An Introduction to An Introduction to Forces: Forces:

More PracticeMore PracticeDraw a free-body Draw a free-body

diagram for each of diagram for each of the following the following objects:objects:

(a) A car engine (a) A car engine being lifted from a being lifted from a car by a rope car by a rope attached to a pulleyattached to a pulley

An Introduction to An Introduction to Forces: Forces:

More PracticeMore PracticeDraw a free-body Draw a free-body

diagram for each of diagram for each of the following the following objects:objects:

(b) an car moving (b) an car moving with constant with constant velocity on a level velocity on a level roadroad

An Introduction to An Introduction to Forces: Forces:

More PracticeMore PracticeDraw a free-body Draw a free-body

diagram for each of diagram for each of the following the following objects:objects:

(c) an apple hanging (c) an apple hanging from a tree branchfrom a tree branch

An Introduction to An Introduction to Forces: Forces:

More PracticeMore PracticeDraw a free-body Draw a free-body

diagram for each of diagram for each of the following the following objects:objects:

(d) a skydiver being (d) a skydiver being slowed by a slowed by a parachuteparachute

An Introduction to An Introduction to Forces: Forces:

More PracticeMore PracticeMs. Rosebery is pulling across level Ms. Rosebery is pulling across level

snow a sled on which is sitting her snow a sled on which is sitting her daughter Ivy. Tied to the back of daughter Ivy. Tied to the back of Ivy's sled is another tiny sled on Ivy's sled is another tiny sled on which Ivy's baby doll is sitting. which Ivy's baby doll is sitting.

Draw the free-body diagrams for (a) Draw the free-body diagrams for (a) Ms. RoseberyMs. Rosebery

An Introduction to An Introduction to Forces: Forces:

More PracticeMore PracticeMs. Rosebery is pulling across level Ms. Rosebery is pulling across level

snow a sled on which is sitting her snow a sled on which is sitting her daughter Ivy. Tied to the back of daughter Ivy. Tied to the back of Ivy's sled is another tiny sled on Ivy's sled is another tiny sled on which Ivy's baby doll is sitting. which Ivy's baby doll is sitting.

Draw the free-body diagrams for (b) Draw the free-body diagrams for (b) IvyIvy

An Introduction to An Introduction to Forces: Forces:

More PracticeMore PracticeMs. Rosebery is pulling across level Ms. Rosebery is pulling across level

snow a sled on which is sitting her snow a sled on which is sitting her daughter Ivy. Tied to the back of daughter Ivy. Tied to the back of Ivy's sled is another tiny sled on Ivy's sled is another tiny sled on which Ivy's baby doll is sitting. which Ivy's baby doll is sitting.

Draw the free-body diagrams for (d) Draw the free-body diagrams for (d) Ivy’s sledIvy’s sled

An Introduction to An Introduction to Forces: Forces:

More PracticeMore PracticeMs. Rosebery is pulling across level Ms. Rosebery is pulling across level

snow a sled on which is sitting her snow a sled on which is sitting her daughter Ivy. Tied to the back of daughter Ivy. Tied to the back of Ivy's sled is another tiny sled on Ivy's sled is another tiny sled on which Ivy's baby doll is sitting. which Ivy's baby doll is sitting.

Draw the free-body diagrams for Draw the free-body diagrams for

(d) Ivy’s doll’s sled(d) Ivy’s doll’s sled

Newton’s 1Newton’s 1stst Law of Motion: Law of Motion: Learning GoalLearning Goal

The student will be able to state The student will be able to state Newton’s 1Newton’s 1stst Law and apply it in Law and apply it in qualitative and quantitative terms to qualitative and quantitative terms to explain the effect of forces acting on explain the effect of forces acting on objects. (B3.4)objects. (B3.4)

Newton’s 1Newton’s 1stst Law Law of Motionof Motion

Net ForceNet Force

The net force is the The net force is the sumsum of all forces of all forces acting on an object.acting on an object.

ExampleExample: A weightlifter holds a weight : A weightlifter holds a weight above his head by exerting a force of above his head by exerting a force of 1.6 kN [up]. The force of gravity 1.6 kN [up]. The force of gravity acting on the weight is 1.6 kN acting on the weight is 1.6 kN [down]. Draw a FBD of the weight. [down]. Draw a FBD of the weight. What is the net force on the weight?What is the net force on the weight?

Net ForceNet Force

FA=1.6 kN

Fg=1.6 kN

Net ForceNet Force

The net force is zero: 1.6 kN + (-1.6 The net force is zero: 1.6 kN + (-1.6 kN) = 0kN) = 0

FA=1.6 kN

Fg=1.6 kN

Newton’s First Law of Newton’s First Law of MotionMotion

If there is no If there is no netnet force acting on an force acting on an object, the object will remain at restobject, the object will remain at rest

Newton’s First Law of Newton’s First Law of MotionMotion

If there is no If there is no netnet force acting on an force acting on an object, the object will remain at restobject, the object will remain at rest

Newton’s First Law of Newton’s First Law of MotionMotion

If there is no If there is no netnet force acting on an force acting on an object, the object will remain at rest object, the object will remain at rest or will keep moving at the same or will keep moving at the same constant velocity.constant velocity.

Newton’s First Law of Newton’s First Law of MotionMotion

If there is no If there is no netnet force acting on an force acting on an object, the object will remain at rest object, the object will remain at rest or will keep moving at the same or will keep moving at the same constant velocity.constant velocity.

(Conversely, if an object is at rest or is (Conversely, if an object is at rest or is moving at constant velocity, there is moving at constant velocity, there is no net force acting upon it.)no net force acting upon it.)

InertiaInertia

This is the principle of This is the principle of inertiainertia, first , first articulated by Galileo:articulated by Galileo:

InertiaInertia

Inertia is the property of matter that Inertia is the property of matter that causes it to resist changes to its causes it to resist changes to its motion.motion.

The greater the mass of an object, the The greater the mass of an object, the greater its inertia. greater its inertia.

“Inertia is a property of

matter.”

Newton’s First Law of Newton’s First Law of MotionMotion

QuestionQuestion: An object is being pushed : An object is being pushed along at constant velocity by a force along at constant velocity by a force of 5 N [left]. What is the force of of 5 N [left]. What is the force of friction acting on the object?friction acting on the object?

Newton’s First Law of Newton’s First Law of MotionMotion

QuestionQuestion: An object is being pushed : An object is being pushed along at constant velocity by a force of along at constant velocity by a force of 5 N [left]. What is the force of friction 5 N [left]. What is the force of friction acting on the object?acting on the object?

If the velocity is constant, there is no If the velocity is constant, there is no net force, so the force of friction must net force, so the force of friction must be equal in magnitude and opposite in be equal in magnitude and opposite in direction to the applied force:direction to the applied force:

Newton’s First Law of Newton’s First Law of MotionMotion

Question: An object is being pushed along : An object is being pushed along at constant velocity by a force of 5 N at constant velocity by a force of 5 N [left]. What is the force of friction acting [left]. What is the force of friction acting on the object?on the object?

If the velocity is constant, there is no net If the velocity is constant, there is no net force, so the force of friction must be force, so the force of friction must be equal in magnitude and opposite in equal in magnitude and opposite in direction to the applied force:direction to the applied force:

FFff = 5 N [right] = 5 N [right]

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Explaining Why with Newton’s 1Explaining Why with Newton’s 1stst Law Law of Motionof Motion