Download 4.1 Forces and the Law of Inertia

Force
Chapter 4 (Ewen et al. 2005)
Objectives:
•Related force and the law of inertia.
•Apply the law of acceleration.

What causes an object to accelerate?
 Answer: forces

Force is any push or pull
 Forces , when unbalanced, change the motion of
an object.
 When balanced, motion stays constant.

Newton (N) in the SI system
Pounds (lb) in the English system

4.45 N = 1 lb


Newton discovered three relationships or
laws between forces and motion.
 Law of inertia: Newton’s first law
 Law of acceleration: Newton’s second law
 Law of action and reaction: Newton’s third law

An object in motion continues that motion
with the same velocity, and a body a rest
continues at rest unless an unbalance force
acts on it.

The property of a body that causes it to
remain at rest if at rest or to continue moving
with a constant velocity.
 It is an object’s resistance to a change in motion.

Inertia is directly related to mass of an object.
 More mass, more inertia
 Units for mass: kilogram (SI), slug (US)
 1kg = 0.0685 slug


Relates the applied force, the mass, and the
acceleration of an object.
It states that
 Acceleration is directly proportional to force
 And, inversely proportional to mass.

Units for force
 1 N = 1 kg m/s2
 1 lb = 1 slug ft/s2

What force is necessary to produce an
acceleration of 6.00 m/s2 on a mass of
5.00kg?

What force is necessary to produce an
acceleration of 2.00 ft/s2 on a mass of 3.00
slugs?

Find the acceleration produced by a force of
500 N applied to a mass of 20.0 kg.