time interval, a 20-N object free-falls 10 meters. The object gains _____ Joules of kinetic energy during this interval.

12. A rope is attached to a 50.0-kg crate to pull it up a frictionless incline at constant speed to a height of 3-meters. A diagram of the situation and a free-body diagram are shown below. Note that the force of gravity has two components (parallel and perpendicular component); the parallel component balances the applied force and the perpendicular component balances the normal force.

A. Of the forces acting upon the crate, which one(s) do work upon it?

B. Based upon the types of forces acting upon the system and their classification as internal or external forces, is energy conserved? Explain.

C. Calculate the amount of work done upon the crate.

STUCK? Here's the answers/solutions: LINK

At Home Learning (HW)

1. Complete the practice problems for #045B. I will check them for credit tomorrow.

2. Introduce yourself to our new concepts of momentum and impulse! :-) Take notes, and answer EdPuzzle questions:

• (3:18) You Can't Run From Momentum! (a momentum introduction) - EDpuzzle
• (3:57) Force of Impact Equation Derivation - EDpuzzle