Newton’s Second Law of Motion

Force and Acceleration Force is a push or a pull. Acceleration is when the motion of an object changes, speed or direction Examples: Speed up Slow down Changes direction Mass is the amount of matter in an object.

Force and Acceleration a is the acceleration m is the mass, and F is the net force. F = ma

What does F = ma mean? Force is directly proportional to mass and acceleration. Imagine a ball of a certain mass moving at a certain acceleration. This ball has a certain force. Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball has twice the force of the old ball. Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.

More about F = ma If you double the mass, you double the force. If you double the acceleration, you double the force. What if you double the mass and the acceleration? (2m)(2a) = 4F Doubling the mass and the acceleration quadruples the force.

What does F = ma say? Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force. Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.

Plug in Practice A book with a mass of 2.0kg is pushed along a table. If the net force on the book is 1.0N, what is the book’s acceleration? Answer:.5m/s 2

Force and Acceleration Unit of Force = Newton (N) Unit of mass is kg Unit for acceleration is m/s 2

Gravity & Weight Gravity is the force of attraction that exists between any two objects that have mass. The force of gravity depends on the mass of the objects and the distance between them.

Gravity & Weight The force of gravity causes all objects near Earth’s surface to fall with an acceleration of 9.8 m/s² or 10.0m/s 2. Your weight on Earth is the gravitational force between you and Earth.

Gravity and Weight How are weight and mass different? Weight is a force, like the push of your hand is a force, and is measured in newtons. Mass is the amount of matter in an object, and doesn’t depend on location. Weight will vary with location, but mass will remain constant.

Newton’s 2 nd Law Newton’s Second Law states: an object acted upon by an unbalanced force will accelerate in the direction of the force. If you kick the ball, it starts moving. The ball accelerates only while your foot is in contact with the ball.

Circular Motion A rider on a merry-go-round ride moves in a circle. This type of motion is called circular motion. If you are in circular motion, your direction of motion is constantly changing. This means you are constantly accelerating.

Circular Motion If you are constantly accelerating, there must be a force acting on you the entire time. The force exerted is the centripetal force and always points toward the center of the circle.

Everyday language What do you say when a sports team is on a roll? They may not have the lead but they may have ___________ MOMENTUM A team that has momentum is hard to stop.

Momentum The momentum of an object is the product of its mass and velocity. Momentum = mass x velocity p=mv Momentum = mass x velocity p=mv

Momentum In symbols: p = mv p m v

How hard is it to stop a moving object? To stop an object, we have to apply a force over a period of time. This is called Impulse Impulse = FΔt Units: N∙s F = force (N) Δt = time elapsed (s)

Impulse In symbols: I = F Δ t I F ΔtΔt

How hard is it to stop a moving object? Using Newton’s 2 nd Law we get FΔt= mΔv Which means Impulse = change in momentum

Impulse & Momentum The impulse exerted on an object equals the object’s change in momentum.

Impulse & Momentum In symbols: I =  p