HPP Activity A29.v1 5
This is a lot of work
You can now understand and calculate the motion of any object undergoing constant acceleration. You can determine how far, how fast, starting location, starting velocity, and the acceleration of any object. You've jumped into the air and measured your position and velocity as a function of time. You have also related accelerated motion to applied forces, drawn free-body diagrams, and constructed equations of motion using those free-body diagrams and Newton's laws of motion. You have related these ideas to various physical activities. What gives you the ability to jump, or do any other physical activities in the first place? It is one of the most essential components of the universe and is critical to all people. We are talking about energy.
Exploration
Let’s get to work!
GE 1.
1. Write a definition for Energy.
2. How do you get energy in order to do you physical activities?
3. What unit do you use to measure that energy?
4. Define the unit for measuring food energy. If you don’t know it from chemistry, go look it up.
5. Can you relate this energy unit to any other units for energy measurement?
6. So how much physical exertion does it take to “burn off” that energy you took in at lunch? Let’s be specific, how many steps do you think you would have to run up to “burn off” lunch?
7. Is your estimate a guess or is it based on a calculation? If so, show the calculation.
In order to answer this question more definitively, we better do a little exploring first. Let’s investigate the factors that affect the amount effort you exert during an activity.
GE 2.
1. Begin by placing the physics cart on the floor and have a group member sit on the cart. Attach the spring scale on to the cart and make sure the friction plate is not down. Pull on the spring scale so that the cart moves at a constant velocity for one meter. Record the spring scale reading when the cart is moving at a constant velocity. Repeat with the friction plate down so that you notice a moderate amount of friction, and again with a considerable amount of friction. Record your results.
Amount of friction / F [N]Small
Medium
large
2. Suppose there was truly no friction at all on the cart. What force would be required to keep the cart moving at constant velocity?
3. Determine which trial required the most effort and support your conclusion with data from your investigation.
4. Now do one more trial. Keep everything the same as trial three, but increase the distance to 4 meters.
5. How does increasing to 4 meters affect the amount of effort you exerted?
Invention
GE 3.
1. From your explorations, what are the two main factors that affected the effort required to pull the cart?
2. What kind of relationship exists between each of these factors and the effort required?
3. If you had to pull this cart around all day, you might consider this some pretty tough work. In fact, this is Work, even by the definition in the physics world. Using what you now know about Work, come up with a formula that solves for Work. Check with your instructor.
4. Now substitute SI base units of measure in the formula to come up with the unit for Work. What derived SI unit is this equal to? If you don’t know you can ask the instructor.
Application
To the stairmaster!
GE 4.
1. Now back to the question of how much Work you have to do to “burn off” lunch. Let’s say you had a Big Mac for lunch (about 700 Calories). We want to determine how many steps you must run up to burn this meal off. Start with moving up 1 meter. How much work must you do to move vertically upwards one meter at constant velocity?
2. How many steps must you go up to burn off lunch?
3. How much energy does an average person take in everyday? Based on your results from the Big Mac calculation, do you think you do enough physical activity to work off all the energy you take in everyday?
4. What are some other important functions your body uses energy for? Which one do you think requires the most energy? Defend your answer.
Exploration
You have investigated how force and distance are related to the amount of Work you do. How does changing the angle the force is applied at affect the Work?
GE 5.
1. Use the same cart set up as in the earlier exploration. Set the friction plate so there is some light friction on the cart. Attach the spring scale to the cart making sure the scale is always parallel to the floor while you are pulling the cart. Using a protractor to measure the angle, find the force needed to pull the cart at a constant velocity when the spring scale is at angles of 0, 30, 60, and 90 degrees. Record your data.
q / Fp [N]2. What happens to the force you must apply in order to achieve constant velocity as the angle is increased?
Invention
GE 6.
1. Using the forces and angles from the exploration, determine the component of the pulling force parallel to the direction of motion. Make sure you make a sketch of the forces on the cart and write the formula that you are using.
Top View of Cart
2. How do the parallel components of pulling force for each trial compare?
q / Component of Fp [N]4. How much work does the perpendicular component of pulling force do on the cart? Think about the trial for q = 90.
5. How much work does the parallel component of pulling force do on the cart for each trial?
6. Write the formula for finding work when the pulling force is at angle q with respect to the direction of travel. Check with instructor.
Application
GE 7.
1. Write the work done on the cart in terms of the friction force on the cart.
2. Using the results from above, write the work in terms of the pulling force, at an arbitrary angle.
3. Try to find an equation that relates the ratio Ff/Fp to the angle.
4. Graph your experimental values of Ff/Fp versus angle and theoretical values.
5. How does the experimental curve compare to the theoretical curve?
Application
GE 8.
1. Suppose you are pulling the cart at an angle of 30 degrees in the vertical direction with a force of 40 [N]. How much work is done on the cart after pulling it 4 meters?
Activity Guide
Ó 2010 The Humanized Physics Project