1 / 190

Forces between objects

P2. LO: calculate the forces acting on an object. Forces between objects.

bridget-ireland
Download Presentation

Forces between objects

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. P2

  2. LO: calculate the forces acting on an object Forces between objects When two objects push or pull on each other, they exert equal and opposite forces on one another e.g. you are all pushing down on the floor, but the floor is also pushing up on you….if it didn’t you’d fall straight through the floor!

  3. LO: calculate the forces acting on an object Resultant force If you have multiple forces acting on an object, you can replace them with one single force that has the effect of all the other forces combined together. This single force is called the resultant force

  4. LO: calculate the forces acting on an object Calculating the resultant A rocket produces a thrust of 2000N. It has a weight of 1000N. What is the resultant force acting on the rocket?

  5. LO: calculate the forces acting on an object Calculating the resultant A rocket producing a resultant force of 1000N hits a wall, causing it to come to a stop. What force does the wall exert on the rocket and the rocket exert on the wall. Explain the reasoning for your answer.

  6. LO: calculate the forces acting on an object Calculating the resultant A car of weight 5000N produces a driving force of 2000N. It experiences friction force from the ground of 500N and air resistance of 300N. What are the horizontal and vertical resultant forces acting on the car?

  7. LO: calculate the forces acting on an object Rules for calculating the resultant Forces that act in the same direction can be added together Forces that act opposite to each other must be taken away Forces that act vertically and horizontally CAN NOT be added and taken away from each other and MUST be considered separately.

  8. LO: calculate the forces acting on an object Effects of forces 1 What is the resultant force on this book? What would happen if the force was not zero? • The resultant force on a stationary (not moving) object is zero! • If a resultant force is applied to an object, it will accelerate in the direction of the force

  9. LO: calculate the forces acting on an object Effects of forces 2 An arrow is fired from a bow. What are the forces acting on the arrow as it flies towards its target? • If an object is moving with constant speed, the resultant force on it is zero • If a resultant force is applied to a moving object, it will accelerate in the direction of the force

  10. LO: calculate the forces acting on an object Calculating forces F = m x a F • F = force (N) • m = mass (kg) • a = acceleration (m/s2) m x a

  11. LO: calculate the forces acting on an object Example 1 A car of mass 400kg is accelerating at 5m/s2. What is the driving force produced by the engine?

  12. LO: calculate the forces acting on an object Example 2 A novice skier is being pulled along a horizontal section of a nursery slope. Given that her acceleration of 1.3m/s2 is provided by a force of 70N, calculate her mass.

  13. LO: calculate the forces acting on an object Example 3 A man pushes a car with a force of 200N along a straight horizontal road. He manages to accelerate the car by 0.1m/s². Find the mass of the car.

  14. LO: understand how to draw and interpret graphs of motion Graphs of motion Graphs of motion are a visual representation of the motion of a body They can either show the change in displacementor change in velocityof an object

  15. LO: understand how to draw and interpret graphs of motion Can you draw… Mr R cycles into work. The journey takes him 15 minutes (900s) and is a total distance of 3km (3000m). We will try to represent his journey using a graph…

  16. LO: understand how to draw and interpret graphs of motion Can you draw… Mr R cycles to the first traffic light, a distance of 500m away. It takes him 180 seconds to do this. He waits at the traffic lights for 120 seconds while the light is red When the light turns green, he cycles for 2000m without stopping. This takes him 5 minutes to do. After 2000m, Mr R has to stop at another traffic light. He waits for 180 seconds. Realising that he is about to be late, he sprints the last 500m in 120 seconds.

  17. LO: understand how to draw and interpret graphs of motion Mini-plenary Usain Bolt ran the 100m race in London 2012 in approximately 9.6 seconds. He ran the first 20m in approximately 2.7 seconds after accelerating and running the final 80m in 6.9 seconds. It took him 20 metres to come to a stop, which he covered in 5 seconds. Draw a distance-time graph to show this journey. Explain what a horizontal line on a distance-time graph represents Extension: What do you think the steepness of a line on a distance-time graph represents?

  18. LO: understand how to draw and interpret graphs of motion Using distance-time graphs • How steep the line is (the gradient) on a distance-time graph tells you the speed that an object is moving • The steeper the line, the faster something is moving • Speed is measured in m/s

  19. LO: understand how to draw and interpret graphs of motion Calculating the gradient Change in y Gradient = Change in x ∆y Gradient = ∆x Lets have a go at working out the speed that Mr C was travelling at during his journey to school!

  20. LO: understand how to draw and interpret graphs of motion One last definition Two cars are travelling on a road in opposite directions. One is travelling east at 20m/s and the other is travelling west at 20m/s. Their speeds are exactly the same. However, their velocity’s are different. What do you think their velocity’s are? Velocity is the speed of an object in a given direction. Two objects can have the same speed, but very different velocities

  21. LO: understand how to draw and interpret graphs of motion Can you draw… Mr R has brought himself a slick new ride! He has also moved house and is now living in the leafy suburbs. His journey takes him 1200 seconds, his top speed is 50m/s and his lowest is -30m/s. Let’s plot his journey into school on a velocity-time graph…

  22. LO: understand how to draw and interpret graphs of motion Can you draw… Mr R leaves his house. He is happily driving along on country roads at a steady speed of 30m/s for two minutes… D’oh! He’s forgotten his lunch. He turns round and drives back at 30m/s for two minutes. He is at home for 60s. Back on the road, Mr R drives at 30m/s for 300s Now on the motorway, Mr R is able to drive at 50m/s, which he does for 5mins Coming off the motorway, he stops at a traffic light for 120s Realising he is going to be late, he steadily increases his speed for the next 180 seconds from 0 to 50m/s. He arrives JUST on time!

  23. LO: understand how to draw and interpret graphs of motion Task What does a horizontal line on a velocity-time graph represent? How do you know if an object has stopped by looking at the velocity-time graph? How can you tell if an object is accelerating using a velocity time graph? Draw the velocity-time graph for the following journey: 0-10s = 50m/s 10-25s = 0m/s 25-50s = 60m/s 50-80s = acceleration to 80m/s

  24. LO: understand how to draw and interpret graphs of motion Acceleration Acceleration is the change in speed of a body over a given amount of time

  25. LO: understand how to draw and interpret graphs of motion Acceleration Acceleration can be calculated using the following equation: Change in velocity Acceleration = Time taken Final velocity – initial velocity Acceleration = Time taken • a = acceleration (m/s2) • v = final velocity (m/s) • u = initial velocity (m/s) • t = time (s) v - u a = t

  26. LO: understand how to draw and interpret graphs of motion Example 1 A car accelerates from a velocity of 10m/s to a velocity of 25m/s in 15 seconds. What is the acceleration of the car?

  27. LO: understand how to draw and interpret graphs of motion Example 2 A runner starts at rest and accelerates to a top speed of 10m/s. If he does this in 2 seconds, what is his acceleration?

  28. LO: understand how to draw and interpret graphs of motion Example 3 A train accelerates at 2m/s² for 30 seconds. If its initial velocity was 10m/s, calculate what the final velocity will be after 30 seconds.

  29. LO: understand how to draw and interpret graphs of motion Task What is the acceleration of a car that starts at rest and reaches a top speed of 50m/s in 25 seconds? A plane starts at rest. It takes 8 seconds to take off and accelerates at a constant rate of 10m/s². What is the final take-off velocity of the aircraft? A runner starting at rest reaches a speed of 11m/s in 2.2 seconds during the drive phase of his 100m sprint. What is his acceleration during this phase? Assuming that his speed remains constant for the rest of the race, sketch the velocity-time graph for his journey A car accelerates at 5m/s² for 12 seconds, reaching a final velocity of 80m/s. What was the car’s initial velocity before it started accelerating?

  30. LO: understand how to draw and interpret graphs of motion How are they linked? Change in y Gradient = Change in x ∆y Gradient = ∆x The gradient of a velocity time graph represent the acceleration of an object! Go back and calculate the acceleration of Mr C in the final part of his journey

  31. LO: understand the factors that affect the stopping distance of a car Streamlining Most of the resistance forces that act on a car are due to air resistance. Streamlininga car will increase the top speed, even if the engine is giving the same power output

  32. LO: understand the factors that affect the stopping distance of a car Stopping distance The stopping distance of a car is the minimum distance that a car can safely stop in Stopping distance = thinking distance + braking distance

  33. LO: understand the factors that affect the stopping distance of a car other drugs and some medicines alcohol distractions, such asmobile phones tiredness speed Thinking distance The thinking distance is the distance travelled by the vehicle in the time it takes for the driver to react What factors will affect The thinking distance

  34. LO: understand the factors that affect the stopping distance of a car weather condition of tyres/brakes condition of road speed Stopping distance The stopping distance is the distance travelled by the vehicle during the time the braking force acts What factors will affect The stopping distance

  35. LO: understand the factors that affect the stopping distance of a car Typical stopping distances What effect would factors such as drugs, alcohol, tiredness, higher speed, adverse weather, poor road conditions or worn out breaks have on these stopping distances?

  36. LO: understand the factors that affect the stopping distance of a car Braking force Which of these would need the bigger force to stop if the stopping distance remained the same? Why?

  37. LO: understand the factors that affect the stopping distance of a car QWC Practice A local authority is worried about the number of road deaths occurring in the area. They have imposed a ban on mobile phones while driving, imposed a speed limit of 30mph and installed speed cameras. Explain how the changes may lead to fewer people being hit by cars. • 5-6 marks criteria: • Knowledge of accurate information appropriately contextualised • Detailed understanding, supported by relevant evidence and examples • Answer is coherent and in an organised, logical sequence, containing a wide range of appropriate or relevant specialist terms used accurately • The answer shows almost faultless spelling, punctuation and grammar.

  38. LO: understand what is meant by terminal velocity What is happening? Thegraph below shows the velocity-time profile for a skydiver falling through the air. Discuss with the people on your pod what you think is happening and why. Think about the forces that are involved at each stage

  39. LO: understand what is meant by terminal velocity Moving in a fluid Let’s think about what happens when an object moves through a fluid by considering a skydiver When the skydiver FIRSTjumps out of the aircraft, gravitycauses him to accelerate. The acceleration is a constantso the line on v-t graph will have an unchanging steepnessat the beginning

  40. LO: understand what is meant by terminal velocity Moving in a fluid As the speed of the skydiver increases, the air resistance on him increases. The increased air resistance causes his acceleration to decrease. However, his velocity is still increasing i.e. he’s speeding up slower than before, but he’s NOT slowing down.

  41. LO: understand what is meant by terminal velocity Moving in a fluid After a certain amount of time, the weight of the skydiver and the air resistance on the skydiver will be balanced. At this point, the skydiver will be moving at a constant velocity. This is the MAXIMUM velocity it is possible for him to move with and is known as the TERMINAL VELOCITY.

  42. LO: understand what is meant by terminal velocity Moving in a fluid The process that we have just considered is relevant for ANY object that is moving in a fluid e.g. a car driving along a road, a plane flying at 2000ft, a submarine underwater etc. What factors will affect the terminal velocity of an object?

  43. LO: understand what is meant by terminal velocity Moving in a fluid • The factors that will increase/decrease the terminal velocity of an object are: • The driving force that the object can generate • How streamlinedthe object is • The fluidthat the object is travelling through

  44. LO: understand what is meant by terminal velocity Calculating weight The weight of an object is the force that acts on an object due to gravity. It can be calculated using the following equation: W = m x g W = weight (Newtons) M = mass (kg) g = gravitational field strength (N/kg) g has a value of 9.81 on earth

  45. LO: understand the link between force and extension of an object Changing shape When a force is applied to an object, it may accelerate. However, a second effect that the force may cause is a change in shape of the object

  46. LO: understand the link between force and extension of an object Stretching objects What happens when you stretch an object? When an object is stretched, it stores elastic potential energy. Some objects are better at storing this energy than others. Which of the materials on your pod is better at storing elastic potential energy?

  47. LO: understand the link between force and extension of an object Material properties Beyond a point, the material will start to show plastic behaviour. A small increase in force will give a large increase in extension. The deformation will be irreversible(the material will not go back to the original shape when the force is taken away) Beyond the proportional limit, the material shows plastic behaviour. The extension is now much harder to predict

  48. LO: understand the link between force and extension of an object Real world application Knowing how materials change shape under force is essentialto most aspects of university. The flexing of aircraft wings can dramatically change the lift generated. It also needs to be within limits to make sure the wings don’t break off!

More Related