What information do these detectives collect that helps them reconstruct the accident? Why is the speed of a vehicle such an important factor in accidents? On your way home from school you notice that the traffic ahead has slowed. You notice the flashing lights of fire trucks and police cars ahead. There has been an automobile accident.
Friction can prevent motion If an object is not moving and a force attempts to slide it across a surface, the force of friction will also act on the object. Friction between the object and the surface tries to keep the object from moving. This is one effect of friction, to stop objects from slipping.
Friction can oppose motion If one object is sliding across another, the force of friction will always try to stop this slipping. The force of friction acts in the direction opposite to the direction in which the object moves. Friction will either slow down the object, or make it more difficult for other forces to keep the object moving.
Friction can create motion There are certain times when friction stops slipping from taking place but actually creates motion! Friction prevents car tires from slipping. Without the friction between your car tires and the road, the tires would slip and the car would not move. This is an example of friction creating motion. When friction causes an object to move we usually call it a traction force. But remember traction is really friction.
Investigating The Force of Friction Your task will be to investigate the factors that influence the size of the force of friction and how it affects energy flow in a simple system. Focus Question: What factors influence the force of friction?
Investigation Reflection What effect did changing the weight of the block have on the force of friction? What effect did changing the surface texture have on the force of friction? What would you suggest to do so that a sliding block transforms a very small amount of KE to HE? Two surfaces are tested by a student. Surface A produces a 50 N frictional force, but surface B only produces a 10 N frictional force. Which surface can transfer more energy in one minute? Explain. In the car accident scene, one road is wet and the other is dry. Will the force of friction be the same for both the wet and the dry road? What evidence did you collect from the experiment would support your answer?
Focus Question: What mathematical relationship shows how mass and speed are used to determine KE? Investigating Kinetic Energy We have already learned some important concepts and facts about energy. 1.An elevated object has gravitational potential energy (GPE) because of Earth's gravity. The energy needed to raise the object is stored as GPE. 2.The size of the GPE of an object depends on its mass (m), and its height (h). A numerical value for the object's GPE can be calculated by using the equation GPE = mgh 3.A moving object has an energy called kinetic energy (KE). The size of the KE of an object depends on its mass (m) and its speed (v). 4. When the object moves downward its GPE is transformed into KE.
Pre-Investigation Questions 1.Is the plot of GPE vs. Release Height a simple or a complex relationship? Explain how you arrive at your answer. 2.How should the GPE in the beginning compare to the KE upon impact with the ground? 3.What would the plot of KE at the bottom of the ramp vs Release Height look like? Why? Use a colored pencil to sketch this plot on the graph of GPE vs. Release Height.
4.What variables affect the amount of KE? 5.The equation for calculating GPE is mgh. What are some possible equations that might be used to calculate KE?
Open the spreadsheet file KE Equation Investigation. Guidelines for the investigation: You may adjust the coefficient (the number in front of the variables) and you may adjust the exponents of the mass & speed variables. You may only enter whole numbers or half numbers (i.e., the numbers 0.5, 1 or 2.5 can be entered, but not 0.25 or 3.6). In your journals keep a record of the combinations that you use in the investigation and the results.
Investigation Reflection 1.What is the correct equation for KE? 2.Could you get the KE plot to exactly match the GPE plot? Explain. 3.Look at the KE equation. Is the relationship between speed and KE a simple or a complex relationship?
Using Knowledge of Speed & KE to Reconstruct an Accident The mass of each car is 1000kg. The force of friction on the dry road is 10,000 N The force of friction on the wet road is 3,000 N
Directions: 1. Complete the data table. 2. Construct a Stopping Distance (meters) vs. Speed (mi/hr) graph. Plot both the stopping distance for a dry road and for a wet road on the same graph. 3. Use the graphs to determine the initial speeds of both vehicles. 4. Write a paragraph that describes how KE, speed, and stopping distance are related. 5. Write a paragraph that describes the results of the evidence analysis (who is at fault? how can you tell?).
Equations: To determine the KE of the vehicle use the equation: KE = ½mv 2 To determine the stopping distance use the equation: Stopping Distance = (KE / stopping force)
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