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# Energy

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A set of slides created to teach Energy to grade 10 students following the South African National Science curriculum (NSC CAPS) in Cape Town.

A set of slides created to teach Energy to grade 10 students following the South African National Science curriculum (NSC CAPS) in Cape Town.

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### Transcript

• 1. Mass & Weight
• Mass :
• This is a measure of the amount of matter (stuff) that makes up a body.
• Weight :
• This is the force of gravity exerted on a body’s mass by the earth (planet).
• 2. Mass & Weight - table Investigation: To determine the relationship between an object’s mass and weight. Method: We are going to weigh a number of different masses on a spring balance. We will record the mass and weight in each case. (In a table.) We will then analyse the results to see if any patterns (relationship) can be seen. Mass/kg Weight/N
• 3. Mass & Weight - table Investigation: To determine the relationship between an object’s mass and weight. Method: We are going to weigh a number of different masses on a spring balance. We will record the mass and weight in each case. (In a table.) We will then analyse the results to see if any patterns (relationship) can be seen. 2.8 0.30 2.3 0.25 1.8 0.20 1.3 0.15 0.8 0.10 0.3 0.05 Weight/N Mass/kg
• 4. Mass vs Weight - Graph From the graph we can see that the weight rises as the mass is increased. The straight line indicates a direct proportion between mass and weight. (Double the one the other will also double). Line corrected for systematic error scales were not zero’d correctly. Graph should pass through (0;0)
• 5. Mass & Weight - Results
• From the table we can see:
• As the mass increases the weight increases.
• The weight is (roughly) equal to the ten times the mass of the object.
• W = mass x 10
• Conclusion: The weight of an object in Newtons is equal to the object’s mass in kilograms multiplied by ten.
2.8 0.30 2.3 0.25 1.8 0.20 1.3 0.15 0.8 0.10 0.3 0.05 Weight/N Mass/kg
• 6. Work
• The unit of work is N.m called a Joule(J).
• The joule is the amount of work done when a force of 1N acts over 1m.
• 1J =
• Work is a scalar quantity.
• From the formula we see that if there is no movement then there is no work done.
• If the displacement is perpendicular to the force then there is no work done either.
• Q How many joules of work does it take to lift your body up a 4m staircase?
Work is done when a resultant force is applied to an object and causing the object to be displaced in the direction of the force. W = ............... 4m
• 7. Work
• The unit of work is N.m called a Joule(J).
• The joule is the amount of work done when a force of 1N acts over 1m.
• 1J = 1N.1m
• = 1kg.m.s -2 .m
• = 1kg.m 2 .s -2
• Work is a scalar quantity.
• From the formula we see that if there is no movement then there is no work done.
• If the displacement is perpendicular to the force then there is no work done either.
• Q How many joules of work does it take to lift your body up a 4m staircase?
Work is done when a resultant force is applied to an object and causing the object to be displaced in the direction of the force. W = F x s 4m
• 8. Energy
• Energy is the …………………. to do work .
• Energy is a scalar quantity and measured in ……………….
• Energy is ……………….. to an object when work is done on it by a resultant force.
• The energy transferred is ………………. to the work done .
• A 200N force moves a block (M) over a distance of 5m. How much energy is required?
Force – F (200N) M m Displacement s (5m) V (10m.s -1 )
• 9. Energy
• Energy is the ABILITY to do work .
• Energy is a scalar quantity and measured in JOULES
• Energy is TRANSFERRED to an object when work is done on it by a resultant force.
• The energy transferred is EQUAL to the work done .
• A 200N force moves a block (M) over a distance of 5m. How much energy is required?
Force – F (200N) M m Displacement s (5m) V (10m.s -1 )
• 10. Gravitational Potential Energy m Potential energy is the energy a body has because of its ................... or …................ . Any object that is above the ground has the POTENTIAL to fall Down. It has potential energy (E P ) - given by: E p = ………………. where; m = the …………. of the object (kg) g = the ……………….. due to gravity (10m/s 2 ) h = ………………. above the ground (m) How much energy is gained by a 50g tennis ball which is hit 6m up into the air? g (9.8m/s 2 ) E K Potential Energy (mgh) m m E p Potential Energy GAINED Object could fall down h
• 11. Gravitational Potential Energy m Potential energy is the energy a body has because of its position or state . Any object that is above the ground has the POTENTIAL to fall Down. It has potential energy (E P ) - given by: E p = mgh where; m = the mass of the object (kg) g = the acceleration due to gravity (10m/s 2 ) h = height above the ground (m) How much energy is gained by a 50g tennis ball which is hit 6m up into the air? g ( 9.8 m/s 2 ) E K Potential Energy (mgh) m m E p Potential Energy GAINED Object could fall down h
• 12. Kinetic Energy (E k ) m KINETIC ENERGY (E K ) is the energy a body has when it is ………………………... The kinetic energy is given by: E k = ……………………… where; m = the ………………… of the object (……………) v = the ……………………… (speed) the object is moving (…………..) What is the kinetic energy of a 1 tonne car moving at 60km.h -1 ? E K Kinetic Energy = …………..……. m
• 13. Kinetic Energy (E k ) m KINETIC ENERGY (E K ) is the energy a body has when it is moving. The kinetic energy is given by: E k = 1 / 2 mv 2 where; m = the mass of the object (kg) v = the velocity (speed) the object is moving (m/s) What is the kinetic energy of a 1 tonne car moving at 60km.h-1? E K Kinetic Energy ( 1 / 2 mv 2 ) m
• 14. Mass and speed If you increase the mass of a moving object you increase the kinetic energy. If you double the mass, you double the kinetic energy. If you increase the speed of a moving object you increase the kinetic energy. BUT, If you double the speed, you quadruple the kinetic energy. This is why even if you are slightly above the speed limit, you increase the kinetic energy of a moving car a lot, this means it is harder to stop the car and there is more chance of an accident.   
• 15. Energy Relationships TASK: Write an equation to express each of the relationship between Potential energy (E p ) and Kinetic energy (E k ) and each of constituent variables, i.e.; mass, (m), height (h), gravitational acceleration (g), and velocity (v). Express each of these relationships as an equation and draw a sketch graph to show this relationship graphically.
• 16. Velocity You may be asked to rearrange the kinetic energy formula so you can calculate the mass of a moving object or the velocity of a moving object, if you do not think you can rearrange the formula in the examination, remember the rearranged formulae. Do you know what the two formula would be for mass and velocity? Mass = 2E k (v) 2 Velocity = 2E k m
• 17. Mechanical Energy m Any object that is above the ground has the POTENTIAL to fall Down. The potential energy E P =mgh (Height) lost would be converted into speed (kinetic energy) E K . E p (Top) = E k (Bottom) If the object is lifted up then it gains potential energy. At any time the sum of these two is called the MECHANICAL ENERGY - and it stays constant. E p Potential Energy LOST E K gained = E p lost! E = E p + E k m 4kg 8m E p = mgh E k = 320J E = E p + E k = 320 + 0 = 320J E = E p + E k = 0 + 320 = 320J 4m E = E p + E k = 160 + 160 = 320J E p = 0 E k = 0J
• 18. Pendulum Investigation
• Plan an investigation to determine all the factors which affect the time taken for a pendulum to complete one full swing. (Period)
• 19. Energy
• 20. A pendulum
• A person of 60kg is lifted to a height of 30m on a slingshot pendulum and then released what is its maximum speed?
m h E Top = E p + E K = mgh + 1 / 2 mv 2 = (60)(10)(30) + 0 = 18000J Total Mechanical Energy (Top) E Bottom = E p + E k = mgh + 1 / 2 mv 2 18 000 = 0 + 1 / 2 (60)v 2 v 2 = 18000 / 30 = 600 v =  600 = 24.5 m.s -1