Forces and Motion The unit used to measure distance is centimeters. An athlete is running a 5-km race at an average speed of 8 km/h. She has to stop to tie her shoe. After tying her shoe, she continues running at 8 km/h. Why would her average speed for the entire race be less than 8 km/h. The total time she took to complete the race decreased because she stopped. An elevator traveled 15 floors (150 ft) at a speed of 4.5 m/s. What additional information is needed to determine the velocity of the elevator. The direction the elevator traveled. The definition of motion is A change in position over time. Tyrone wants to determine how fast a bowling ball travels when it is bowled. He uses a stopwatch to record, in seconds, the time it takes for the bowling ball to reach the pins. What unit would be most appropriate to measure the length of the bowling lane? meters. What two measurements are needed to determine the speed of an object? Distance and time What is acceleration? The rate at which velocity changes. What is an example of acceleration? A bus moves on a straight road and then makes a right turn. What is an example of centripetal acceleration. An object moving in a circle at 20m/s. An example of a contact force is Friction between an object and air. An index card is placed across the top of a drinking glass. A coin is then placed on top of the index card. A student quickly flicks the card sideways off the glass. Which of Newton’s laws explains why the coin falls into the glass rather than moving to the side with the index card. Newton’s first law of motion. A moving object collides with a stationary object. What is true according to Newton’s third law of motion? Each object exerts a force on the other, and the two forces are equal and in opposite directions. Objects with the greatest mass and the closest will have the greatest gravitational force between them. In order for a space shuttle to leave Earth, it must produce a great amount of thrust. Its rocket boosters create this thrusting force by burning great amounts of fuel. However, once in space, the shuttle needs very little fuel. It circles Earth while gravity pulls it toward Earth. What term describes the motion of the shuttle around Earth? orbit High and low tides are the regular pattern of rising and sinking ocean-water levels. They are formed as a result of the gravitational forces of the moon and sun on Earth’s oceans. The moon has a greater effect on Earth’s oceans than the sun. Why would the moon have a greater effect than the sun? The moon is closer to Earth than the sun. What is gravity? The force that pulls all objects toward one another. What is true of an object in free fall? Only the force of gravity is accelerating it. Two objects have a certain gravitational force between them. What would happen if the mass of the objects suddenly increased? The gravitational force between the two objects would increase. Melanie watched the path a baseball followed after a pitcher threw it. She noticed that the ball traveled horizontally away from the pitcher, as well as downward toward the ground. What force caused the ball to accelerate in the downward direction when it was thrown? Gravitational force A skydiver jumps out of a plane. She falls downward at a very fast speed. When she opens her parachute, she slows down. What force pulled the skydiver to the ground? gravity What is air that moves from regions of high pressure to regions of low pressure? wind How are water pressure and atmospheric pressure similar? They both result from the downward pull of gravity A student submerges a cube in a container of water. How can the student determine the buoyant force on the cube? Measure the weight of the water displaced by the cube A weather forecaster is determining the air pressure in order to describe the weather conditions. What characteristic of air is the weather forecasting analyzing? The force it exerts on a given area A hiker is climbing a tall mountain. How will atmospheric pressure at the top of the mountain compare to the atmospheric pressure at the bottom of the mountain? The pressure decreases with elevation so it will be lower at the top of the mountain than at the bottom. What equation describes the relationship between pressure, force, and area? Pressure = force/ area What results from a difference in fluid pressure? A child drinks milk through a straw. Rajiv made a model of a boat. When he places it in water, it sinks. According to Archimedes’ principle, why does the boat sink? The buoyant force is less than the boat’s weight.