Conservation of Energy and Momentum Physics 6(D)

Conservation of Energy and Momentum

Physics 6(D)

Learning Objectives• Demonstrate the laws of conservation of

energy and momentum in one dimension• Apply the laws of conservation of energy and

momentum in one dimension

Conservation of Energy and Momentum

• Kinetic energy (KE ) – the energy an object has due to motion– Dependent on mass (m ) and velocity (v )

• Gravitational potential energy (PEg) – the energy an object has due to its position– Dependent on mass (m ), height above ground

(h), and acceleration due to gravity (g = 9.8 m/s2 )

• Law of conservation of energy – states that when no external work is done on a system, the total mechanical energy is conserved

Law of Conservation of Energy

Ex) Consider a sled going down a snowy hill. Assume there are no external forces at work.

(a) Where will the sled have the most potential energy?

(b) Where will the sled have the most kinetic energy?

(c) Where will the sled have the most mechanical energy?

Conservation of Energy Conceptual Example

Conservation of Energy Calculation

Ex) If the sled in the previous problem starts from rest and travels down a hill that is 17 m high, what is the final velocity of the sled?

1. Write unknown and givens

2. Identify the formula and rearrange, if needed

3. Convert units and find intermediates, if needed

4. Plug in and solve

5. Make sure the answer is reasonable

Conservation of Energy Calculation

Ex) A bowling ball starts from rest and is rolled down a hill. When the bowling ball reaches the ground, it is moving at 13 m/s. What is the height of the hill?1. Write unknown and

givens

2. Identify the formula and rearrange, if needed

3. Convert units and find intermediates, if needed

4. Plug in and solve

5. Make sure the answer is reasonable

• Momentum ( p ) – the product of an object’s mass (m ) and its velocity (v )

• Law of conservation of momentum – states that the total momentum of a system before a collision is equal to the total momentum after the collision

Law of Conservation of Momentum

Ex) A waiter moving to the left with 150 of momentum runs into a hostess moving to the right with 120 of momentum. What is the combined momentum of the two people after the collision?

Conservation of Momentum Example

1. Write unknown and givens

2. Identify the formula and rearrange, if needed

3. Convert units and find intermediates, if needed

4. Plug in and solve

5. Make sure the answer is reasonable

Ex) A 4,500 kg truck traveling at 25 m/s runs into a 270 kg golf cart parked on the side of the road. If the final velocity of the golf cart is 12 m/s, what is the final velocity of the truck?1. Write unknown and

givens

2. Identify the formula and rearrange, if needed

Conservation of Momentum Example

3. Convert units and find intermediates, if needed

4. Plug in and solve

5. Make sure the answer is reasonable

Conservation of Momentum Example

Ex) An astronaut with a mass of 95 kg floating away from the space station can throw an object in the opposite direction to propel herself back. If the astronaut wants to move with a velocity of 1.5 m/s and can throw an object with a velocity of 15 m/s, how much mass would she need to throw?

1. Write unknown and givens

2. Identify the formula and rearrange, if needed

3. Convert units and find intermediates, if needed

4. Plug in and solve

5. Make sure the answer is reasonable

Learning Objectives• Demonstrate the laws of conservation of

energy and momentum in one dimension• Apply the laws of conservation of energy and

momentum in one dimension

Conservation of Energy and Momentum