Newton’s LawsForces and Motion

Balanced and Unbalanced Forces

The acceleration of an object is produced by a net force that is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Newton’s 2nd Law

Fnet = m * a

or Forcenet = Mass * Acceleration

One unit of force causes a 1kg mass to accelerate 1 m/s2

One Force unit has the dimensions 1kg·m/s2

One Force unit is called a Newton, N.

Calculating Force

Net Force (N) Mass (kg)

Acceleration (m/s2)

1. 10 2 2. 20 2 3. 20 4 4. 2 5 5. 10 10

1. What acceleration will result when a 12-N net force is applied to a 3-kg object? A 6-kg object?

4 m/s2

2 m/s2

2. A net force of 16 N causes a mass to accelerate at the rate of 5 m/s2. Determine the mass.

m = 3.2 kg

Elephant and Feather - Free Fall Suppose that an elephant and a feather are dropped off a very tall building from the same height at the same time. Suppose also that air resistance could be eliminated such that neither the elephant nor the feather would experience any air drag during the course of their fall.

Which object - the elephant or the feather - will hit the ground first? The animation at the right accurately depicts this situation. The motion of the elephant and the feather in the absence of air resistance is shown. Further, the acceleration of each object is represented by a vector arrow.

The only force acting upon the two objects is the force of gravity.

The force of gravity experienced by an object is dependent upon the mass of that object.

Due to its greater mass, the elephant also experiences a greater force of gravity. That is the Earth is pulling downwards upon the elephant with more force than it pulls downward upon the feather.

When figuring the acceleration of object, there are two factors to consider - force and mass.

We can say that the elephant experiences a much greater force

The greater mass of the elephant (which tends to produce small accelerations) offsets the influence of the greater force

The greater mass of the elephant requires the greater force just to maintain the same acceleration as the feather.

Summary A simple rule to bear in mind is that all objects (regardless of their mass) experience the same acceleration when in a state of free fall.

When the only force is gravity, the acceleration is the same value for all objects. On Earth, this acceleration value is 9.8 m/s/s.

This is such an important value in physics that it is given another name - the acceleration of gravity.

Newton’s 3rd Law"For every action, there is an equal and opposite reaction."

The size of the force on the first object equals the size of the force on the second object.

The direction of the force on the first object is opposite to the direction of the force on the second object.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards.

Example

While driving, Anna observed a bug striking the windshield of her car. Obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?

Example

A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is ...

greater than the acceleration of the bullet. smaller than the acceleration of the bullet. the same size as the acceleration of the

bullet.

Example

Rockets are unable to accelerate in space because ...

there is no air in space for the rockets to push off of.

there is no gravity in space. there is no air resistance in space. ... nonsense! Rockets do accelerate in

space.

Example In the top picture, a physics student is pulling upon a rope which is attached to a wall. In the bottom picture, the physics student is pulling upon a rope which is held by the Strongman. In each case, the force scale reads 500 Newtons.

Is the physics student is pulling . . .with more force when the rope is attached to the wall.

with more force when the rope is attached to the Strongman.

the same force in each case.

Identifying Action and Reaction Force Pairs

consider the interaction between a baseball bat and a baseball.

The baseball forces the bat to the right (an action); the bat forces the ball to the left (the reaction).

Note that the nouns in the sentence describing the action force switch places when describing the reaction force.