4.1 Forces and the Law of Inertia4.2 Force and the Law of Acceleration

ForceChapter 4 (Ewen et al. 2005)

Objectives:•Related force and the law of inertia.•Apply the law of acceleration.

Forces

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.

Units for measuring force

Newton (N) in the SI system Pounds (lb) in the English system

4.45 N = 1 lb

Isaac Newton (1642-1827)

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

Law of inertia

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.

Inertia

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

Law of Acceleration: Newton’s 2nd Law

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

Example 1

What force is necessary to produce an acceleration of 6.00 m/s2 on a mass of 5.00kg?

Example 2

What force is necessary to produce an acceleration of 2.00 ft/s2 on a mass of 3.00 slugs?

Example 3

Find the acceleration produced by a force of 500 N applied to a mass of 20.0 kg.