Section 1 review
A force is an action upon an object that can change the object’s velocity. The SI unit of force is the newton (N). One newton is the force required to accelerate a 1 kg object by 1 m/s2. Force is a vector: It has direction as well as magnitude (or strength). Weight is one example of a force. An object is in equilibrium when the net force (the vector sum of all external forces) on that object is equal to zero. Read the text aloud
force, newton (N), pound (lb), weight, free-body diagram, normal force, center of mass, net force, equilibrium

F w =mg
F 1 + F 2 + F 3 +=0

Review problems and questions

  1. Ariel’s puppy weighs 15 lb. Heather’s puppy weighs 50 N. Whose puppy weighs more? Read the text aloud Show
  1. Imagine that a spacecraft has been designed, built, and launched to explore the atmosphere of the planet Mars. To enter a circular orbit around Mars, the craft has to be slowed down. The spacecraft’s reverse thruster (its “brakes”) is supposed to deliver a force of 1,000 N whenever it is fired. The thruster, however, actually delivers 1,000 lb of force. Is it too weak or too strong? Read the text aloud Show
  1. The acceleration due to gravity at the surface of Mars is 38% of that on Earth. If a Mars rover has a mass of 950 kg on Earth, how much does it weigh on Mars? Read the text aloud Show
  1. A 100 kg astronaut weighs 500 N on a different planet.
    1. What is the astronaut’s mass on that planet?
    2. What is the acceleration due to gravity on that planet? Read the text aloud Show
Woman sitting on a stool
  1. A 120 lb woman sits on a 20 lb stool with her feet above the floor, as shown here. (Assume that the stool rests on a kitchen floor.)
    1. Is the woman in equilibrium? How about the stool?
    2. What is the net force on the woman? On the stool?
    3. Describe how to draw a free-body diagram for the woman.
    4. How strong is the normal force that the stool exerts upon the woman? Read the text aloud Show
  1. A student places a 15 kg box on a level floor.
    1. What is the net force on the box? What is the normal force on the box?
    2. The student now presses directly down on the box with a force of 30.0 N. What is the net force on the box? What is the normal force on the box?
    3. The student now pulls directly up on the box with a force of 80.0 N. What is the net force on the box? What is the normal force on the box? Read the text aloud Show

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