Successfully reported this slideshow.
Upcoming SlideShare
×

# Chapter 9

522 views

Published on

• Full Name
Comment goes here.

Are you sure you want to Yes No
• Be the first to comment

• Be the first to like this

### Chapter 9

1. 1. Work and Energy Changes <ul><li>How do we do work and how does it affect energy? </li></ul>
2. 2. We will find out <ul><li>How to calculate changes in energy types in different scenarios </li></ul><ul><li>How Newton’s Laws of Motion can be used to calculate the braking forces and distances (work done in stopping a vehicle) </li></ul><ul><li>Newton's Laws of Motion </li></ul><ul><li>Objects continue at the same velocity unless acted on by a resultant force </li></ul><ul><li>Force = Mass x Acceleration </li></ul><ul><li>Every Action has an equal and opposite reaction </li></ul>
3. 3. Falling Masses - Acceleration <ul><li>Why do 2 different masses fall at the same rate? </li></ul><ul><li>Think about Newton’s 2 nd Law!!! </li></ul><ul><li>Force = Mass x Acceleration </li></ul>
4. 4. Falling Masses - Acceleration <ul><li>Why do 2 different masses fall at the same rate? </li></ul><ul><li>F = ma therefore a = F / m </li></ul><ul><li>But Force is the weight of the mass = mg </li></ul><ul><li>Acceleration = Force / Mass = Mass x g / Mass = g </li></ul><ul><li>Acceleration is always g independent of Mass!!! </li></ul>LARGE MASS - M Small mass - m
5. 5. Free Fall <ul><li>What are the forces acting on a 100Kg parachutist? </li></ul><ul><ul><li>When he first jumps </li></ul></ul><ul><ul><li>When he stops accelerating </li></ul></ul><ul><ul><li>What is the maximum air resistance? </li></ul></ul><ul><li>How long will it take him to fall 4km? (ignore air resistance) </li></ul><ul><li>Can you hurry up?!?!?! </li></ul>
6. 6. Free Fall <ul><li>What are the forces acting on a 100Kg parachutist? </li></ul><ul><ul><li>When he first jumps </li></ul></ul><ul><ul><li>WEIGHT mg = 1000N </li></ul></ul><ul><ul><li>When he stops accelerating – WEIGHT and DRAG Both = 1000N </li></ul></ul><ul><ul><li>What is the maximum air resistance? 1000N </li></ul></ul>
7. 7. Free Fall <ul><li>It takes him this long… </li></ul><ul><li>We know </li></ul><ul><ul><li>Distance – s = 4000m </li></ul></ul><ul><ul><li>Acceleration – g = 9.8 ms -2 </li></ul></ul><ul><ul><li>Initial Velocity – u = 0 </li></ul></ul><ul><ul><li>Use s = u + ½ at 2 </li></ul></ul><ul><ul><li>4000 = 0 + ½ (9.8) t 2 </li></ul></ul><ul><ul><li>9.8 x 8000 = t 2 </li></ul></ul><ul><ul><li>t = 280s = 4.7minutes </li></ul></ul>
8. 8. Forces at Angles <ul><li>What happens if forces act at angles to each other? </li></ul><ul><li>We look at the components of each force – like we did with vectors… </li></ul><ul><li>Example – what is the tension in the cable in the picture? </li></ul><ul><li>The car has a mass of 1500Kg and the helicopter is flying level. </li></ul>
9. 9. Forces at Angles <ul><li>What happens if forces act at angles to each other? </li></ul><ul><li>We look at the components of each force – like we did with vectors… </li></ul><ul><li>Tension = Weight = mg = 1500 x 9.8 = 14.7kN </li></ul>
10. 10. Forces at Angles <ul><li>What happens if forces act at angles to each other? </li></ul><ul><li>We look at the components of each force – like we did with vectors… </li></ul><ul><li>The helicopter starts to accelerate vertically at 3m/s 2 . </li></ul><ul><li>What is the tension in the cable now? </li></ul>
11. 11. Forces at Angles <ul><li>What happens if forces act at angles to each other? </li></ul><ul><li>We look at the components of each force – like we did with vectors… </li></ul><ul><li>Force (Resultant) = ma = 1500 x 3 = 4500N </li></ul><ul><li>But it also has to overcome the weight, so add it! </li></ul><ul><li>4500 + 14700 = 19.2kN </li></ul>
12. 12. Forces at Angles <ul><li>The helicopter now moves forward. </li></ul><ul><li>The cable tilts – but the mass obviously does not change! </li></ul><ul><li>What is the tension now? </li></ul>
13. 13. Forces at Angles <ul><li>The helicopter now moves forward. </li></ul><ul><li>The cable tilts – but the mass obviously does not change! </li></ul><ul><li>Weight = Component of Tension </li></ul><ul><li>mg = T Cos 15 </li></ul><ul><li>14700 = T Cos 15 </li></ul><ul><li>T= 14700 / 0.76 = 19300N </li></ul>
14. 14. Forces at Angles <ul><li>The helicopter now moves forward. </li></ul><ul><li>The cable tilts – but the mass obviously does not change! </li></ul><ul><li>What is the Air Resistance on the car? </li></ul><ul><li>Which direction is it acting? </li></ul>
15. 15. Forces at Angles <ul><li>The helicopter now moves forward. </li></ul><ul><li>The cable tilts – but the mass obviously does not change! </li></ul><ul><li>Air Resistance = T Sin 15 </li></ul><ul><li>= 19300 Sin 15 </li></ul><ul><li>= 12500N </li></ul><ul><li>Which direction is it acting? </li></ul>
16. 16. Gravitational Field <ul><li>Gravity is proportional to mass </li></ul><ul><li>It acts on all masses </li></ul><ul><li>‘ Gravitational field Strength’ and ‘Acceleration due to Gravity’ are the same thing! </li></ul><ul><li>The units are either m/s 2 or N/Kg </li></ul><ul><li>Can you derive it from the equation: </li></ul><ul><li>F = ma ? </li></ul><ul><li>N = Kg x a therefore a = N/Kg </li></ul>
17. 17. Energy Changes <ul><li>Potential Energy = mgh </li></ul><ul><li>Kinetic energy = ½ m v 2 </li></ul><ul><li>If potential energy is converted to kinetic, how does v vary with the change in h? </li></ul><ul><li>mgh = ½ m v 2 therefore gh = 1/2 v 2 </li></ul><ul><li>Therefore v 2 = 2gh </li></ul>
18. 18. Energy Changes <ul><li>Draw a ramp on a piece of graph paper – side view </li></ul><ul><li>Draw a mass at the bottom of the ramp. Label it 150Kg </li></ul><ul><li>Measure or calculate the height at 3 different points on the ramp – it can be to scale… </li></ul><ul><li>Draw the mass at these points </li></ul>
19. 19. Skateboard Worksheet
20. 20. Forces and Momentum <ul><li>Can you re-arrange the following to make a new equation? </li></ul><ul><li>F = ma </li></ul><ul><li>a = v /t </li></ul><ul><li>Force x Time = mv </li></ul><ul><li>This tells us how large a force is needed over a given time to stop something </li></ul>
21. 21. Stopping Distances To stop an object, a force has to be applied for a certain time proportional to its moments – mv Force x Time = mv
22. 22. Stopping Distances e.g. how long does a force of 100N have to be applied to stop a car of mass 1000Kg travelling at 10m/s? Time = mv / Force = 1000 x 10 / 100 = 100s How far will it travel in this time? – s = (u + v / 2) x t = 50 x 100 = 5000m