Motion

1
Motion ForcesLesson 3

• Action and Reaction
• Newtons Third Law
• Momentum
• Conservation of Momentum

2
Newtons Third Law

• Newtons 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
Newtons 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?

• Arent these balanced forces resulting in no
  acceleration?

4
Newtons 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 Dont 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 Dont 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 Newtons third law, the
• gases exert a force on the
• rocket and push it forward.

9
Newtons 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
Momentum
p mv

• Momentum
• quantity of motion

• Use the unit kgm/s

p momentum (kgm/s) m mass (kg) v velocity
(m/s)
12
Force and Changing Momentum

• By combining these two relationships, Newtons
  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 doesnt 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 doesnt 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
Newtons Third Law

• Action-Reaction Pairs

• Both objects accelerate.
• The amount of acceleration depends on the mass of
  the object.

• 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 doesnt
  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
Momentum

• 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
kgm/s
20
Momentum

• The momentum of a second bumper car is 675
  kgm/s. What is its velocity if its total mass
  is 300 kg?

GIVEN p 675 kgm/s m 300 kg v ?
WORK v p m v (675 kgm/s)(300 kg) v
2.25 m/s
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 kgm/s
p 0
v p m v (21 kgm/s) (7 kg) v 3 m/s
pbefore 21 kgm/s
pafter 21 kgm/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 v 0 m/s
AFTER Clown m 50 kg v 20 m/s Cannon m 250
kg v ? m/s
p 0
p 1000 kgm/s
p 0
p -1000 kgm/s
pbefore 0
pafter 0
23
Conservation of Momentum

• Sonow we can solve for velocity.

GIVEN p -1000 kgm/s m 250 kg v ?
WORK v p m v (-1000 kgm/s)(250 kg) v
- 4 m/s (4 m/s backwards)
24
Rocket Challenge

• After I check your fill in the blank notes. Come
  and get the instructions for the paper rockets.
  Depending on time you might have time to do all
  3. If not you will complete them on Tuesday.
• You will use the best design and make it for your
  larger one we will do next week.