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Forces and Equilibrium in Physics

Learn about field forces and contact forces, with examples of objects in equilibrium and constant velocity. Understand the concept of net force and balanced forces.

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Forces and Equilibrium in Physics

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  1. Chapter 4.1 Review

  2. 1. What is the difference in a field force and a contact force? Give one example of each.

  3. Field forces do not require contact, gravity.Contact forces require contact, friction.

  4. 2. Draw a free body diagram of a book on a table being pushed at a constant velocity.

  5. Ff and FP are equal in magnitude, mg and FN are also equal in magnitude. The net force on the book is zero, so it is in equilibrium.

  6. 3. Draw a free body diagram of a baseball being hit by a bat.

  7. The bat is applying a force FB and gravity is pulling downward mg. The forces are NOT balanced so the ball is NOT in equilibrium. It is accelerating.

  8. 4. Draw a free body diagram of a box sitting on an incline.

  9. Since the box is not moving, it is not accelerating. The net force on the box must be zero, so these three forces must balance each other. The box is in equilibrium.

  10. 5. Draw a free body diagram of a box sliding down an incline at a constant speed.

  11. Even though the box is moving, it is not accelerating because the speed is constant. The box is in equilibrium, so the free body diagram is the same as in the previous question.

  12. 6. What is the minimum number of forces (besides zero) that can be applied to an object in equilibrium?

  13. Two. The forces must be equal and opposite.

  14. 7. A cat runs at a constant velocity, 10 m/s north. What is the net force on the cat?

  15. Zero. At a constant velocity, there is no acceleration. No acceleration means no net force.

  16. 8. A ball is falling through the air at a constant speed of 35 m/s. If the weight of the ball is 5 Newtons, what is the force of air resistance on the ball?

  17. 5 Newtons. Since the object is falling at a constant velocity, it is not accelerating. The forces must be balanced, so air resistance must equal gravity.

  18. 9. Dalyn is pushing upfield with a force of 200 N. If a linebacker hits him from the left with a force of 150 N, what is the net force on Dalyn?

  19. 2002 + 1502 = c2c = 250 Ntanq = 150/20037°to the right of upfield

  20. 10. With what force would Charles have to hit Dalyn to hold Dalyn at a constant velocity in question 9?(Better grammar!)

  21. Constant velocity means all the forces must be balanced. Charles must apply an equal force in the opposite direction (this is called an equilibrant force), 250 N, 37°left of back downfield.

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