Speed & Velocity Review

Velocity differs from speed in that we also know the direction of the moving object.

Velocity is both speed and direction. Velocity is a vector quantity. Speed is a scalar quantity. Constant speed doesn’t mean constant

velocity… the opposite is true though.

A student practicing for a track meet ran 250.0 meters in 30.00 seconds. What was

her average speed?

Light and radio waves travel through a vacuum

in a straight line at a speed of very nearly

3.00 x 108 m/s. How far is a light year (the

distance light travels in a year)?

A motorist travels 406. km during a 7.00 hr

period. What was the average speed in

km/hr and m/s?

A bullet is shot from a rifle with a speed of 720.

m/s. What time is required for the bullet to strike a target 3240. m

away?

WHAT IS FORCE?

FORCE = Any push or pull which can cause something to move (change its speed or direction)

Motion

A change in position in relation to an object.

Velocity A moving sidewalk is going forward at 10m/s. If you are

at rest in the airport what would you measure the velocity of a person on the sidewalk to be if the person is a) standing still on the sidewalk? b) walking with a speed of 2 m/s forwards? c) walking with a speed of 2 m/s backwards?

What would the person on the sidewalk measure your velocity to be if they aren’t walking on the sidewalk?

If you are on another moving sidewalk traveling the same direction at 10m/s what would you measure the velocity of the person to be if neither one of you are walking on the sidewalk?

If you are on another moving sidewalk traveling the opposite direction at 10 m/s what would you measure the velocity of the person to be if neither one of you is walking on the sidewalk?

Frame of reference

® An object or point from which motion is determined.

® The most common frame of reference is

ALTHOUGH THEY ARE MOVING AT MORE THAN 30,000 KM/HR ASTRONAUTS ON THE SPACE

SHUTTLE FEEL NO SENSATION OF MOVEMENT. WHY?

WHY DOES DRIVING 55 MPH FEEL DIFFERENT IN THE CITY

COMPARED TO THE COUNTRY?

NAME AN INSTANCE WHEN YOU FEEL LIKE YOU ARE MOVING

WHEN YOU ACTUALLY ARE NOT.

YOU’RE IN A SPACESUIT IN THE MIDDLE OF SPACE WITH NOTHING AROUND YOU (NO STARS, NO METEORS) AND

AN OBJECT GOES BY - ARE YOU MOVING OR IS THE

OBJECT MOVING?

History of Motion

ARISTOTLE GALILEO Greek scientist (Born 384

BCE) Natural motion Unnatural motion Believed that it was

“natural” for heavy objects to fall faster than light objects

Motion continues so long as there is only an applied motion (force) to an object. Removing the motion (force) stops the object.

Aristotle’s ideas lasted almost 2000 years…

Italian scientist (1564-1642 CE).

Leaning Tower of Pisa Said that a force is

required to change the motion of an object

1st scientist to formulate

idea of inertia

Inertia A property of matter The tendency of an object to resist any

change in its motion The greater the mass the greater the inertia The greater the speed the greater the inertia

Newton

Sir Isaac Newton (1642-1727) Mathematician and physicist Discovered many things:

Laws of motion Optics Gravity Calculus

Newton’s first law of motionThe Law of InertiaAn object at rest . . . and an object in motion . . .at . . .unless acted upon . . .

According to Newton’s 1st law

An object in motion stays in motion at constant velocity

An object at rest stays at rest

unless acted upon by an outside force.

Describe where the outside force is and how it changes the motion

Describe how the object is not affected by an outside force

or

Use Newton’s 1st law to explain why using seatbelts is important.

According to Newton’s 1st law, an object in motion stays in motion at constant velocity until acted upon by an outside force. The person in the car is traveling at the same velocity as the car. When an outside force is applied to the car to stop the car no outside force is applied to the person so the person continues to travel at the initial velocity until an outside force stops

him/her. The seatbelt is a much safer force to stop the motion of the person than the windshield or the ground.

What is a Force? Forces are vector quantities with direction Balanced forces are equal in size and

opposite in direction Unbalanced forces are not equal in size

and/or opposite in direction. Free body diagrams are used to show all

forces acting on an object Draw a free body diagram that shows

balanced forces and one that shows unbalanced forces

Force It is the net force that changes the state

of motion of an object.

5 N

5 N 5 N

5 N

10 N

10 N

10 N

15 N

0 N

is the same as

is the same as

is the same as

Net Force

5 N

Net Force Sum of all the forces on an object ( F) Net force on each box?

ΣF = 15 N ΣF = -9 N

Force

A body can have many forces acting on it and still have a zero net force.

And no net force means no change in

motion!

Equilibrium Rule

F = 0 when an object is at a constant velocity (stopped or moving)

If ΣF = 0 then there is no “outside force” so no change in motion

Static Equilibrium

Equilibrium for an object at rest Object hanging from spring scale Book on table:

Support or normal force

Equilibrium for moving objects(Dynamic equilibrium)

Objects traveling at constant velocity have F = 0 because of Newton’s 1st law – an outside force is needed to change the motion of an object

Computer simulation

When the pellet fired into the spiral tube emerges, which path will it follow? (Neglect gravity).

When the ball at the end of the string swings to its lowest point, the string is cut by a sharp razor.

Which path will the ball then follow?

Chapter questions Correct your friend who says, "The race-car

driver rounded the curve at a constant velocity of 100 km/h."

If a huge bear were chasing you, its enormous mass would be very threatening. But if you ran in a zigzag pattern, the bear's mass would be to your advantage. Why?

Consider a ball at rest in the middle of a toy wagon. When the wagon is pulled forward, the ball rolls against the back of the wagon. Interpret this observation in terms of Newton's first law.

Push a shopping cart and it moves. When you stop pushing, it comes to rest. Does this violate Newton's law of inertia? Defend your answer.

Consider a pair of forces, one having a magnitude of 20 N, and the other 12 N. What maximum net force is possible for these two forces? What is the minimum net force possible?

The sketch shows a painting staging in mechanical equilibrium. The person in the middle weighs 250 N, and the tensions in each rope are 200 N. What is the weight of the staging?

A different staging that weighs 300 N supports two painters, one 250 N and the other 300 N. The reading in the left scale is 400 N. What is the reading in the right hand scale?

Nellie Newton hangs at rest from the ends of the rope as shown. How does the reading on the scale compare to her weight?

If you toss a coin straight upward while riding in a train, where does the coin land when the motion of the train is uniform along a straight-line track? When the train slows while the coin is in the air? When the train is turning?

Review questions If the speedometer of a car reads a constant

speed of 50 km/hr, can you say that the car has a constant velocity?

A space probe may be carried by a rocket into outer space. What keeps the probe going after the rocket no longer pushes it?

Why do you lurch forward in a bus that suddenly slows? Why do you lurch backward when it picks up speed?

When a car moves along the highway at constant velocity, the net force on it is zero. Why, then, do you continue running your engine?

As you stand at rest on a floor, does the floor exert an upward force against your feet? If so, what exactly is this force?

Review Questions

Harry the painter swings year after year from his bosun's chair. His weight is 500 N and the rope, unknown to him, has a breaking point of 300 N. Why doesn't the rope break when he is supported as shown at the left above? One day Harry is painting near a flagpole, and, for a change, he ties the free end of the rope to the flagpole instead of to his chair as shown at the right. Why did Harry end up taking his vacation early?

A horizontal force of 100 N pushes a box across a floor at a constant speed. What is the net force acting on the box? What is the force of friction on the box?