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# Forces and gravity part 1

## by hornerjm1 on Jun 18, 2012

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• Teacher notes Forces during take off include gravity, thrust and air resistance (friction). Photo credit: NASA Marshall Space Flight Center (NASA-MSFC)
• Teacher Notes The gradient of the graph increases when the 6 Newton weight is added because the spring has reached its elastic limit.
• Teacher Notes Encourage pupils to predict the outcome of each simulation.
• Photo credit: © 2008 Jupiterimages Corporation
• Photo credit: © 2007 Jupiterimages Corporation
• Photo credit: © 2008 Jupiterimages Corporation
• Worksheet 1 accompanies this slide. It provides further practice at calculating weights in different gravitational fields.

## Forces and gravity part 1Presentation Transcript

• 1 of 54 © Boardworks Ltd 2008
• 2 of 54 © Boardworks Ltd 2008
• What are forces? 3 of 54 © Boardworks Ltd 2008
• Push and pull 4 of 54 © Boardworks Ltd 2008
• Types of forcesAll forces involve interactionsbetween objects. There areseveral different types.Gravity and magnetismare forces that can act overdistances.Friction and upthrust areforces that involve directcontact between objects.All types of forces can occur whether objects are still or moving.What forces are acting during this rocket launch? 5 of 54 © Boardworks Ltd 2008
• Forces affecting objects 6 of 54 © Boardworks Ltd 2008
• Forces changing an object’s shape 7 of 54 © Boardworks Ltd 2008
• Balanced forcesThere are two wind machines applying forces to the ice skater.The forces acting on the skater are equal in magnitude andopposite in direction.The forces are balanced, so they cancel each other out.The skater does not move. 8 of 54 © Boardworks Ltd 2008
• Unbalanced forcesWhat if only one wind machine is blowing on the skater?The forces acting on her are no longer balanced so shewill start to move to the left. Her speed will change – thisis called acceleration.Unbalanced forces lead to a change in speed or direction. 9 of 54 © Boardworks Ltd 2008
• Measuring forces 10 of 54 © Boardworks Ltd 2008
• 11 of 54 © Boardworks Ltd 2008
• Force pairs 12 of 54 © Boardworks Ltd 2008
• Balanced and unbalanced forcesImagine a car travelling at a constant speed of 50 km/h.The engine provides sufficient force to balance all thefrictional forces that are acting to decrease the speed. 500 N 500 N 13 of 54 © Boardworks Ltd 2008
• Balanced and unbalanced forcesA crosswind acting on the car produces a sideways force. 500 N 500 N 100 N cross windThe crosswind causes the direction of the car to change– this happens because the sideways forces on the carare not balanced. The car will veer sideways.If the car turns right so that the wind is now behind thecar, what will happen to the speed? 14 of 54 © Boardworks Ltd 2008
• Resultant forceThe sum effect of more than one force is called theresultant force.The resultant force is calculated by working out thedifference between opposing forces in each direction.What is the resultant force on this truck?A resultant force of 100 N is accelerating the truck. 400 N 500 N 15 of 54 © Boardworks Ltd 2008
• Balanced and unbalanced forces – summary If the forces on an object are balanced:  and the object is stationary, it will remain stationary  and the object is moving, it will continue to move at the same speed and in a straight line. In other words, the object will continue to do what it is already doing without any change. If the forces are unbalanced, two things can happen:  The speed can change. This is called acceleration.  The direction of motion can change. 16 of 54 © Boardworks Ltd 2008
• Resultant forces – question 11. What is the resultant force on the satellite? 5N 5NResultant force = 20 N – 10 N = 10 N downThe satellite will acceleratedownwards. 20 N 17 of 54 © Boardworks Ltd 2008
• Resultant forces – question 22. What is the resultant force on the bird? The forces acting in each direction horizontally are equal in size, so there is no resultant force in this direction. Resultant force = 5 N – 5 N = 0 N 5N 5 N The vertical forces are not balanced, the bird will accelerate in a downwards direction. 5N Resultant force = 5N – 0N = 5 N down 18 of 54 © Boardworks Ltd 2008
• Resultant forces – question 33. What is the resultant force on the yacht? 10 N 10 N 13 N 20 N 10 NThe vertical forces are equal in size and opposite in directionso there is no resultant force in the vertical direction.The horizontal forces are not balanced, so the yacht willaccelerate to the right. Resultant force = (20 N +10 N) – 13 N = 17 N right 19 of 54 © Boardworks Ltd 2008
• 20 of 54 © Boardworks Ltd 2008
• What is gravity?When the netball is thrown, why does it fall back down?There is a gravitational forcepulling it towards the Earth.Gravity is a force that occurs between all objects.Gravity always acts to pull objectstowards each other.The Earth and the ball arepulling each other together.However, the ball moves muchmore than the Earth becauseit has a much smaller mass. 21 of 54 © Boardworks Ltd 2008
• Sir Isaac NewtonSir Isaac Newton is a veryfamous physicist who lived inEngland 1643–1727.Newton wrote down his ideasin the Philosophiae NaturalisPrincipia Mathematica; a veryimportant book about forcesand gravity.Some accounts suggest that oneof Newton’s greatest discoveriesoccurred when an apple fell onhis head and it made him thinkabout the reason it fell… 22 of 54 © Boardworks Ltd 2008
• Gravity and NewtonNewton realized that the motionof falling objects and objectsorbiting in space must be causedby the same force – Gravity!He wrote in the PhilosophiaeNaturalis, “It is an attractiveforce that makes apples fallfrom trees and the planets orbitthe Sun.”Other scientists had alreadynoted the effects of gravity butNewton was the first to calculatethe force of gravity on objects. 23 of 54 © Boardworks Ltd 2008
• What is gravity?Gravity is an attractiveforce that acts between Satellite’s gravityall objects that havemass. The size of theforce depends on themasses of the objects. Earth’s gravityAll objects produce agravitational force. Thisis very large for hugemasses such as planets.When you jump, the gravitational force of the Earth pulls youdown. Your gravitational force also pulls the Earth towardsyou, but you don’t notice it because the Earth is too heavy tobe visibly affected by your gravity. 24 of 54 © Boardworks Ltd 2008
• Gravity and distanceThe force of gravity between twoobjects depends on their massesand the distance between them.Spacecraft produce a verylarge force, called thrust, toovercome the force of gravity.As a spacecraft gets furtheraway from Earth, the forceof gravity gets smaller.Why do spacecraft lose their large fuel tanks and boosterrockets once they have left the Earth’s surface? 25 of 54 © Boardworks Ltd 2008
• Gravity during a rocket launch 26 of 54 © Boardworks Ltd 2008
• What are mass and weight?Mass is the amount ofmatter in an object and ismeasured in kilograms.Mass is not a force.An object, such as thissatellite, has the samemass at any point in theUniverse.Weight is a force and is caused by the pull of gravity actingon a mass.Weight is measured in newtons and has both magnitudeand direction. An object’s weight changes depending onwhere it is in the Universe. 27 of 54 © Boardworks Ltd 2008
• Investigating mass and weight 28 of 54 © Boardworks Ltd 2008
• Mass and weight on the MoonThe force of gravity on the Moonis only one-sixth of that on Earthbecause the Moon has a muchsmaller mass.Any object on the Moon weighsone-sixth of the amount it wouldweigh on Earth.Astronauts can jump up 20 feeton the Moon due to there beingsuch a low gravitational force.However, the astronaut still hasthe same mass – they just weighless because gravity is weaker. 29 of 54 © Boardworks Ltd 2008
• Mass and weight on different planets 30 of 54 © Boardworks Ltd 2008
• Weight and mass activity 31 of 54 © Boardworks Ltd 2008
• Missing words about gravity 32 of 54 © Boardworks Ltd 2008