1. Which force keeps everything on Earth firmly on the ground? A. Gravity B. Friction C. Mass D. Weight 2. Where is the gravitational force directed as you stand on the floor? A. Upward B. forward C. backward D. downward 3. Who was the scientist discovered the principle of gravity? A. Sir Isaac Newton C. Sir Galileo Galilei B. Sir Alexander Graham Bell D. Sir Archimedes 4. Which of the following equals the force exerted by the earth on you? A. Your mass C. half your mass B. Your weight D. half your weight 5. When you throw stone upward, the stone will fall down. What strong attractive force explains this statement? A. Friction B. Gravity C. Mass D. Weight 6. Which of the following statements is not true about friction? A. it acts upward opposing the pull of gravity B. it prevents things from sliding off surfaces C. it keeps things that are in contact with each other D. it holds objects that are separated by the great distance 7. What is the direction of the frictional force that acts on an object moving to the left? A. upward B. to the right C. downward D. to the left also 8. Which of the following is an example of friction that is helpful? A. brakes used in a bike B. Tires from a car on the road C. Tread on the bottom of your shoes D. All are examples that are helpful 9. Which of the following statements is true? A. Friction pulls objects towards the center of the earth. B. Friction primarily affects objects that contain iron C. Friction slows down or stops objects in motion D. Friction does not affect objects in motion, 0. Where is the gravitational force directed when you are holding a glass? A. upward C. into the glass B. downward D. away from the glass​