July 13 - Force & Motion 1 Name_____________________________Date________________Partners_____________________________ FORCE AND MOTION A vulgar Mechanik can practice what he has been taught or seen done, but if he is in an error he knows not how to find it out and correct it, and if you put him out of his road, he is at a stand; whereas he that is able to reason nimbly and judiciously about figure, force and motion, is never at rest til he gets over every rub. –Isaac Newton OBJECTIVES • To develop a method for measuring forces reliably. • To learn how to use a force probe to measure force. • To explore how the motion of an object is related to the forces applied to it. • To understand the relationship between the direction of the force applied to an object and the direction of the acceleration of the object. • To understand how different forces can act together to make up a net force. This net force is that which changes an object's motion according to Newton's second law. • To understand the motion of an object with no net force applied to it and how Newton’s first law describes this motion. • To find a mathematical relationship between the acceleration of an object and its mass when a constant force is applied–Newton’s second law. • To develop consistent statements of Newton’s first and second laws of motion for one-dimensional motion (along a straight line) for any number of one-dimensional forces acting on an object. OVERVIEW In the previous labs, you have used a motion detector to display position—time, velocity—time, and acceleration—time graphs of different motions of various objects. You were not concerned about how you got the objects to move, that is, what forces (“pushes” or “pulls”) acted on the objects. From your own experiences, you know that force and motion are related in some way. To start your bicycle moving, you must apply a force to the pedal. To start up your car, you must step on the accelerator to get the engine to apply a force to the road through the tires. Modified from P. Laws, D. Sokoloff, R. Thornton Supported by National Science Foundation and the U.S. Dept. of Education (FIPSE), 1993-2000 PHYS 635, Summer 2005 University of Virginia Physics Department2 July 13 - Force & Motion But exactly how is force related to the quantities you used in the previous lab to describe motion–position, velocity, and acceleration? In this lab you will pay attention to forces and how they affect motion. You will first develop an idea of a force as a push or a pull. You will learn how to measure forces. By applying forces to a cart and observing the nature of its resulting motion graphically with a motion detector, you will come to understand the effects of forces on motion. Another major goal of this lab is to continue to develop the relationship between force and acceleration: the first two of Newton’s famous laws of motion. INVESTIGATION 1: MOTION AND FORCE You can use the force probe to apply known forces to an object. You can also use the motion detector, as in the previous two labs, to examine the motion of the object. In this way you will be able to explore the relationship between motion and force. You will need the following additional materials: • motion detector • PASCO motion cart without friction pad • rectangular brass piece to increase cart’s mass to 1 kg; • 0.5 kg masses, rectangular black • Elevated 2-m motion track • level • low-friction pulley, lightweight string, table clamp, variety of hanging masses Activity 1-1: Pushing and Pulling a Cart In this activity you will move a low-friction cart by pushing and pulling it with your hand. You will measure the force, velocity, and acceleration. Then you will be able to look for mathematical relationships between the applied force and the velocity and acceleration, to see whether either is (are) related to the force. 1. Set up the cart, force probe, and motion detector on the 2-m track as shown below. The cart should have a mass of about 1 kg with force probe included. Use the specially constructed thin brass piece to place in the tray on top of the force probe to reach about 1.0 kg. The force probe should be fastened securely to the cart so that its body and cable do not extend beyond the end of the cart facing the motion detector. (Tape the force probe cable back along the body to ensure that it will not be seen by the motion detector.) Modified from P. Laws, D. Sokoloff, R. Thornton Supported by National Science Foundation and the U.S. Dept. of Education (FIPSE), 1993-2000 PHYS 635, Summer 2005 University of Virginia Physics DepartmentJuly 13 - Force & Motion 3 Prediction 1-1: Imagine that you grasp the force probe hook and move the cart forward and backward in front of the motion detector. Do you think that either the velocity or the acceleration graph will look like the force graph? Is either of these quantities related to force? (That is to say, if you apply a changing force to the cart, will the graph of the velocity or acceleration resemble the graph of the force?) Explain. 2. To test your predictions, open the experiment file called Motion and Force L4.1-1. This will set up velocity, force, and acceleration axes with a convenient time scale of 5 s. 3. Zero the force probe by pressing the “Tare” button located on the side of the force probe. Grasp the force probe hook and begin graphing. When you hear the clicks, quickly pull the cart away from the motion detector, let it roll on its own for a couple of seconds, then quickly stop it. Pull and push the force probe hook along a straight line parallel to the ramp. Do not twist the hook. Be sure that the cart never gets closer than 0.2 m to the motion detector. 4. Print out one copy of the graph for your group and include it with your report. Question 1-1: Does either the velocity vs. time or acceleration vs. time graph resemble the force graph? Which one? Explain why or why not.