• In this chapter we will learn about the relationship between the force exerted on an object and the acceleration of the object.

• Forces

• Newton’s three laws.

Chapter 5: Force and Motion – I

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II.

Reading assignment: Chapter 5.5-5.8

Homework : (due Wednesday, Sept. 14, 2005)

Chapter 5: 34, 38, 39, 48, 58, 66

Contact forces- Involve physical contact between _________________.

Field forces:

-No _______________________ between objects

- Forces act through __________ space

gravity

electricmagnetic

Measuring forces

- Forces are often measured by determining the ___________________ of a calibrated spring.

- ______________ are vectors!! Remember vector addition.

- To calculate ________ force on an object you must use vector addition.

Newton’s first law:

In the absence of external forces (no net force):

• an object at rest __________ at rest

• an object in motion continues in motion with constant _____________ (constant ____________, straight line)

(assume no _____________).

Or: When no force acts on an object, the acceleration of the object is ____________________.

Inertia: Object resists any attempt to change its _____________

Inertial frame of reference:

-A frame (system) that is not ____________________.

- Newton’s laws hold true ____________ in non-accelerating (inertial) frames of reference!

Are the following inertial frames of reference:

- A cruising car?

- A braking car?

- The earth?

- Accelerating car?

Mass

- Mass of an object specifies how much inertia the object has.

- Unit of mass is ___________.

- The greater the mass of an object, the _____________ it accelerates under the action of an applied force.

- Don’t confuse mass and ______________ (more later).

Newton’s second law(Very important)

The acceleration of an object is:

- directly ____________ to the net force acting on it

- and ___________ proportional to its mass.

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xxnet amF , yynet amF , zznet amF ,

m

Fa

Unit of force:

• The unit of force is the Newton (1N)

• One Newton: The force required to _______________ a 1 kg mass by 1m/s2.

• 1N = 1______/s2

Two forces act on a hockey puck (mass 0.3 kg) as shown in the figure.

(a) Determine the magnitude and direction of the net force acting on the puck

(b) Determine the magnitude and the direction of the pucks acceleration.

(c) What third force (direction and magnitude) would need to be applied to the puck so that its acceleration is zero?

Black board example 5.1F2 = 8.0 N

2 = 60°

F1 = 5.0 N

1 = - 20°

The force of gravity and weight

• Objects are attracted to the Earth.

• This attractive force is the force of gravity Fg.

• The ________ of this force is called the weight of the object.

• Thus, the weight of an object is _______ (Force required to keep

mass m from falling to the ground).

gmFg

The weight of an object can very with _____________ (less weight on the moon than on earth, since g is smaller).

The mass of an object does not vary. Don’t confuse mass and weight