Forces

• Def – an action exerted on an object to change the object’s state of motion (resting or moving)– Units: Newtons (N)

• Net force – total force on an object

Balanced forces• Net force = 0, no movement

– An object standing still will not move– An object moving will not stop

Unbalanced Forces

• Net force is greater than zero– Object will move

Friction• Force that opposes motion between two surfaces

that are in contact

2 types– Static – friction between two forces at rest, initial

friction an object must overcome– Kinetic – friction between two moving objects

Newton’s Laws• He was born the same

year that Galileo died• Newton had ideas about

motion, which he called his three laws of motion– Also had ideas about

gravity, the diffraction of light, and forces

• Physics and Math• Helped create Calculus

First LawLaw of Inertia

• An object at rest will stay at rest and an object in motion stays in motion unless acted on by an external force

• Inertia – tendency of an object to stay at rest or stay in motion– Mass is a measure of inertia– Small mass = less inertia

• Seatbelts

1. Explain how this activity demonstrates the law of inertia.

The penny was at rest, and when you thumped the card from under it quickly, the penny remained at rest. It simply fell into the cup, rather than flying away with the card.

2. Do you think the texture of the card is important?The rougher the texture of the card, the less likely the trick will work. The

friction of the card could place a force on the penny, causing it to move with the cloth

rather than staying.3. What would happen with a quarter rather than a penny?

Second Law• The force acting on an object equals the object’s

mass times its acceleration– F=ma– Force is measured in Newtons (N)

• 1 N = 1 kg x 1 m/s2

2nd Law Problem1. What is the force necessary for a 1600 kg car to

accelerate forward at 2.0 m/s2 ?2. Mike's car, which weighs 1,000 kg, is out of gas.

Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s2 . How much force Mike is applying to the car.

Third Law

• For every action there is an equal and opposite reaction– Equal force but opposite in direction

• Example: Balloon full of air being released

Write Newton’s 3 laws.

Agenda for Wednesday Feb 19th 1. Review Newton’s laws2. Bozeman Science vid3. Gravity

Force of Gravity

• All objects are attracted to each other through gravitational force

• Bigger mass increases – larger force of gravity• Distance increases – force of gravity decreases• Universal Gravitation Equation

F=G(m1m2) d2

What is the force of gravity when…

What happens to the force of gravity if….

Objects start close

Then move farther away

Force of Gravity

• Free fall – only force of gravity is affecting an object

• Calculate the acceleration at which an object free falls– Earth – 9.8 m/s2

Force of Gravity

• Weight is not the same as mass– Depends on gravity– Different locations have different gravities and

therefore different weights

• Force of gravity on an object is weight– weight = mass x free fall acceleration– w=mg

• Newtons (N)

A person that weighs 50 kg runs and hits a wall with a force of 50 N. What was the person’s acceleration as they hit the wall?With what force did the wall hit the person?

Agenda for Friday Feb 21st 1. Go over Force Problems & Newton’s laws

worksheet2. Whiteboard questions3. Energy notesQuiz Wednesday – newton’s laws

WHITEBOARD QUESTIONS

Need a whiteboard, marker, paper towel, and calculator.

1. Who was the scientist who gave us the Laws of Motion?

2. Which law explains why we need to wear seatbelts?

3. Which law says that force is equal to mass times acceleration (F=MA)?

4. Which law explains how rockets are launched into space?

5. Which law says that heavier objects require more force than lighter objects to move or accelerate them (Throwing a bowling ball requires more force than throwing a baseball)?

6. In the absence of air resistance, describe the acceleration of a 1.5 kg book compared to the acceleration of a 15 kg rock if the objects were dropped from the same height?7. What happens to the gravitation force on an object as it gets closer to another object?8. There are 2 rocks. Rock A has a mass of 50 kg and rock B has a mass of 1000 kg. There would be a greater gravitational force between you and which rock?

9. If a 50 kg person hits a wall with a force of 25 N, with what force does the wall hit the person back?

10. What is the weight of a person on earth who weighs 25 kg?

11. A person kicks a ball with a force of 11 N. It travels at a rate of 3 m/s2. What is the ball’s mass?

12. What force is applied to a 2 kg ball to make it travel at 32 m/s2?

13. What object would have a greater inertia?A. A carB. A semi – truck

14. Why?

Gravity

• Read pages 352 – 359• Answer questions 1-6 on page 359

Projectile Motion• Vertical and horizontal components

– No affect on each other– Combined they form a curved path

• Same acceleration downward because of gravity and will land at the same time

Momentum

• Def. – mass and velocity of an object– P=mv– Units: kg x m/s AND direction

• Objects with larger mass have more momentum

• Objects moving faster have more momentum

Calculate Momentum

Calculate the momentum of a 6.00 kg bowling ball moving at 10 m/s toward the pins.

Calculate the momentum of the following objects:a. a 75 kg speed skater moving forward at 16 m/sb. a 135 kg ostrich running north at 16.2 m/sc. a 5.0 kg baby on a train moving east at72 m/sd. a seated 48.5 kg passenger on a train that is stopped

Force and Momentum

• To catch a ball you use a force to stop the ball– You change the ball’s momentum– If you increase the time to catch the ball you

reduce the force

Momentum and Collisions

• The total amount of momentum is an isolated system in conserved– m1v1 = m2v2

• During a collision, momentum can be transferred– Hitting billiard ball