Test 3 – Forces

Study Guide

Conceptual Knowledge

  1. What is equilibrium?
  2. Can an object be moving if it is at equilibrium?
  3. What is friction?
  4. What two factors influence the magnitude of friction?
  5. A 20-N force and a 40-N force act in the same direction on an object. What is the net force on the object?
  6. What is Newton’s 1st law?
  7. What is Newton’s 2nd law?
  8. What is Newton’s 3rd law?
  9. A boxer preparing for a fight punches a sheet of paper. Which experience the greater force? Why?
  10. What is terminal velocity?
  11. If an object is at rest, can we conclude that no external forces are acting on it?
  12. An object thrown into the air stops at the highest point in it’s path. Is it in equilibrium?
  13. Which is usually greater, the maximum force of static friction or the force of kinetic friction?
  14. If Nellie Newton pushes an object with twice the force for twice the distance, she does ______.
  15. Which type of friction opposes the start of motion?
  16. What is another name for Newton’s first law?
  17. Mass is measured in what unit?
  18. What is the difference between mass and weight?
  19. The SI unit for force is ______.
  20. The law of inertia says that an object in motion remains in motion unless acted upon by an ______.
  21. What is friction?
  22. Draw a free body diagram for you pushing a box down the hall way.
  23. What is Newton’s third law in your own words?
  24. What is air resistance?
  25. In Newton’s third law, one force is called the _____ force and the other is the _____ force.
  26. If a tire pushing against the road is an action force, what is the reaction force?
  27. Weight is a ______and mass is a measure of the amount of ______an object contains.
  28. Describe the relationship between your weight and your mass on Earth and on the Moon.

Applying to the real world

  1. A satellite in free fall orbit around the Earth experiences a force about 80% of the force of gravity at the Earth’s surface. If the satellite has a mass of 500 kg. What is its weight in free fall?
  2. What is the acceleration of a 200 kg car if it is pushed with a 5 N force?
  3. Four forces act on a hot-air balloon. From the south 400N, from the north 5000N from the west 1000N, from the east 900N. What is the magnitude of the resultant force on the balloon.
  4. What net force is required to give a 25kg suitcase an acceleration of 3 m/s2 to the right?
  5. An 2000 kg car is moving to the right at a constant speed of 2 m/s. What is the net force on the car?
  6. A 5 kg bag of groceries is in equilibrium on an incline of angle Θ = 20°. What is the magnitude of the normal force on the bag.
  7. A desk has a mass of 71.25 kilograms. If the coefficient of static friction between the desk and the floor is 1.14, what force must be used to move the desk from rest?
  8. A hockey puck has a coefficient of kinetic friction of μk = .10. If the puck feels a normal force (FN) of 5 N, what is the frictional force that acts on the puck?
  9. Suppose a 10 N force is applied to the side of a 4.0 kg block that is sitting on a table. The block experiences a frictional force against the force that is applied. What is the weight of the block (FG)?
  10. A 52 N sled is pulled across a cement sidewalk at constant velocity. A horizontal force of 36 N is exerted. What is the coefficient of kinetic friction.
  11. If you have a mass of 37kg on earth, what is your weight? (W=m x g where g=9.81m/s2)
  12. A medium sized American car has a weight of 3000 pounds. What is its mass in kg? (Hint 1kg=2.2 pounds)
  13. A car has a mass of 1050kg and with acceleration it produces a force of 2200N. What is the acceleration?

Problems to know

  1. Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. There is a frictional force of 11N, a normal force of 18N and gravitational force of 22N. What is the net force?
  2. A student is designing a support to keep a tree upright. Two wires have been attached to the tree and placed at right angles to each other. One wire exerts a force of 30N on the tree; the other wire exerts a 40N force. Determine where the third wire should be placed and how much force is should have so that the net force acting on the tree is zero.
  3. A student pushes a box 20kg box with a force of 50N. The kinetic coefficient of friction between the box and the floor is .11. What is the force of kinetic friction? What is the acceleration of the box?
  4. A girl coasts down a hill on a sled, reaching the bottom with a speed of 7.0 m/s. If the coefficient of friction between runners and snow is 0.050 and the girl and sled together weigh 600 N. how far does the sled travel on the level surface before coming to a stop?(Hint: Find the overall acceleration first and then use your four kinematic equations)
  1. On takeoff, the propeller of a helicopter exerts an 8000-N force on the helicopter, directed upward at an angle of 65 degrees above the horizontal. The helicopter rises with constant velocity in the vertical direction while continuing to accelerate in the horizontal direction.

a) What is the weight of the helicopter?

b) What is the horizontal acceleration?

  1. A student attaches a rope to a 40kg box of books. He pulls with a force of 110N at an angle of 30° with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is 0.50. Find the acceleration of the box. (Hint: Break it up into it’s component form and use ΣF)