Forces and MotionForces and Motion

Newton’s Laws of MotionNewton’s Laws of Motion

11stst Law – Law of Inertia Law – Law of Inertia

An object at rest will remain at rest until an An object at rest will remain at rest until an outside force acts upon it.outside force acts upon it.

An object in motion will remain in motion at An object in motion will remain in motion at a constant speed and direction until an a constant speed and direction until an outside force acts upon it.outside force acts upon it.

An object will just keep doing what it’s An object will just keep doing what it’s doing already.doing already. If it sits still – it stays stillIf it sits still – it stays still If it’s moving – it stays movingIf it’s moving – it stays moving

InertiaInertia

Inertia is an object’s tendency to resist Inertia is an object’s tendency to resist change in motion.change in motion.

All objects have inertia, because all All objects have inertia, because all objects have mass.objects have mass.

The greater the mass, the greater the The greater the mass, the greater the inertia.inertia.

The greater the inertia, the greater the The greater the inertia, the greater the force needed to overcome the inertia.force needed to overcome the inertia.

22ndnd Law of Motion Law of Motion

The acceleration of an object by a force is The acceleration of an object by a force is inversely proportional to the mass of the inversely proportional to the mass of the object and directly proportional to the object and directly proportional to the force.force.

Force = Mass x AccelerationForce = Mass x AccelerationSo…So…

The smaller the mass of an object, the greater The smaller the mass of an object, the greater the acceleration when a certain force is the acceleration when a certain force is applied to the object.applied to the object.

The greater the force applied to an object, the The greater the force applied to an object, the greater the object’s acceleration.greater the object’s acceleration.

22ndnd Law of Motion Law of Motion

bowling ball vs. golf ball

25 kg

1 kg

10 m 38 m

22ndnd Law of Motion Law of Motion

There is a relationship between There is a relationship between acceleration, mass, and force.acceleration, mass, and force.

Force = Mass x Acceleration or F = m x aForce = Mass x Acceleration or F = m x a

F

M A

x

÷÷

Object # 1: mass = 35 kg & acceleration = 12 m/sec

M x A = F

35 x 12 = 420 Newtons

Object # 2: mass = 5 kg & force = 110 Newtons

F ÷ M = A

110 ÷ 5 = 22 m/sec

22ndnd Law of Motion Law of Motion

F

M A

x

÷÷Object # 3: acceleration = 6 m/sec & force = 90 N

F ÷ A = M

90 ÷ 6 = 15 kg

Practice Problems:

1. F = 24 Newtons M = 45 g A = ?

2. M = 80 kg A = 195 m/sec F = ?

3. M = 16 g F = 35 Newtons A = ?

4. A = 52 m/sec F = 110 Newtons M = ?

33rdrd Law of Motion Law of Motion

If one object exerts a force on another If one object exerts a force on another object, then the second object’s force is of object, then the second object’s force is of equal strength in the opposite direction on equal strength in the opposite direction on the first.the first.

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

33rdrd Law of Motion Law of Motion

Example:Example:Two people are ice skating.Two people are ice skating.One skater pushes on the other skater. Both One skater pushes on the other skater. Both

skaters move even though only one skater skaters move even though only one skater exerted the initial motion.exerted the initial motion.

Other Examples:Other Examples:TrampolineTrampolineBeing in a swimming poolBeing in a swimming pool

33rdrd Law of Motion Law of Motion

Momentum = mass x velocityMomentum = mass x velocityMomentum – measure of the motion of an Momentum – measure of the motion of an

objectobjectWhich sledgehammer has more Which sledgehammer has more

momentum?momentum?A 3 kg sledgehammer swung at 1.5 m/sec?A 3 kg sledgehammer swung at 1.5 m/sec?A 4 kg sledgehammer swung at 0.9 m/sec?A 4 kg sledgehammer swung at 0.9 m/sec?

a) 3 kg x 1.5 m/sec = 4.5 kg · m/sec ***

b) 4 kg x 0.9 m/sec = 3.6 kg · m/sec

33rdrd Law of Motion Law of Motion

p

m v

x

÷ ÷p = momentum

m = mass

v = velocity

Practice Problems:

1) m = 15 kg v = 4 m/sec p = ?

2) v = 12 m/sec p = 140 kg · m/sec m = ?

3) p = 21 kg · m/sec m = 50 kg v = ?