Forces

Remember the penny falling off the Empire State Building Remember the penny falling off the Empire State Building? What caused the penny to accelerate toward the ground?

Force A FORCE is a push or a pull. Gravity pulls us down toward the Earth. There are many different kinds of forces Magnetic – two opposite poles attract or repel Gravity – like our falling penny Electrical – static electricity Frictional – rubbing between any two surfaces Applied – like pushing on a wall

When you look at all of the forces acting on an object, some things cancel each other out. When you play tug-o-war, two sides are each applying a force on the flag in the middle. Two things can happen. One team is stronger than the other and pulls the flag their way Both teams are equally strong, and the flag stays right in the middle.

Force diagram

When you find that some things cancel out, the result is called a NET FORCE. The net force determines the motion of an object. Any non-zero net force results in ACCELERATION.

If two forces cancel each other out, then they are called BALANCED FORCES.

If two forces DO NOT cancel each other out, then they are called UNBALANCED FORCES.

Where do you see forces at work? Individually, come up with 2 examples of forces at work in life situations. With a partner, share your examples. Try to identify if they are balanced or unbalanced forces. Where is the net force pointing? As a class, we will discuss a few examples.

We will begin measuring forces – comparing balanced and unbalanced forces using numbers. This makes it easier for us to tell for sure whether one force is stronger than another. For force, the units we measure in are called “NEWTONS,” named after Sir Isaac Newton.

Constant Velocity If a non-zero NET FORCE causes an acceleration, then what does a net force of 0 cause? A car drives down the highway with a CONSTANT SPEED. Draw a force diagram for this car. When driving, you always need to press the gas (applied force) just a little to keep up your speed. A NET FORCE of 0 means that the object’s speed isn’t changing – THIS DOES NOT MEAN THE SPEED IS 0 m/s!!!

Center of Mass When we draw a force diagram, do we ALWAYS have to draw the picture? I mean…my art skillz aren’t so good…. And sometimes the stuff is hard to draw…. When we draw a force diagram, we should always draw the force acting on the CENTER OF MASS – the point where the mass is all balanced out. For this reason, you can draw every object as a box or a sphere, if you’d like! 

Practice Draw a force diagram for a bird that is flying forward through a windstorm. Show that some forces are more powerful than others by their length or width. Are the forces balanced or unbalanced? What is the net force on the bird? Will the bird accelerate? If so, in which direction?

Draw a force diagram for a baseball being hit by a player Draw a force diagram for a baseball being hit by a player. Show that some forces are more powerful than others by their length or width. Are the forces balanced or unbalanced? What is the net force on the ball? Will the ball accelerate? If so, in which direction?

Draw a force diagram for a car traveling at EXACTLY 60 mph down the highway. Show that some forces are more powerful than others by their length or width. Are the forces balanced or unbalanced? What is the net force on the car? Will the car accelerate? If so, in which direction?

Draw a force diagram for a steel beam being suspended by a crane Draw a force diagram for a steel beam being suspended by a crane. Show that some forces are more powerful than others by their length or width. Are the forces balanced or unbalanced? What is the net force on the beam? Will the beam accelerate? If so, in which direction?

Draw a force diagram for a sled being pushed down a hill Draw a force diagram for a sled being pushed down a hill. Show that some forces are more powerful than others by their length or width. Are the forces balanced or unbalanced? What is the net force on the sled? Will the sled accelerate? If so, in which direction?

Draw a force diagram for a box being pushed across a desk Draw a force diagram for a box being pushed across a desk. The box has a weight of 50 N. The force of friction is 45 N, and the force of the push is 60 N. Show that some forces are more powerful than others by their length or width. In case you forgot what a box looks like. Are the forces balanced or unbalanced? What is the net force on the box? Will the box accelerate? If so, in which direction?

Draw a force diagram for an apple hanging from a branch on a tree Draw a force diagram for an apple hanging from a branch on a tree. The weight of the apple is 0.1N. Show that some forces are more powerful than others by their length or width. Are the forces balanced or unbalanced? What is the net force on the apple? Will the apple accelerate? If so, in which direction?

Newton’s Laws Isaac Newton 1st law – An object at rest stays at rest (an object in motion stays in motion) until an outside force acts on it. 2nd law – F = mass*acceleration 3rd law – For every action, there is an equal and opposite reaction.

A basketball is sitting on a table A basketball is sitting on a table. Jake walks by and taps it on the side. Draw a force diagram for the ball when Jake’s hand hits it. What will happen to the ball immediately after the hit? 5 seconds later? 5 minutes later? Which law does this demonstrate?

Ally likes fishing. After seeing the bobber dip under the water, she pulls back on the rod and starts to reel in this fish, who is struggling to get away. Draw a diagram for the fish, assuming the line is straight forward. What happens if the fish pulls back too hard? Which law does this demonstrate?

Look at Ally’s fish again. The weight of the fish is 20 N Look at Ally’s fish again. The weight of the fish is 20 N. The fish is pulling back with a force of 60 N, and Ally is pulling forward with a force of 70 N. What is the value of the applied force on the fish that helps it swim? If the fishing line is unbreakable, which direction does the fish go? What if Ally only pulls with 60 N? 50 N? What happens if the fishing line can only hold 150 N of force? 100 N?

Driving down the highway, you are looking forward and a bug splats on the windshield. Draw a diagram for the bug as it hit, assuming it was not moving. What happens when the bug hits the windshield? Does the bug slow down? Does the car slow down? What law do these things demonstrate?

Identifying Newton’s Laws Where do you see Newton’s Laws acting in these pictures? 1) Object in motion stays in motion until an outside force acts on it 2) F =m*a 3) For every force, there is an equal and opposite force.