Mass and Acceleration

Types of Forces Noncontact ForcesContact Forces GravitationalNuclear PushesPulls ElectricMagnetic FrictionNormal Tension W M T N f perpendicular to surface Electromagnetic

Newton’s 1 st Law: No net force - no acceleration No acceleration = constant velocity

Motion of the Block at rest uniform motion to the right moves to the right and speeds up without changing direction moves to the right, slows down and stops without changing direction VelocityAccelerationPush or Pull Necessary to Maintain Motion Dry Ice on Slate

Pretest #3 (“the crate in the snow”) F by gravity F by snow ?

Pretest #3 (“the crate in the snow”) F by gravity F by snow

Newton’s 2 nd Law: Net force causes acceleration (not velocity) The net force is the vector sum of all the individual forces on an object. W E,b N r,b F net W E,b a Acceleration is in same direction as the net force. a

Types of Forces Noncontact ForcesContact Forces GravitationalNuclear PushesPulls ElectricMagnetic FrictionNormal Tension W M T N f Electromagnetic Which one is the net force?

What’s the mass? A measure of “how much stuff” A measure of how hard it is to speed up or slow down Very different than weight!

Hammer of Death

More net force - more acceleration (constant mass) F a F a  aF net

More mass - less acceleration (constant net force) F a a F  a M 1

Putting it all together a  M F net Constant × Newton is defined such that constant = 1.  a M F net  aM

1.0 kg 10 N Scale reading = ?

2.0 kg 10 N 20 N Scale reading = ?

Free body diagram of 1 kg crate: W E,C T S,C (Reading on the scale)

In General The force that the Earth exerts on an object of mass m is given by W = m (10 N/kg)

Question: If Jeff jumps off the physics building in a vacuum, what will his acceleration be? (use Newton’s 2nd law) 10 N/kg is just g Weight force is given by mg W E,J F net = m a W E,J = m a m (10 N/kg) = m a W = m (10 N/kg) = m g

2nd Law is the translator M = 5 kg T = 100 N a = 4 m/s/s Block W E,B T rope,B = 100 N N floor,B f floor,B = m g = 50 N a = 4 m/s/s How big is the friction force? = 50 N F net = m a In the x-direction: T - f = m a 100 N - f = (5 kg) (4 m/s/s) f = 80 N = 80 N