Gravitational Potential Energy (filled in)

Transcription

1 Name: Date: 4/3 Period: Unit 5 Gravitational Potential Energy (filled in) Essential Questions: Why is energy and work useful to learn? What does work mean in physics? What does energy mean in physics? What are eamples of how energy changes form ( transforms )? Review Work and Change in KE (T/F) Work done on an object is equal to the object s change in KE. (T/F) Negative work done means work is to the left. (T/F) Positive work done means object gains KE energy. (T/F) Negative work done means object loses KE energy. Slide

2 Overview of Work, Energy, and Power Work: W W = F d Units: Joules ( J ) Work changes Energy Energy can do Work Total Energy: E of or more objects Units: Joules ( J ) Power: P How fast you do work or use energy Units: Watts ( W ) Mechanical Energy = KE + PE Other Types of Energy: Thermal (Heat), Radiation, Electric, Magnetic, Nuclear Kinetic Energy: Energy of motion KE = ½ m v Potential Energy: Stored energy PE W Fd E P t t t g 0 m s Gravitational Potential Energy: PE = m g Elastic Potential Energy: Stored in springy material Slide 3 Objectives Define Gravitational Potential Energy or PE using physics language. Calculate the PE of an object. Include units. Calculate the change in PE. Connect change in PE to work done on an object. Slide 4

3 What is Gravitational Potential Energy or PE? Everyday meaning: Stored energy that can do work Physics meaning: System has at least objects with mass One mass is separated by a distance from the center of the other object A B Objects are attracted by F g Objects want to come together, but. F g B on A F g A on B when the objects are separated, energy is stored as Gravitational Potential Energy Slide 5 More separation, more PE stored Less separation, less PE stored A B If you do work to separate objects more, more energy is stored in the system. F g A on B F g B on A A B F g A on B If object s are allowed to come together, less energy is stored in the system. F g B on A Slide 6 3

4 How to calculate PE gained m kg 0.5m Mass PE mg On Earth, g = 0 m/s Height change Earth PE mg ( kg)(0 m / s )(0.5 m) 0J Ball moves farther away from the Earth PE is stored Slide 7 Work must be done on an object to change its PE Work = Force Distance or W = F g d The minimum work you do to lift an object is raising it at constant speed F a balances gravity force F g. Work Done = Change in PE called the Work Energy Theorem F a Work can be positive or negative: Positive work adds PE system stored PE Negative work takes away KE energy system loses PE F g Slide 8 4

5 Calculating work done to store PE m kg 0.5m Mass PE mg On Earth, g = 0 m/s Height change Earth W PE mg ( kg)(0 m/ s )(0.5 m) 0J 0 J of Work is done to move ball farther away from the Earth 0 J of PE is stored Slide 9 Compare the Work done lifting the ball in these 3 cases 0-N ball W PE mg Ball has the same weight, F g = 0 N So, mass is the same in all 3 cases Ball is lifted the same height of = 3 m g = 0 m/s Weight: Fg = mg 0N = m (0m/s ) m = k kg PE is the same in all 3 cases. Thus, Work done on ball is the same. Path doesn t matter, only height change matters. Slide 0 5

6 Calculating PE Mass changes m. You lift a 0-kg bo a vertical distance of m. How much work do you do on the bo? W PE mg h ( 0 kg)(0 m / s )( m) 00 J ( )(0 / )( ) m. You lift a 0-kg bo a vertical distance of m. How much work do you do on the bo? (Double the mass of #) m m W PE mg 0 m 400J ( kg)(0 / s )( m) 3. Divide the PE in # by #. How many times is PE in # larger? PE 400J PE 00J Slide Calculating PE Height lifted changes m. You lift a 0-kg bo a vertical distance of m. How much work do you do on the bo? W PE mg h kg m ( 00 J (0 )(0 / s ) m ) m. You lift a 0-kg bo a vertical distance of 4m. How much work do you do on the bo? (Double the mass of #) W PE mg h kg m (4 00J (0 )(0 / s ) m) 4 3. Divide the PE in # by #. How many times is PE in # larger? PE 400J PE 00J m 4m Slide 6

7 Based on the previous calculations If you triple the mass of an object, by how much does the work and PE change? (Lifted through the same height.) W PE mg h mg h 3 3 Mass triples 3 the original PE If you triple the speed of a moving object, by how much does its KE change? (Mass stays the same.) W PE mg h mg h 3 3 Height lifted triples 3 the original PE PE mg Slide 3 How to do the same amount of work easier.. Who does more work to life the ice block from the ground to the top of the ramp? Answer: Both do the same work. Work is related to change in PE, which is related to vertical height lifted. Both lift the ice block through the same height.. Who eerts less force to lift the ice block? Answer: Girl eerts less force. Work is related to F d. Because both do the same work. The person who lifts through less distance eerts the most force. The ramp is longer and requires less force. W = F d = F d Girl Boy Slide 4 7

8 Write a summary of key concepts in your own words: Slide 5 8