Download KINE 480 – LAB 1

Lab #3:
Impulse and Ground Reaction Forces (GRF)
In class, you have been introduced to the relationship that exists between ground reaction forces
(GRF), force, time, impulse and velocity. Newton's second law (F = ma) can be written in a form
which includes the definition of acceleration:
F = m ( v/ t)
With simple algebraic rearrangement, this can be written as:
F  t = m  v or
Impulse = Change of Momentum
The "Impulse" part of this expression is a new quantity in our study of mechanics. It depends on
both force and the time over which the force was applied. Units of impulse involve force and
time: N*s. This impulse-momentum relationship is an important means of determining what
motion results from an applied force. In the case of constant force application, impulse is
particularly easy to calculate (see figure 1). However, if force is not constant (which is typical of
most real situations) a slightly different approach must be used as shown in Figure 2.
Since the impulse is essentially the area under the force-time curve, it can be approximated by a
series of rectangles. Each rectangle would represent the impulse over the time interval  t.
Each area is calculated by taking the average force over a time interval ( t) and computing and
multiplying it by  t:
Area of rectangle = ((F1 + F2)/2) *  t
The purpose of this lab will be to estimate the horizontal impulse associated with the impact of
the front wheel of a mountain bike with a bump using the concept described above. With the data
set provided to you, you will:
1.
Graph both force data sets vs time on a single graph.
2.
For each fork, determine the maximal braking force. Make sure to highlight it in
your data sets.
3.
For each fork, determine the braking impulse using the approximation technique
described above.
4. For each fork, determine the change in velocity ( v) associated with the impact. Show
how  v was calculated.
5.