Unit 4 - Unbalanced Force Particle Model

Skateboard Challenge

Initial Trends and Patterns:

- The object accelerates when more force is applied

- The more force you apply, the faster it accelerates.  The less force you apply, the slower it accelerates

- The lighter the object is, the faster the acceleration (and vice versa pertaining to heavy objects)

- There's an inverse relationship between mass and acceleration

- There's a direct relationship between force and acceleration

Elevator Challenge Question

An elevator is moving up at constant velocity of 2.5 m/s.  The passenger has a mass of 85kg.

The elevator now accelerates upwards at 2m/s^2.

What's the equation of the vertical forces on the passenger?

Fnet = Fg + Fn

Newton's 2nd Law Key Points:

Situation - You have a 2kg box being pushed with a net force of 4N.

1) Calculate the box's acceleration:

a = Fnet/mass

a=4/2

a=2m/s^2

2) How would the acceleration compare if the mass of the box was cut in half?

The acceleration would increase/double.

3) How would the acceleration compare if the force was doubled?

The acceleration would increase/double.

4) How would the acceleration compare if the force was cut in half?

The acceleration would decrease/be cut in half.

5) How would the acceleration compare if the mass was doubled?

The acceleration would decrease due to no change in the force.

6) How would the acceleration compare if the mass and force was doubled?

The ratio would be higher between mass and force compared to the original, but essentially the object would be moving with the same acceleration.

7) How would the acceleration compare if the force and mass was cut in half?

The ratio would be lower between mass and force compared to the original, but the object would still move with the same acceleration.

8) How would the acceleration compare if the force was doubled and mass was cut in half?

The object would accelerate at a quick rate because a lot of force would be exerted on a light object.

9) How would the acceleration compare if the force was cut in half and the mass doubled?

The acceleration would decrease by a lot, because not a lot of force would be exerted on a heavier object.

Practice Problems!

1) A race car has a mass of 710kg.  It starts from rest and travels 40 meters in 3 seconds.  How big is the net force on the car?

First, solve for the acceleration.  Plug in all of the variables you have:

When you find the acceleration, solve for the Fnet.  This will be the total force acting on the car:

Here's what a force diagram would look like: 

2) A rollercoaster car has a mass of 300kg with passengers.  It accelerated down a 65 degree hill.  Draw a force diagram, determine the gravitational force parallel to the hill, and find the acceleration.

3) A sled weighing 300N is moved at constant velocity over a horizontal floor by a push force of 50N.  Determine the coefficient of kinetic friction between the sled and the floor.  Then, determine the sled's acceleration if the kinetic friction was 0.

Formulas to be familiar with:

vf = at + vi

Δx = 1/2 aΔt^2 + viΔt

vf^2 = vi^2 + 2aΔx

a = Δv/Δt

a = Fnet/mass

Ffriction = µ (Fnormal)