Newton’s Laws of Motion Use the Force Luke!

Force

A force is anything that can cause an object with mass to accelerate Mass = MASS ACCELERATE Acceleration = How much stuff Increasing or decreasing your speed If you are going 50 km/h and stay that speed your acceleration = 0 = Weight

Force and Mass Mass is the measure of how hard it is to change an object’s velocity. (Inertia) Mass can also be thought of as a measure of the quantity of matter in an object.

Force Measured in Newtons Weight: is a force The effect of earths gravity on mass Force = MASS * ACCELERATION (F=ma) Mass = amount of stuff in the object Acceleration = gravity 10N = 1kg (on earth)

On the Moon Mass stays the same Gravity is different –Acceleration is different Gravity Moon = 1/6 th that of earth So if you weigh 50lbs on earth, how much will you weigh on the moon? –Remember weight is a force, and F=m*a F = 50lbs * (1/6) = 8.35lbs How many Newtons is this?

Law 1 An object at rest will stay at rest An object in motion will stay in motion Unless acted upon by an unbalanced force Newton’s Laws of Motion

Law 1 Newton’s Laws of Motion

5-2 Newton’s First Law of Motion Question: If you stop pushing an object, does it stop moving? Answer: Only if there is friction! In the absence of any net external force, an object will keep moving at a constant speed in a straight line, or remain at rest. This is also known as the law of inertia.

INERTIA Inertia – an object’s resistance to a change in it’s motion. Related to the mass of an object More mass = more inertia! Example: The heavier a shopping cart is, the harder it is to get moving or stop moving.

Tablecloth

In order to change the velocity of an object – magnitude or direction – a net force is required. An inertial reference frame is one in which the first law is true. Accelerating reference frames are not inertial. 5-2 Newton’s First Law of Motion

Not Inertial Frame of Reference You are on a train that suddenly stops while holding a tray with an ice cube on it. From your point of view the ice cube seems to have accelerated off the tray even though no visible force has acted upon it. This appears to violate the First Law of Motion

Law 2 The acceleration of an object is caused by an unbalanced force Acceleration is directly proportional to the force and in the same direction F=ma Newton’s Laws of Motion

5-3 Newton’s Second Law of Motion Two equal weights exert twice the force of one; this can be used for calibration of a spring:

5-3 Newton’s Second Law of Motion

Free-body diagrams: A free-body diagram shows every force acting on an object. Sketch the forces Isolate the object of interest Choose a convenient coordinate system Resolve the forces into components Apply Newton’s second law to each coordinate direction

5-3 Newton’s Second Law of Motion Example of a free-body diagram:

Law 3 For every action there is an equal and opposite reaction Example: Think Bumper Cars! –When you crash into a bumper car, that car pushes back on you, which is why you feel a sudden stop and a “jerk”. Newton’s Laws of Motion

5-4 Newton’s Third Law of Motion Some action-reaction pairs:

5-4 Newton’s Third Law of Motion Although the forces are the same, the accelerations will not be unless the objects have the same mass. Contact forces: The force exerted by one box on the other is different depending on which one you push.

While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus? Answer: trick question! They are equal Newton’s Laws of Motion

Momentum ALL moving objects have momentum Momentum depends on the mass of the object and the velocity Momentum = mass x velocity Momentum is in the direction of the velocity

Conservation of Momentum When two or more objects collide, momentum is never lost! Momentum before collision = momentum after collision Example: The momentum from a bat is transferred to the ball once the ball has been hit. Momentum of the bat = momentum of the ball

Momentum Momentum = Mass * Velocity P = M * V P i = 60 kg cm/s P f = 60 kg cm/s

Question? When fighting fires, a firefighter must use great caution to hold a hose which emits large amounts of water at high speeds. Why would such a task be difficult? Answer: The hose is pushing lots of water (large mass) forward at a high speed. This means the water has a large forward momentum. In turn, the hose must have an equally large backwards momentum, making it difficult for the firefighters to manage.

Problems Katie pushes a cart of physics text books with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0N? F = ma a = F / m a = 60.0N / 40.0g = 1.50 m/s 2

Problem A 25 N force accelerates Garrett on a bicycle at 0.5m/s 2 What is the mass of Garrett and the bicycle? F = ma m = F/a m = 25N/0.5m/s 2 = 50kg

Collisions Elastic –Bounces off each other Inelastic collisions –Stick Together

Inelastic Collision M 1 V 1 = M 2 V 2