1. PA2001: Time and Energy Energy and Momentum Work and energy Power Potential energy Conservation of momentum Pressure in a fluid Tipler Chapters 6,7,8 and 13 Energy and Momentum Dr Mervyn Roy, S6

2. PA2001: Time and Energy Energy and Momentum Work is a scalar quantity that is positive if Δx and F is the same sign, and negative if they are opposite – units are Joules = Newton metres Work A force does work when it acts on an object moved through a distance. Motion in 1D with constant force:

3. PA2001: Time and Energy Energy and Momentum Energy and work are related When work is done by one system on another energy is transferred between the two systems. Total energy is always conserved (read Tipler Chapter 7). Types of Energy Kinetic energy – associated with the motion of an object. Potential energy – associated with the configuration of a system. Many types of potential and kinetic energy – e.g. Thermal energy.

4. PA2001: Time and Energy Energy and Momentum Work

5. PA2001: Time and Energy Energy and Momentum Work done by a variable force

6. PA2001: Time and Energy Energy and Momentum Work done by a variable force Work done = area under the graph of F x against x

7. PA2001: Time and Energy Energy and Momentum Power is defined as the rate of doing work Units are Joules/second (or Watts) Power

8. PA2001: Time and Energy Energy and Momentum At constant power, acceleration decreases with velocity e.g. Ford Fiesta max power = 50 hp mass = 1000 kg At 70 mph. Acceleration = Power and acceleration

9. PA2001: Time and Energy Energy and Momentum Potential energy is energy stored within a physical system. exists when there is a force which tends to pull an object back towards its original position when it is moved. e.g. a stretched spring, or a weight suspended weight Potential energy If the spring is released or the weight dropped, the stored energy will be converted back to kinetic energy and the restoring force will accelerate the object back towards its original position.

10. PA2001: Time and Energy Energy and Momentum Potential energy define e.g. spring

11. PA2001: Time and Energy Energy and Momentum Momentum is conserved Energy is conserved Before collisionAfter collision

12. PA2001: Time and Energy Energy and Momentum Examples Inelastic collision

13. PA2001: Time and Energy Energy and Momentum Examples Elastic collision

14. PA2001: Time and Energy Energy and Momentum Pressure in a Fluid P = F / A When a body is submerged in a fluid, the fluid exerts a force perpendicular to the surface of the body at each point on the surface. h P PoPo area, A Variation of pressure with depth

15. PA2001: Time and Energy Energy and Momentum Archimedes Principle Buoyant force equal to the weight of the displaced fluid PA PoAPoA FsFs FsFs mg FsFs B

16. PA2001: Time and Energy Energy and Momentum F = mg - B

17. PA2001: Time and Energy Energy and Momentum Examples