FORCE

DEFINITION TO REMEMBER: A force is a push or a pull!

Forces

cause

masses (objects)

to

accelerate.

F=m·aIn other words… Don’t forget this!

f=ma

m=f/a a=f/m

A 10kg cart…

accelerates at 3m/s2…

How much force is acting on the cart?

A force Problem

F=ma

F= 10kg 3m/s2

F= 30kgm/s2

Or 30Newtons (N)

Because 1kgm/s2 equals 1Newton (N)

Another force Problem

A 10kg cart…

…is accelerated by a 14N Force

…what is the acceleration of the cart?

a=f/m

a= 14kgm/s/s / 10kg

a= 1.4m/s2

Note that the “kg” cancels out leaving only the acceleration units.

And yet Another force Problem

A cart… …is accelerated at 2.0m/s2 by a by a 12N Force.

…what is the mass of the cart?

m= f/a

m=12kgm/s2 / 2.0m/s2

m= 6kg

WEIGHT is a measure of the force of attraction between the Earth and any object.

Fw= m Ag

**Force of weight equals mass times the acceleration due to gravity.

What is the weight of the 10kg cart?

Fw= m ·Ag

Fw= 10kg · 9.80m/s2

Fw= 98kgm/s2 or 98N

IntermissionClick: A Short Musical Interlude

Now a Little Quiz

1. What is the mass of the cart shown above?

F= 200NM= 100kg

2. What is the acceleration of the cart in the direction of the arrow?

3. In the SI ( metric) system weight is measured in: A. grams B. Kilograms C. Pounds D. Kgm/s2

4. In what direction does weight act on the cart?

M= 100kgF= 200N

5. What is the weight of the cart shown above?

a= 14m/s2

6. How much force does the rocket engine produce?

7. What is the acceleration of the rocket?

8. What is the mass of the rocket?

a= 14m/s2

9. What is the weight of the rocket?

10. In the equation Fw=mAg the Fw stands for ___?__.

11. In the equation Fw=mAg the Ag stands for ___?__.

1. 100kg given!

2. 2m/s2 use a=f/m

3. D same as a Newton

4. down duh!

5. 980N Fw = m·Ag 100kg x 9.8m/s/s

6. 600N given!

7. 14m/s2 given!

8. 43kg use m=f/a

9. 421N use Fw= m·Ag

10. Weight see the top

11. Accel. due to gravity…see the top

And now…FORCES in

EQUILIBRIUM

( Balanced Forces )

Here is a cart. The cart is sitting still. The cart has a mass of 20kg. If we consider the forces acting on the cart we show them using an arrow. Here is the weight of the cart.

The weight acts downward and is the product of mass times gravity.

The ground pushes up on the cart with a force equal to the weight, but in the opposite direction.

The weight acts downward and is the product of mass times gravity.

What happens if we try to pull the cart from left to right by applying a force?

The weight stays the same…

But the pulling force is opposed by a friction force!

pulling forcefriction force!

+ pulling force- friction force!

If both the pulling force and the friction force are equal, the cart is in equilibrium! This means the forces are balanced.

In this case the friction force is negative (-) and the pulling force is positive (+).

+ equilibrant force

- friction force!

When the pulling ( or pushing force) equalsequals the friction force it becomes the equilibrant force.

NOW the cart is in equilibrium…the two opposing forces are BALANCED ( that means EQUAL!)

•This cart is now in EQUILIBRIUM.

•The force of weight is balanced by the upward push of the ground.

•The friction force is balanced by the pulling force.

•The NET FORCE is the sum of the applied forces.

•The NET FORCE acting on this cart is ZERO.

+ upward push of the ground

- friction force

+ pulling force

-

+ upward push of the ground

- friction force

+ pulling force

-

When any object is in equilibrium the object will NOT ACCELERATE.

When an object is in equilibrium it can sit still or move with a constant velocity…

but it won’t accelerate.

What is the net force acting on the ball shown here!

Net force = (-20left )+ (+20right)+ (-10down) + (+5Up)= -5down

1. What is the mass of the cart?

2. What is the weight of the cart?

3. What is the net force acting on the cart?4. What is the acceleration of the cart?

30kg

Fw=mag=294N 3N

Right

.1m/s2

a = f/m a=

+3kgm/s2/30kga=.1m/s2

Guess What…

Quiz Tomorrow!