Physics on Friday

9-28-07

To do…Friday

• Answers for first test

• Questions on Homework handout

• Class example problem

• Hand in the graph created by you walking before the motion detector with the analysis

• Talk about the causes of motion

To do Monday

• Hand in Handout #3• Hand in Graph work

– Divide up, calculate velocity, calculate acceleration, determine overall velocity

Forces- general informationForces –laws of motionForces- free body diagram

Homework #4Test thursday

Any questions on the homework handout?

Results are due on Monday,

if needed

New homework

• 3 more question on Handout #4

• Grade old homework and pass back tomorrow

• Test on Friday (primarily constant acceleration problems and review)

Do Now problem• Joe runs with a constant velocity of

5.4 m/s for 20 s before he reaches a pile of leaves. It takes 1.5 s for Joe to come to a stop in the pile. What is his rate of acceleration in the leaf pile and how far did Joe travel overall?

Answer

• Vf = Vi + at

• 0 = 5.4 + a(1.5)

• = -3.6 m/s2

What causes objects to move or change their motion?

Forces

• Contact (applied) Forces– Electromagnetic forces (most common)

• Non-contact Forces– Gravitational (most common)– Fields

Anything that pushes or pulls is a forceUnits in Newtons (for this class)

Weight is a Force

• Force due to the pull of gravity

• Weight (W) = mass X acceleration due to gravity

• On earth, everything in free-fall accelerates at the same rate (9.8 m/s2) towards Earth

Demonstration that all objects are pulled at the same rate

• Paper and book

• Paper-vs-paper

• The only reason why things do not fall at sam rate is air resistance

Determine your weight in Newtons

• Take you weight in lbs and divide by 2.2

• The 1st action converts from lbs kg

Kg is the common unit for Mass

Mass is the amount of stuff that makes up an object

Mass is the measure of the inertia of an object

• Inertia = the amount resistance to change in motion

• The object that is the most massive, changes its velocity the least when a constant force is applied

• Golf, Magic Tricks

Difference between mass and weight

• Mass = measure of the amount of matter that makes up an object

• Weight is the amount of Force generated due to the pull of gravity

To find your weight in Newtons

• Weight = mass x 9.8

• 9.8 = acceleration due to gravity on earth

• Change the location (moon, into space, on Jupiter): Change the acceleration value– Moon 1/6th of earth = 1.67– Jupiter about 2.5 times Earth = 25

What weighs 1000 N?

Football player (220 lbs)

What weighs 100 N?

Freshman backpacks

What weighs 10 N?

The average textbook

What weighs 1 N?

A smallish apple