Download Forces - Storming Robots

Physics: Forces
Summer of Innovation Zero Robotics
Forces
You Push Me, I’ll Push You
1
What Is a Force?
What is a force? A force is an interaction between any two objects. Forces can be the result of direct contact,
such as pushing a shopping cart through a grocery store or pulling on a rope during tug-of-war. A force can
also act at a a distance, such as gravity.
Forces cause acceleration in an object. When you push a cart, the cart accelerates from rest (zero velocity)
to some final speed. However, forces can also be present when nothing is moving. Imagine that you and your
identical twin are standing on either side of a large box, such as one holding a refrigerator. If you both push
with the same amount of force, the box does not move at all. Even though nothing is moving, there are still
forces present in the system.
The easiet way to understand and analyze forces is to draw them out. Below, draw you and your twin pushing
on a box. Then, draw an arrow showing which way each person is applying the force to the box.
Wow, you just drew a free body diagram! Get used to it–scientists and engineers draw them all the time. See
how the two forces cancel each other out?
Now, imagine you and your older brother are pushing on the box. What would happen if your brother (on the
right) pushed a lot harder than you did (on the left)? Would the box move?
1
Week 1
Physics: Forces
2
Summer of Innovation Zero Robotics
Gravity
Gravity is a constant force in our lives. Everywhere on Earth, no matter what we are doing, gravity is acting on
us, pulling us toward the Earth. Even an Olympic high jumper, seen below, cannot escape the force of gravity
(that’s why they have mattresses behind the jump bar!). If Harriet the high jumperi leaps into the air, gravity
will push her back down, accelerating her to the ground as she goes. However, once she reaches the ground
(or, rather, the matress that is directly on top of the ground), she stops moving. In the pictures below, draw
the forces acting on the person in the air, as opposed to the person on the ground. As we discovered with the
(a) High jumper Tia Hellebaud.
(b) Donut connosseiur Homer
Simpson.
Figure 1: Gravity at work.
box, there was no movement when the forces acting on the box cancelled each other out. This must mean that
there is another force acting besides gravity when a person is just standing on the ground. In order for us to
avoid continuously falling toward the center of the Earth, another force must be there to push us back up when
gravity is yanking us down. This force is known as the normal force. Normal forces keep books from falling
through tables, clothing from falling off of hangers, and buildings from sinking into the ground. If you haven’t
done so already, draw an arrow representing the normal force acting on Homer Simpson.
3
Friction
Friction is another force that is constantly present in daily life. Think about how hard it is to push a toy car
on the carpet. Now, think about how it feels to push the same car on a tile floor. Even though you push the
car with the same force, the car travels faster and farther on the tile floor. The carpet exerts a greater force
of friction on the car than the tile does. The friction causes the car to slow down and to stop sooner. Draw a
picture of a car below (don’t worry, it doesn’t have to be pretty!), and draw what direction the force of friction
must be acting in to slow the car down.
2
Week 1
Physics: Forces
Summer of Innovation Zero Robotics
Friction is caused by two objects rubbing against each other. A frictional force is always exerted in a direction
opposite of the motion of the object. This causes the object to slow down and eventually stop. Although friction
is present everywhere on Earth (it’s even helping to keep your pen attached to the paper as you draw pictures
of friction), friction is not present for an object floating in space. Why?
4
Summary
• Force: A push or pull on an object;
• Gravity: A force that pulls everything toward the center of the Earth;
• Normal force: A force that helps push things back up when gravity is pulling them down;
• Friction: A force that is caused by two objects rubbing together;
• Free-body diagram: A drawing that helps visualize forces acting on a body;
• Remember: If forces are not balanced, the object is accelerating in the direction of the stronger force.
5
Resources
Figure 1a from http://www.topnews.in/sports/files/Tia-Hellebaud.jpg. Retrieved June 15, 2011.
Figure 1b from http://images.wikia.com/simpsons/images/0/02/Homer_Simpson_2006.png. Retrieved June
15, 2011.
3
Week 1