Friday PS 10-19

Test Review

Hand in day

Work/power

Hand in

• Yellow workbook

• Seat-belt position paper

Review for test on Monday

• ForcesDefinition- any push or pull on an objectUnits are NEWTONSIdentification

Applied forcesFrictionGravitational forcesMagnetic, electric forces

Forces

• Balanced forces = when all the forces acting on an object cancel each other out

• Unbalanced forces = when all the forces do not cancel each other out (more force on object in one direction than in others)

Net force

• The combination of all forces on an object

• For balanced forces Fnet = 0

– At rest or at constant velocity

• For unbalanced forces Fnet ≠ 0

– acceleration

Weight, mass

• Weight = the force due to gravitational pull on an object

W = mg g = 10 m/s2 on earth

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

• Differences? Similarities?

Inertia

• Is the measure of an objects resistance to change in its motion

• Directly related to the amount of mass

Newton’s 3 laws

• !st

• 2nd

• 3rd

• When does an object change its motion (accelerate)?

Collisions and the laws

• Be able to use the laws of motion to explain what happens in different types of collisions

• Front crash• Rear crash• At least 2 reasons for head rests• Does the bus and the car exert the same

amount of force on each other?• Why are people less injured on the bus?

Collisions and the laws

• Seat-belt configurations

• What is wrong with just lap-belts?

• Why are seat-belts flat and wide?

• What are uses for EKG’s, MRI’s and Xrays?

Free body diagrams

• FBD’s purpose: To visually show how the forces are arranged on an object

• Components of a free-body diagram– Forces– Object– Direction arrow

FBD

• Where do all but the direction arrow originate in the diagram?

• How are the forces of air resistance or friction drawn on FBD?

• How is the force of gravity drawn on any FBD?

2nd law problems

• Using Fnet = ma

• Units for each part are…?

• Triangle set-up

Advanced 2nd law problems

#1 Have to use the acceleration equation first then use 2nd law

#2 Have to determine Fnet first, then use 2nd law

Example

• Nicole slows down in her BMW from 15 m/s to 3 m/s in 42 seconds. What is the mass of her car if the net force on it is 7312 N?

Law Universal Gravitation

• There is a gravitational attraction between any two items that have mass.

• The amount of the force is related to the mass of each object and the distance their centers are apart

• More mass, less distance greater force

Free Fall

• Any object that moves under the influence of only the force due to gravity

• Thrown, Dropped objects

• Acceleration = 10 m/s2 or (9.8)

Terminal velocity

• The maximum velocity reached by a dropped object

• Occurs when the downward gravitational force is matched by the upward force of air resistance

• Air resistance depends on surface area and speed of object

Action and reaction forces

• Identification in the real world

• Equal and opposite

• Applied to different objects

• May have different effects

Momentum

• A quantity of motion

• The value of an objects mass times its velocity

• To have momentum the object has to be ___________.

Change in momentum

• To change an object’s momentum requires a net force being applied

• The greater the force and the longer the time applied the bigger change in momentum

• Baseball/softball examples• Fender benders vs totaling the car

Momentum change in a accident

• The amount of change in momentum is constant

• Change can be brought about different ways

More time less forceLess time more force

Seat Belts, crumple zones, air bags, Padded dashboards

Conservation of momentum

• In a system, momentum can not be created or destroyed, but can be transferred

• Momentum is direction oriented

• Billiards balls

Metric conversion

• 0.302 km _________ mm

What is the difference between distance and displacement?

Homework

• P368-370

Problems 7-11

Problems 18-20

Problems 32,34,39,40

Answers to homework

• P368-370

7 B

8 D

9 B

10 D

11 A

• 18 Free fall means there is only the force of gravity

Weightlessness = no force of gravity

19 less force of gravity, so acceleration due to gravity is less than 9.8

20 when the upward force of air resistance = force of gravity

• 32 a 195 kg m/s• 32 b 440 kg m/s• 32 c 0 kg m/s• 32 d 12 kg m/s

• 34 gravitational force is much less (1/16th)• 39 acceleration would be doubled• 40 a. Train, b. ball, c. equal, equal but

opposite