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# 4.1 Forces and the Law of Inertia 4.2 Force and the Law of Acceleration PowerPoint PPT Presentation

Force Chapter 4 ( Ewen et al. 2005) . 4.1 Forces and the Law of Inertia 4.2 Force and the Law of Acceleration. 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

4.1 Forces and the Law of Inertia 4.2 Force and the Law of Acceleration

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Force

Chapter 4 (Ewen et al. 2005)

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

• Objectives:

• Related force and the law of inertia.

• Apply the law of acceleration.

### Forces

• What causes an object to accelerate?

• 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.