Objectives

1. Measure the net force on and acceleration of a cart on a flat track with different amounts of attached hanging mass
2. Calculate the predicted acceleration based on application of Newton’s 2^nd Law

Resources

• Flat dynamics cart track
• Cart with force sensor attached
• Pulley and string
• Mass hanger with some mass
• Heavy mass blocks for carts
• Science Workshop interface and DataStudio software
• Motion sensor
  • Datastudio setup file: newtonslaw_motionsensor.ds

Background

Force can be defined simply as anything that causes acceleration. The source of the force could be electric, magnetic, gravitational, or physical contact with another moving object (which is actually also an electromagnetic force if you look at the atomic level), to name only a few examples.

Isaac Newton formed three laws of motion that describe quantitatively how force effects acceleration:

1. An object will remain still or continue moving at its current velocity unless a force acts on it. Essentially, changes in velocity (acceleration) come from force and nothing else.
2. Acceleration produced by a force is inversely proportional to the object’s mass, specifically F=ma.
3. When an object exerts force on another object, the second object exerts an equal and opposite force on the first object. This means that force must be overcome to produce force on an object – or stated more crudely, it takes work to move things.

In this lab, you will be measuring the acceleration and the force applied to a cart on a horizontal track. A mass will be attached to the cart by a piece of string and allowed to hang from a pulley. In the diagram, choosing downward along the hanging part of the string and to the right on the track as the positive direction, the sum of the forces on the hanging mass is ma=mg-T. The only force along the string on the cart is the tension from the string Ma=-T. These two equations can be combined to show that the acceleration of the cart and the tension in the string, respectively, are a=g\frac{m}{M+m} T=g\frac{Mm}{M+m}

Hints:

Check that the force sensor measures zero Newtons when there is nothing pushing or pulling on the hook – if not press the TARE button to reset the zero point of the sensor. You should check occasionally that the sensor’s zero point is still correctly set – it can become miscalibrated if a large force is applied to the sensor.