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

Newton’s 1st and 2nd 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

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

Newton’s 1st 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

1st Law cont’d

• Sometimes called the law of inertiaInertia- 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

Newton’s 2nd Law of Motion

The acceleration of an object is equal to the net force acting on it divided by the object’s mass

What would happen if we doubled the mass, how would that affect acceleration?

Triple the Force?

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?

Example

• A 25 N force accelerates a boy in a wheelchair at 0.5 m/s2 . What is the mass of the boy and the wheelchair?

Weight and Mass

• NOT THE SAME THING!!• Weight- the force of gravity acting on an

object

-Essentially Newton’s 2nd Law

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