Module 2

What Causes an Object to Accelerate?

Student Guide

Amanda Beasley, Angela Chomokos, John Zikopoulos, Mangala Joshua, Pat Shontz

Motion is a common, everyday experience, whether we walk, run, ride a bicycle or drive an automobile. When you start your automobile engine, and press the "accelerator" pedal, the car begins to move. You can control your car's motion using the accelerator and brake pedals. At times you cause the car to move with constant speed. Sometimes you cause the car to speed up to pass another car. When you see a stop sign, you press on the "brake" pedal, and the vehicle slowly comes to a stop. What are the factors that determine how something moves? What can cause an object to change its motion? Can we predict an object's motion and how it will change with time? What causes objects to accelerate to greater speeds than other objects?

1. To investigate various factors that affect the motion of a cart on a horizontal track.
2. To understand and be able to interpret the motion graphs of the cart.
3. To find a relationship among the variables that affect the cart's motion.
   

           

1. Discuss with your group the relationship you predict between the net acceleration of the cart on the track as shown in the diagram and the net force acting on the cart due to the mass(M) hanging from the string on the pulley. Sketch a graph and write a mathematical function that predicts the relationship between acceleration and force.
   
2. Predict the relationship that you would expect between the net acceleration of the cart on the track and the total mass of the system. Sketch a graph and write a mathematical relationship between acceleration of an object and the total mass of the cart (mass of cart + mass of force probe + added mass m)
   
3. Set up the experiment illustrated in the diagram above. Connect the force probe cable to the CH 1 port of the CBL.Connect the motion detector cable to the SONIC PORT of the CBL. The motion detector cannot collect data on objects closer than 0.5 meter. Be sure that the motion detector is securely taped to the track. The cardboard can be used to make a large target for the motion detector. Connect the TI-83 to the CBL. Press the cable plugs FIRMLY into the sockets. Use the level to ensure that the track is horizontal, and that the string passing over the pulley is parallel to the track.
   
4. Use the PHYSICS program in your TI-83 calculator to collect data simultaneously from the motion detector and the force probe.
   
5. Test your group's predicted relationship between a)acceleration and force, b)acceleration and mass, by performing experiments using the force probe and the motion detector.
   
6. Write a single mathematical function which relates the three variables: force, mass and acceleration.
   

1. Before you performed the experiment, you sketched the predicted graph between acceleration and force. Was your prediction correct? What is the significance of the slope of the graph? What are the units of the slope?
2. A box with mass 2 kg is resting on an icy surface. Suppose you pull the box with a force of 4 Newtons at the same time that Maria pulls in the opposite direction with a force of 8 Newtons. Assume that the box can slide without friction on the slippery surface. a) Sketch a diagram to indicate the net force on the box ? b) What is the net acceleration of the box?
3. You are driving along a straight part of a road, and your speedometer indicates that you are traveling at constant speed. What is the net force acting on your car?
4. A rocket becomes easier and easier to accelerate as it travels through space. Explain why.
5. a) A book is resting on a table. Ignore friction, and suppose that you push the book with a force of 6N. When the book reaches a speed of 5 m/s, you stop pushing the book. Describe the motion of the book on the frictionless surface after you remove your hands. b)Sketch distance vs. time, velocity vs time and acceleration vs. time graphs for the book. Begin your sketch while the book is at rest on the table, and end your sketches several minutes after you stop pushing the book.

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Last modified 12 Aug 1997
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