Warm Up Explain how weight, matter, mass, and gravity are related? What units are used for Force? Explain the concept of Inertia (use an example or 2)

Warm Up

• Explain how weight, matter, mass, and gravity are related?

• What units are used for Force?

• Explain the concept of Inertia (use an example or 2)

Ch. 5 Newton’s Second Law

• An object travelling at constant velocity has zero acceleration – Applies to both the state of rest and constant

velocity

• In both cases, there is no change in state of motion because the forces that act on the object are balanced

∑F = 0 then a = 0 and if

a = 0 then ∑F = 0

• Force causes acceleration– How did we previously define acceleration?– Acceleration is the change in velocity over a

period of time

• Acceleration is directly proportional to force.

a F

• Mass is inversely proportional to acceleration

a 1 mass

a mass

2nd Law of Motion

Acceleration produced by a net force is proportional to magnitude of the net force, is in the same direction as the net force, and is

inversely proportional to mass.

Fnet = m · a

Forcenet = Mass x acceleration

Problem solving hints

• Always look at units given

– Mass is given in units of kg or g– Acceleration is given in units m/s2

– Force is given in N or kg·m/s2

Time to problem solve

• A 20 kg object is pulled so that it accelerates at 0.80 m/s2. What is the Force on the object?

• F = m·a• F = (20 kg)(0.8 m/s2)• F= 16 kg·m/s2

• Two girls are playing with a toy that has a mass of 0.30 kg. One of the girls decides that she wants it and tries to pull it away from the other girl. If girl one pulls on the toy with a horizontal force of 10.0 N and the other girl pulls on the dog with a horizontal force of 11.0 N in the opposite direction. What is the horizontal acceleration of the toy?

• Knowns :• F1 = 10 N• F2 = 11 N• m = 0.30 kg

• Equation a = Fnet/mass

• Two girls are playing with a toy that has a mass of 0.30 kg. One of the girls decides that she wants it and tries to pull it away from the other girl. If girl one pulls on the toy with a horizontal force of 10.0 N and the other girl pulls on the dog with a horizontal force of 11.0 N in the opposite direction. What is the horizontal acceleration of the toy?

• a = Fnet = 11. 0 N – 10.0 N

mass 0.30 kg

• a = 3.3 m/s2

A child is learning how to ice skate. The instructor want to pull the child so that she has an acceleration of 0.80 m/s2. If the child’s mass is 27.2 kg, with what force does the instructor

need to pull her?

• F = ma• F = (27.2 kg)(0.80 m/s2)• F = 21.76 N

Calculate the acceleration of a 500 kg object when it experiences a 200 N force.


• a = F/m


• a = F/m• a = 200 / 500• a = 0.4 m/s2

Calculate the horizontal force that must be applied to a 4 kg object to make it accelerate with the same

acceleration as if it were dropped and falling freely.

• If the object was falling freely, what would be its acceleration?




• a = g = 10 m/s2




• a = g = 10 m/s2

• F = m·a• F = (4 kg)( 10 m/s2)

Pressure

• Amount of force per unit area

• P = Force area

• Units are N/m2 or Pascal (Pa)

• If you have a mass that is distributed over a large surface area compared to a mass that has contact at only one small point, which will experience the greatest pressure at the point of contact?

• Sharp point vs. blunt surface – Change in area thus a change in pressure

The end!!!!

Time to practice in groupsRecorder – Records your responses

Documents

Warm Up Explain how weight, matter, mass, and gravity are related? What units are used for Force? Explain the concept of Inertia (use an example or 2)