Quick Quiz

1. Force is defined as?2. Newton’s First Law states?

Quick Quiz Results

1. a push or a pull2. An object at rest stays at rest, and

an object in motion stays in motion, unless acted upon by an external force.

Example #2An unbalanced force of 40 N gives a

stone an acceleration of 8.0 m/s2. What is the mass of the stone?

Given:F = 40 N a = 8.0 m/s2 m = ?

F = mam= F/am = 40/8m = 5 kg

Calculating Net Force1st

Examples 20 N 60

N40N

15 N 20 N25 N

16 N

Net Force = 40-60 = -20 N or 20 N left

Net Force = 20 N

Net Force = 20-15 = 5N Net Force = 25+16 = 41 N

ExampleIf Jane pulls an object to the left with 6N, and John pulls the same object to the right with 3N:

a.) What force will cause the motion of the object to change? (FNet)

b.) At what rate will the object accelerate if its mass is 3kg?

(assume there are no other forces acting on the object)

3N6N

a.) FNet = 3-6 = -3 N or 3N to the left

b.) FNet = maa = FNet/ma = 3N/3kg = 1 m/s2 to the left.

Acceleration due to Gravity

• the acceleration for any object moving under the sole influence of gravity.

• Acceleration due to gravity (g) = 9.8 m/s2

• The weight of an object is defined as the force of gravity on the object and may be calculated as the mass times the acceleration of gravity, Since the weight is a force, its unit is the Newton.

• What is the weight of a 45kg person on Earth and on the moon? gmoon = 1.6 m/s2

Fgearth = mgearth = 45*9.8 =Fgmoon = mgmoon = 45*1.6 =

In Motion!

Newton again..?

Newton’s Third LawYou’ve heard the saying, for every

action, there is an equal and opposite reaction.

This comes from Newton’s Third Law of Motion:

Every action (force) has an equal and opposite re-action (force).

The person pushes against the van, and the van pushes back with the same magnitude, in the opposite direction. Both the person and the van are put in motion because of these forces.

Newton’s 3rd Law Example

Newton’s Third Law Examples

• A balloon filled with air flies across the room when the air is released

• Rockets (the fuel expands, pushes on the rocket, the rocket pushes back on the fuel. These action and reaction forces expel the fuel and propel the rocket.)

• You jump off your skateboard, and the skateboard moves in the opposite direction to that in which you jumped

Momentum and Collisions

Which scenario will cause the bigger dent in your car:a)a Chevette travelling at 50 km/h crashes into your car?

b) a tank travelling at 50 km/h crashes into your car?

The tank will cause more damage because it has more momentum

Momentum and Collisions

• Which scenario will cause the most damage to you:

• a 5g bullet travelling at 30 m/s hits you in the leg (fired from a gun)?

• a 5g bullet travelling at 1 m/s hits you in the leg (thrown at you by a friend)?

Obviously, the bullet fired from the gun will cause you greater pain. In this case, the bullet from the gun has a greater momentum.

Momentum and Collisions

• Momentum depends on two factors, mass and velocity.

• Momentum is the product of mass and velocity, p=mv.

• p=momentum• m=mass• v=velocityMomentum is a vector! It is measured

in kg∙m/s

Calculating Momentum

• Calculate the momentum of a 1000kg car travelling at 6 m/s to the right.

Given:m = 1000 kgv = 6 m/s, rightp = mvp = (1000)(6)p = 6000 kg m/s, right

Practice Makes PerfectA cement truck has a mass of 1.5 x 104 kg, and is moving at 60 m/s. What is its momentum?

Given:m = 1.5 x 104 kgv = 60 m/sp = mvp= (1.5 x 104)(60)p= 9 x 105 kg·m/s

Practice Makes PerfectA bullet travelling at 925 m/s has a momentum of 4.5 kg·m/s. What is its mass?Given:m = ?v = 925 m/sp= 4.5 kg·m/sp = mvm = p/vm= 4.9 x 10-3 kg