1. Brief Review from Yesterday Friction is the force between the surfaces of 2 objects that _____________ the motion of either object 2 students are pushing a box that is at rest The box pushed by the students is now sliding The students put rollers under the box and push it forward

2. Newton’s 1 st and 2 nd Laws of Motion Today, we understand relationships between force and motion Aristotle (384 B.C.-322 B.C.) – proposed that force is required to keep an object moving at constant speed

3. Galileo Italian scientist (1564-1642)- concluded that moving objects not subjected to friction or any other force would continue to move indefinitely Newton- In 1665, the plague broke out and forced Newton into solitude where he would work for 2 yrs building on the work of scientists such as Galileo

4. Newton’s 1 st Law of Motion The state of motion of an object does not change as long as the net force acting on the object is zero Unless an unbalanced force acts, an object at rest remains at rest, and an object in motion remains in motion with same speed and direction

5. 1st Law cont’d Sometimes called the law of inertia Inertia- the tendency of an object to resist a change in motion Think about what happens if you are moving in a car that is involved in a front-end collision

6. Newton’s 2 nd Law of Motion

7. Example An automobile with a mass of 1000 kg accelerates when the traffic light turns green. If the net force on the car is 4000 N, what is the car’s acceleration?

8. Example A 25 N force accelerates a boy in a wheelchair at 0.5 m/s 2. What is the mass of the boy and the wheelchair?

9. Weight and Mass

10. Earth and Moon What is the difference between the gravitational force on the Earth compared to the Moon? Astronaut on Earth: mass = 88 kg; weight = 863 N Astronaut on Moon: mass = 88 kg ; weight = 141 N