Forces &Newton’s Laws

Honors Physics

Newton’s 1st Law

An object in motion stays in motion, and an object at rest stays at rest, unless an unbalanced force acts on it.

An object will maintain a constant velocity if there is no NET force acting on it.

Newton’s 1st Law (The Law of Inertia)

Inertia—the tendency of an

object to resist a change in

its motion

Inertia is associated with an

object’s mass. Moving or

not moving.

Note: Inertia is NOT a force.

1st Law Examples

tablecloth trick—dishes at rest will stay at rest when a table cloth is snapped out from under them. The dishes have inertia.

Motorcycle going 60 mph and suddenly stops, but cyclist continues going forward at 60 mph. The cyclist has inertia.

Mass vs. WeightMass is how much matter is in an object (measured in kg)

Weight is a force that measures how much gravity pulls on that object (measured in Newtons)

𝑤 = 𝑚𝑔

where g is surface gravity

(on Earth, that’s 9.8 m/s2)

Equilibrium

Equilibrium is a state when all

of the forces on an object are

balanced.

Net force = 0 N

Acceleration = 0 m/s2

At rest ORmoving at a constant velocity

Example

Which object has the greatest inertia?

a) A 5-kg mass moving at 10 m/s

b) A 10-kg mass moving at 1 m/s

c) A 15-kg mass moving at 10 m/s

d) A 20-kg mass moving at 1 m/s

Example

Which object has the greatest inertia?

a) A 5-kg mass moving at 10 m/s

b) A 10-kg mass moving at 1 m/s

c) A 15-kg mass moving at 10 m/s

d) A 20-kg mass moving at 1 m/s (more mass, more inertia)

ExampleAn 8.0 kg piece of equipment is brought from Earth to the Moon. Surface gravity on the Moon is 1/6 of the surface gravity of Earth.

What is the weight of this equipment on the Earth?

What is the mass of the equipment on the Moon?

What is the weight of this equipment on the Moon?

ExampleAn 8.0 kg piece of equipment is brought from Earth to the Moon. Surface gravity on the Moon is 1/6 of the surface gravity of Earth.

What is the weight of this equipment on the Earth?

w=(8.0 kg)(9.8 m/s2) = 78.4 N

What is the mass of the equipment on the Moon?

Mass is the mass = 8.0 kg (on Earth and on the Moon)

What is the weight of this equipment on the Moon?

w= mg where g on the moon is (1/6)(9.8 m/s2) = 1.63 m/s2

w=(8.0 kg)(1.63 m/s2) = 13 N

Example

If the net force on a moving object is zero, the object will

a) gradually slow down and stop

b) immediately come to rest

c) continue moving with constant velocity

d) accelerate at a constant rate

Example

If the net force on a moving object is zero, the object will

a) gradually slow down and stop

b) immediately come to rest

c) continue moving with constant velocity (object in motion stays in motion with a constant velocity if no net force acting on it)

d) accelerate at a constant rate

Newton’s 2nd LawThere is a direct relationship between the net force and the acceleration of an object.

There is an inverse relationship between the mass and the acceleration of an object.

F = Net Force (all forces added up) (measured in N)

m = mass (in kg)

a = acceleration (in m/s2)

റ𝐹 = 𝑚 റ𝑎

Example

A force of 1 Newton is equivalent to

a) 1 kgm/s

b) 1 kgm2 /s2

c) 1 kg2m2 /s2

d) 1 kgm /s2

Example

A force of 1 Newton is equivalent to

a) 1 kgm/s

b) 1 kgm2 /s2

c) 1 kg2m2 /s2

d) 1 kgm /s2

Example

A constant net force on an object will cause

a) the object’s inertia to increase

b) the object to move with a constant velocity

c) the object to accelerate uniformly

d) the object’s acceleration to increase

Example

A constant net force on an object will cause

a) the object’s inertia to increase

b) the object to move with a constant velocity

c) the object to accelerate uniformly

d) the object’s acceleration to increase

Explanation: constant net force will cause a constant acceleration (the velocity may be changing, but it will change at a constant rate)

Example

If the net force on an object is constant, then the object’s

a) velocity will increase and the acceleration will remain constant

b) velocity will increase and the acceleration will increase

c) velocity will remain constant and the acceleration will be zero

d) velocity will remain constant and the acceleration will increase

Example

If the net force on an object is constant, then the object’s

a) velocity will increase and the acceleration will remain constant

b) velocity will increase and the acceleration will increase

c) velocity will remain constant and the acceleration will be zero

d) velocity will remain constant and the acceleration will increase

Explanation: constant net force will cause a constant acceleration (the velocity may be changing, but it will change at a constant rate)

Example

Which of these graphs show an object with a constant net force acting on it? (mark all that apply)

Example

Which of these graphs show an object with a constant net force acting on it? Looking for a constant acceleration

Example

A 2.0 kg cart is acted on by a 3.0 N net force. What is the resulting acceleration of the cart?

a) 6.0 m/s2

b) 1.5 m/s2

c) 1.0 m/s2

d) 0.67 m/s2

Example

A 2.0 kg cart is acted on by a 3.0 N net force. What is the resulting acceleration of the cart?

a) 6.0 m/s2

b) 1.5 m/s2

c) 1.0 m/s2

d) 0.67 m/s2

Example

A 2.0 kg cart is acted on by a 3.0 N force East and a 2.0 N force West. What is the resulting acceleration of the cart?

a) 0.5 m/s2

b) 1.0 m/s2

c) 1.5 m/s2

d) 2.5 m/s2

Example

A 2.0 kg cart is acted on by a 3.0 N force East and a 2.0 N force West. What is the resulting acceleration of the cart?

a) 0.5 m/s2

b) 1.0 m/s2

c) 1.5 m/s2

d) 2.5 m/s2

Newton’s 3rd Law

For every force, there is an equal and opposite force.

If A pushes on B, then B pushes on A with the same force in the opposite direction.

Forces occur in pairs. They don’t cancel because they are on two different objects.

Newton’s 3rd Law

While the forces on each object are the same, the accelerations may be different because of their masses.

Example

The Earth is about 80 times more massive than the Moon. Compared to the magnitude of the Earth’s gravitational pull on the Moon, the magnitude of the Moon’s gravitational pull on the Earth is

a) less

b) greater

c) the same

Example

The Earth is about 80 times more massive than the Moon. Compared to the magnitude of the Earth’s gravitational pull on the Moon, the magnitude of the Moon’s gravitational pull on the Earth is

a) less

b) greater

c) the same

(3rd law: Earth pulls on Moon, and Moon pulls on Earth with same force)

Example

A 1000 kg car going 15 m/s East collides with a 20,000 kg truck going 10 m/s West. During the collision, the force on the car is

a) less than the force on the truck

b) greater than the force on the truck

c) the same as the force on the truck

Example

A 1000 kg car going 15 m/s East collides with a 20,000 kg truck going 10 m/s West. During the collision, the force on the car is

a) less than the force on the truck

b) greater than the force on the truck

c) the same as the force on the truck

(3rd law: force of truck on car is same as car on truck)

Example

A baseball bat exerts a force of magnitude F on a ball. If the mass of the bat is three times the mass of the ball, the magnitude of the force of the ball on the bat is

a) F

b) 2F

c) 3F

d) F/3

Example

A baseball bat exerts a force of magnitude F on a ball. If the mass of the bat is three times the mass of the ball, the magnitude of the force of the ball on the bat is

a) F (3rd law: ball on bat is same as bat on ball)

b) 2F

c) 3F

d) F/3