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- Dynamics: force and newton's
- Newton's second law of motion:
Newton’s second law of motion is more than a definition; it is a relationship among acceleration, force, and mass. It can help us make predictions. Each of those physical quantities can be defined independently, so the second law tells us something basic and universal about nature. The next section introduces the third and final law of motion.
Applying the science practices: sums of forces
Recall that forces are vector quantities, and therefore the net force acting on a system should be the vector sum of the forces.
(a) Design an experiment to test this hypothesis. What sort of a system would be appropriate and convenient to have multiple forces applied to it? What features of the system should be held constant? What could be varied? Can forces be arranged in multiple directions so that, while the hypothesis is still tested, the resulting calculations are not too inconvenient?
(b) Another group of students has done such an experiment, using a motion capture system looking down at an air hockey table to measure the motion of the 0.10-kg puck. The table was aligned with the cardinal directions, and a compressed air hose was placed in the center of each side, capable of varying levels of force output and fixed so that it was aimed at the center of the table.
Forces |
Measured acceleration (magnitudes) |
3 N north, 4 N west |
48 ± 4 m/s
2 |
5 N south, 12 N east |
132 ± 6 m/s
2 |
6 N north, 12 N east, 4 N west |
99 ± 3 m/s
2 |
Given the data in the table, is the hypothesis confirmed? What were the directions of the accelerations?
Section summary
- Acceleration,
, is defined as a change in velocity, meaning a change in its magnitude or direction, or both.
- An external force is one acting on a system from outside the system, as opposed to internal forces, which act between components within the system.
- Newton’s second law of motion states that the acceleration of a system is directly proportional to and in the same direction as the net external force acting on the system, and inversely proportional to its mass.
- In equation form, Newton’s second law of motion is
.
- This is often written in the more familiar form:
.
- The weight
of an object is defined as the force of gravity acting on an object of mass
. The object experiences an acceleration due to gravity
:
- If the only force acting on an object is due to gravity, the object is in free fall.
- Friction is a force that opposes the motion past each other of objects that are touching.
Conceptual questions
Questions & Answers
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Source:
OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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