The Laws of Motion Chapter

The First Two Laws of Motion Section 4-1 The British Scientist Isaac Newton published a set of three rules in 1687 that explained how forces and motion are related

The First Law of Motion If the forces acting on an object are balanced (net force is zero) then the velocity of the object does not change If the object is at rest it will stay at rest. If the object is moving in a straight line, it will continue moving in a straight line with constant speed

First Law Simply Stated If the object is at rest it will stay at rest. If the object is moving in a straight line, it will continue moving in a straight line with constant speed

“An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an outside unbalanced force”

Golden Rule #1 ONLY UNBALANCED FORCES CHANGE VELOCITY/MOTION

Example Suppose a toy car is at rest, it does not move unless you give it a push. The velocity (speed and direction) increases as long as you are pushing it. (forces are balanced) When you stop pushing frictional force will slow it down and stop it (forces become unbalanced)

Inertia and Mass All objects have a property called inertia, it is the tendency of that object to resist a change in its motion The inertia of an object depends on the objects mass

Example Bike wanting to move in a straight line when you want to turn (force needed to make it turn against inertia)

Example Which has more inertia and why? vs.

Inertia and the First Law of Motion

What Happens in a Crash? When the car that you are in suddenly stops, your body want to keep moving at the speed that you were traveling (inertia) The car stops due to unbalanced forces, but you keep moving unless you are wearing a seat belt!

The Second Law of Motion Newton’s second law describes how the net force of an object, its mass, and its acceleration are related.

Force and Acceleration

Example Which ball will have greater acceleration and why? vs.

Mass and Acceleration Acceleration depends on the objects mass and the net force exerted on it Net force, acceleration, and mass are related

Which will have a greater acceleration and why? vs.

The Second Law in Equation The acceleration of an object is in the same direction as the net force on the object Net force, acceleration, and mass are related

Algebraic Manipulation m = F/a

Example Problem #1 You push a friend on a sled. Your friend and the sled together have a mass of 70kg. If the net force on the sled is 35N (newton's), what is the sleds acceleration? Identify what the known values are and choose the proper equation…

Example Problem #2 If the mass of a helicopter is 4,500kg, and the net force on it is 18,000N, what is the helicopters acceleration?

Example Problem #3 What is the net force on a dragster with a mass of 900kg if it acceleration is 32.0 m/s 2 ?

Example Problem #4 A car is being pulled by a tow truck. What is the cars mass if the net force on the car is 3,000N and it has an acceleration of 2.0 m/s 2 ?

Gravity Section 4-2

What is Gravity? Gravity is an attractive force between any two objects that depends on the masses of the objects and the distances between them

The Four Basic Forces Gravitational Force Electromagnetic Force (electricity) Strong Nuclear Force (nuclei) Weak Nuclear Force (nuclei)

The Law of Universal Gravitation d

The Limits of Gravity The gravitational forces between two objects decreases rapidly as the distance between the two objects increases The larger the mass the more pull it exerts, the smaller the less pull it exerts

Gravitational Acceleration If air resistance can be eliminated, all objects will fall at the same rate of speed toward the Earth

9.8 m/s 2

Weight and Mass Mass is the amount of matter in an object Weight is a measure of the amount of gravitational force exerted on an object

Moon vs. Earth The gravitational force of the moon is 1/6 as strong as that of the Earth, so you would weight 1/6 as much on the moon Ex. 200 lbs. on Earth is 33lbs. on the moon

Why doesn’t the Earth pull you apart? The force of gravity pulling you down is offset by the floor, ground etc…pushing you back up. Balanced forces keep us together…

Projectile Motion Anything thrown or shot through the air is called a projectile. The Earth’s gravity causes projectiles to follow a curved path

Pitching a Ball The pitcher releases the ball horizontally, but the force of gravity gradually pulls the ball downward giving it a curved path

Centripetal Acceleration Acceleration toward the center of a curved or circular path. Remember that acceleration is not the same as speed. Acceleration is a vector that is made up of speed and direction

Centripetal Force The net force exerted toward the center of a curved path. Anything that moves in a circle is doing so because this force is accelerating it toward the center

Newton’s Third Law of Motion Section 4-3

Action and Reaction When a force is applied in nature, a reaction force occurs at the same time. “For every action, there is an equal and opposite reaction”

Action and reaction forces are not always equal!!! Sometimes forces cancel each other and no movement occurs, but other times the forces can combine to create motion…

Rocket Propulsion Rockets use Newton’s third law to propel themselves forward. The gases expelled by the rocket act to push the rocket up.

Momentum

Example Problem #1 At the end of a race, a sprinter with a mass of 80kg has a speed of 10m/s. What is the sprinters momentum?

Example Problem #2 What is the momentum of a car with a mass of 1,300kg traveling a speed of 28m/s?

Example Problem #3 What is the mass of a person walking a speed of 0.8m/s if there momentum is 52.0kg-m/s?

Example Problem #4 A baseball thrown by a pitcher has a momentum of 6.0kg-m/s. If the baseballs mass is 15.0kg, what is the baseballs speed?

Conservation of Momentum If a group of objects exerts forces only on each other, their total momentum does not change. Momentum gained equals momentum lost!