Forces and Motion Newton’s Laws of Motion

1 st Law – Law of Inertia An object at rest will remain at rest until an outside force acts upon it. An object at rest will remain at rest until an outside force acts upon it. An object in motion will remain in motion at a constant speed and direction until an outside force acts upon it. An object in motion will remain in motion at a constant speed and direction until an outside force acts upon it. An object will just keep doing what it’s doing already. An object will just keep doing what it’s doing already. If it sits still – it stays still If it sits still – it stays still If it’s moving – it stays moving If it’s moving – it stays moving

Inertia Inertia is an object’s tendency to resist change in motion. Inertia is an object’s tendency to resist change in motion. All objects have inertia, because all objects have mass. All objects have inertia, because all objects have mass. The greater the mass, the greater the inertia. The greater the mass, the greater the inertia. The greater the inertia, the greater the force needed to overcome the inertia. The greater the inertia, the greater the force needed to overcome the inertia.

2 nd Law of Motion The acceleration of an object by a force is inversely proportional to the mass of the object and directly proportional to the force. The acceleration of an object by a force is inversely proportional to the mass of the object and directly proportional to the force. Force = Mass x Acceleration Force = Mass x Acceleration So… So… The smaller the mass of an object, the greater the acceleration when a certain force is applied to the object. The smaller the mass of an object, the greater the acceleration when a certain force is applied to the object. The greater the force applied to an object, the greater the object’s acceleration. The greater the force applied to an object, the greater the object’s acceleration.

2 nd Law of Motion bowling ball vs. golf ball 25 kg 1 kg 10 m38 m

2 nd Law of Motion There is a relationship between acceleration, mass, and force. There is a relationship between acceleration, mass, and force. Force = Mass x Acceleration or F = m x a Force = Mass x Acceleration or F = m x a F MA 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

2 nd Law of Motion F MA x ÷÷ Object # 3: acceleration = 6 m/sec & force = 90 N F ÷ A = M 90 ÷ 6 = 15 kg Practice Problems: 1.F = 24 NewtonsM = 45 gA = ? 2.M = 80 kgA = 195 m/secF = ? 3.M = 16 gF = 35 NewtonsA = ? 4.A = 52 m/secF = 110 NewtonsM = ?

3 rd Law of Motion If one object exerts a force on another object, then the second object’s force is of equal strength in the opposite direction on the first. If one object exerts a force on another object, then the second object’s force is of equal strength in the opposite direction on the first. For every action, there is an equal and opposite reaction. For every action, there is an equal and opposite reaction.

3 rd Law of Motion Example: Example: Two people are ice skating. Two people are ice skating. One skater pushes on the other skater. Both skaters move even though only one skater exerted the initial motion. One skater pushes on the other skater. Both skaters move even though only one skater exerted the initial motion. Other Examples: Other Examples: Trampoline Trampoline Being in a swimming pool Being in a swimming pool

3 rd Law of Motion Momentum = mass x velocity Momentum = mass x velocity Momentum – measure of the motion of an object Momentum – measure of the motion of an object Which sledgehammer has more momentum? Which sledgehammer has more 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

3 rd Law of Motion p mv x ÷÷ p = momentum m = mass v = velocity Practice Problems: 1)m = 15 kgv = 4 m/secp = ? 2)v = 12 m/secp = 140 kg · m/secm = ? 3)p = 21 kg · m/secm = 50 kgv = ?