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Balanced and Unbalanced Forces

Balanced and Unbalanced Forces. 14 July 2014 Objectives: To look at balanced or unbalanced forces and describe what will happen to the objects involved. Starter (5 mins). Show some photographs of objects, some stationary and some clearly in motion.

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Balanced and Unbalanced Forces

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  1. Balanced and Unbalanced Forces 14 July 2014 Objectives: To look at balanced or unbalanced forces and describe what will happen to the objects involved.

  2. Starter (5 mins) • Show some photographs of objects, some stationary and some clearly in motion. • Ask the students to describe/draw the forces acting on the objects and explain how they know that the objects are moving

  3. Moving or Stationary?

  4. Force pairs

  5. Balanced and unbalanced forces Imagine a car travelling at a constant speed of 50km/h. The engine provides sufficient force to balance all the frictional forces that are acting to decrease the speed. 500N 500N

  6. Balanced and unbalanced forces A crosswind acting on the car produces a sideways force. 500N 500N 100N cross wind The crosswind causes the direction of the car to change – this happens because the sideways forces on the car are not balanced. The car will veer sideways. If the car turns right so that the wind is now behind the car, what will happen to the speed?

  7. Resultant force The sum effect of more than one force is called theresultant force. The resultant force is calculated by working out the difference between opposing forces in each direction. What is the resultant force on this truck? A resultant force of 100N is accelerating the truck. 400N 500N

  8. Balanced and unbalanced forces – summary If the forces on an object are balanced: • and the object isstationary, it will remain stationary • and the object ismoving, it will continuetomove 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: • Thespeed can change.This is called acceleration. • Thedirectionof motioncan change.

  9. Resultant forces – question 1 1. What is the resultant force on the satellite? 5N 5N Resultant force = 20N –10N = 10N down The satellite will accelerate downwards. 20N

  10. Resultant forces – question 2 2. 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 = 5N – 5N = 0N The vertical forces are not balanced, the bird will accelerate in a downwards direction. 5N 5N Resultant force = 5N – 0N = 5N down 5N

  11. Practical • The students can now attempt a range of ‘force challenges’ using their construction materials. Suggestions include: • Which team can hold a 50 g mass highest? • Which team can suspend a 20 g mass furthest out from the edge of a desk? • Which team can build the longest self-supporting bridge? • Which team can build a boat that can support the greatest mass?

  12. Plenary • Test the students’ designs to destruction by increasing the loads on them. • The students should describe what is happening at the weaker parts of the structure as they collapse.

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