1. Motion & Forces Lesson 3 Action and Reaction Newton’s Third Law
Momentum
Conservation of Momentum

2. Newton’s Third Law Newton’s Third Law of Motion
When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

3. Newton’s Third Law
Problem:
How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force?
Aren’t these “balanced forces” resulting in no acceleration?
NO!!!

4. Newton’s Third Law Explanation:
forces are equal and opposite but act on different objects
they are not “balanced forces”
the movement of the horse depends on the forces acting on the horse

5. Action and Reaction
When a force is applied in nature, a reaction force occurs at the same time.
When you jump on a trampoline, for example, you exert a downward force on the trampoline.
Simultaneously, the trampoline exerts an equal force upward, sending you high into the air.

6. Action and Reaction Forces Don’t Cancel
According to the third law of motion, action and reaction forces act on different objects.
Thus, even though the forces are equal, they are not balanced because they act on different objects.

7. Action and Reaction Forces Don’t Cancel
For example, a swimmer “acts” on the water, the “reaction” of the water pushes the swimmer forward.
Thus, a net force, or unbalanced force, acts on the swimmer so a change in his or her motion occurs.

8. Rocket Propulsion
In a rocket engine, burning fuel produces hot gases. The rocket engine exerts a force on these gases and causes them to escape out the back of the rocket.
By Newton’s third law, the
gases exert a force on the
rocket and push it forward.

9. Newton’s Third Law Action-Reaction Pairs
The hammer exerts a force on the nail to the right.
The nail exerts an equal but opposite force on the hammer to the left.

10. Momentum
A moving object has a property called momentum that is related to how much force is needed to change its motion.
The momentum of an object is the product of its mass and velocity
Momentum is given the symbol p and can be calculated with the following equation:

11. p = mv v Momentum p m Momentum quantity of motion p: momentum (kg·m/s)
Use the unit kg·m/s m p v
p: momentum (kg·m/s)
m: mass (kg)
v: velocity (m/s)

12. Force and Changing Momentum
By combining these two relationships, Newton’s second law can be written in this way:
In this equation mvf is the final momentum and mvi is the initial momentum

13. Law of Conservation of Momentum
The momentum of an object doesn’t change unless its mass, velocity, or both change.
Momentum, however, can be transferred from one object to another.
The law of conservation of momentum states that if a group of objects exerts forces only on each other, their total momentum doesn’t change.

14. Law of Conservation of Momentum
The results of a collision depend on the momentum of each object.
When the first puck hits the second puck from behind, it gives the second puck momentum in the same direction.

15. When Objects Collide
If the pucks are speeding toward each other with the same speed, the total momentum is zero.

16. Newton’s Third Law F m Action-Reaction Pairs Both objects accelerate.
The amount of acceleration depends on the mass of the object. F m
Small mass  more acceleration
Large mass  less acceleration

17. Conservation of Momentum
Law of Conservation of Momentum
The total momentum in a group of objects doesn’t change unless outside forces act on the objects.
pbefore = pafter

18. Conservation of Momentum
Elastic Collision
KE is conserved
Inelastic Collision
KE is not conserved

19. v Momentum p m p = ? p = mv m = 280 kg p = (280 kg)(3.2 m/s)
Find the momentum of a bumper car if it has a total mass of 280 kg and a velocity of 3.2 m/s.
GIVEN:
p = ?
m = 280 kg
v = 3.2 m/s
WORK:
p = mv
p = (280 kg)(3.2 m/s)
p = 896 kg·m/s m p v

20. v Momentum p m p = 675 kg·m/s v = p ÷ m m = 300 kg
The momentum of a second bumper car is 675 kg·m/s. What is its velocity if its total mass is 300 kg?
GIVEN:
p = 675 kg·m/s
m = 300 kg
v = ?
WORK:
v = p ÷ m
v = (675 kg·m/s)÷(300 kg)
v = 2.25 m/s m p v

21. Conservation of Momentum
A 5-kg cart traveling at 1.2 m/s strikes a stationary 2-kg cart and they connect. Find their speed after the collision.
BEFORE
Cart 1:
m = 5 kg
v = 4.2 m/s
Cart 2 :
m = 2 kg
v = 0 m/s
AFTER
Cart 1 + 2:
m = 7 kg
v = ?
p = 21 kg·m/s m p v
p = 0
v = p ÷ m
v = (21 kg·m/s) ÷ (7 kg)
v = 3 m/s
pbefore = 21 kg·m/s
pafter = 21 kg·m/s

22. Conservation of Momentum
A 50-kg clown is shot out of a 250-kg cannon at a speed of 20 m/s. What is the recoil speed of the cannon?
BEFORE
Clown:
m = 50 kg
v = 0 m/s
Cannon:
m = 250 kg
AFTER
Clown:
m = 50 kg
v = 20 m/s
Cannon:
m = 250 kg
v = ? m/s
p = 0
p = 1000 kg·m/s
p = 0
p = kg·m/s
pbefore = 0
pafter = 0

23. Conservation of Momentum
So…now we can solve for velocity.
GIVEN:
p = kg·m/s
m = 250 kg
v = ?
WORK:
v = p ÷ m
v = (-1000 kg·m/s)÷(250 kg)
v = - 4 m/s (4 m/s backwards) m p v

24. JET CAR CHALLENGE
CHALLENGE: Construct a car that will travel as far as possible (at least 3 meters) using only the following materials.
scissors
tape
4 plastic lids
2 skewers
2 straws
1 balloon
1 tray
How do each of Newton’s Laws apply?