Review

F N = 20 N

F G = 20 N

1.) What is the net force on the object?2.) Are the forces balanced or unbalanced?3.) Will the object move?

Review

• Describe an example of Newton’s first law

The SI unit of force is the….

Review

• Team A is pulling with a force of 125 N greater than team B. Which team will win? What is the net force?

What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000N acts on it in an upward direction

Review

• How much force was used to push a car that had a mass of 25000 g and accelerated at 2 m/s2

• What is the mass of someone whose weight is 100 N on earth?

Newton’s 3rd Law

• Remember, a force cannot exist alone. Forces always exist in pairs

• Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object

• For every action, there is an equal and opposite reaction

The 2 forces are called action and reaction forces

Action-Reaction

• Bumper Car example

• Pushing a wall

• Hammer on nail

Action-Reaction Forces and Motion

Action-Reaction Forces Do Not Cancel

• Why didn’t the action-reaction forces acting on the swimmer not cancel each other and produce a net force of zero?

• Action force: acts on the water• Reaction force: acts on the swimmer• Only when equal and opposite forces act on the

same object do they result in a net force of zero

Horse & Wagon example

Momentum

• Which would you prefer to be hit by?: A loaded shopping cart headed your way or a small glass marble rolling toward you at the same speed?

• What if the small glass marble was going 100 times faster?

• We would say the small glass marble going 100 times fast has more MOMENTUM

Momentum

• Momentum is measured in units of kilogram-meters per second (kg x m/s)

• Which has more momentum, a .046 kg golf ball with a speed of 60 m/s, or a 7.0 kg bowling ball with a speed of 6 m/s?

Conservation of momentumBefore collision

10 m/s 5 m/s

M= 300,000 kg*m/s

150,000 kg*m/s

Momentum before collision = 450, 000 kg*m/s

M =150,000 kg*m/s M= 300,000 kg*m/s

Momentum after collision = 450, 000 kg*m/s

5 m/s 10 m/s

Conservation of Momentum

• Law of conservation of momentum: if no net force acts on a system, then the total momentum of the system does not change

• In a closed system, the loss of momentum of one object equals the gain in momentum of another object—momentum is conserved

Ball dropped from bridge

0 0.5 1 1.5 2 2.5 30

1

2

3

4

5

6

7

Momentum of a 0.25 kg ball

Series1Linear (Series1)

Time (sec)

Mom

entu

m (k

g*m

/s)

1.) At what time did the ball have zero momentum? Describe this point in the ball’s motion2.) At what time did ball have greatest momentum? 3.) What is the ball’s speed after 1.25 sec. (Hint: use the graph and the momentum formula)