1. Forces on a Mini-rover Flight and Space
© 2011 Project Lead The Way, Inc.

2. Forces on the Mini-rover
Four Forces of Flight
Gateway To Technology
Unit 4 – Lesson 4.2 – Aeronautics
Forces on the Mini-rover
At Rest
What forces do you think act on a vehicle that is at rest (not being propelled)?
1. Gravity [click]. The weight of the rover due to gravity acts downward toward the center of the planet.
2. Normal Forces [click]. The ground exerts an upward force on the wheels to counteract the gravitational force. We call these “normal forces” because they are normal (or perpendicular) to the surface. The sum of the normal forces equals the weight of the vehicle such that the vehicle is in equilibrium (not accelerating).
We represent these forces with vectors. Notice that the length of the normal forces are each half the length of the weight. Remember that the length of the vector represents the magnitude of the force. If we add the magnitude of the normal forces they equal the magnitude of the weight. W N N

3. Forces on the Mini-rover
Four Forces of Flight
Gateway To Technology
Unit 4 – Lesson 4.2 – Aeronautics
Forces on the Mini-rover
Accelerating
Thrust
Friction
What forces do you think act on a vehicle that is being propelled by a force? Specifically, think about the mini-rover you will design. What forces will act on the mini-rover propelled by balloon power?
Gravity [click] and
Normal Forces [click] act on an accelerating vehicle, too.
Thrust. Some force pushes the car forward. What will cause thrust in our mini-rover? [click] Air rushing out of the balloon.
Friction. Whenever materials rub against each other friction is created. Friction always acts against the direction of movement. What will cause friction in your mini-rover? Most likely your design will include wheels and an axle that move together. However, the axle will rotate and rub against another component of your rover. This contact will create friction — a force that acts against the motion. [click]
Notice that the Friction force is smaller than the Thrust in the diagram. What does this tell you about the motion of the vehicle? If the Thrust is greater than the Friction, the vehicle will move in the direction of the Thrust.
What happens if the Thrust has the same magnitude as the Friction? The vehicle will not move.
Is it possible that the Friction acting on the vehicle can have a larger magnitude than the Thrust? No. The magnitude of the frictional force can be equal to the Thrust, but will never be larger than the Thrust. The Friction will increase with Thrust until the Thrust overcomes the maximum frictional force created between the materials in contact. Then the vehicle will begin to accelerate.
Very Important: The maximum frictional force between two components increases as the force between the components increases. In this case, as the weight of the vehicle increases, the maximum frictional force increases. W N N

4. Four Forces of Flight
Gateway To Technology
Unit 4 – Lesson 4.2 – Aeronautics
Weight
Weight (W)
A force that is always directed toward the center of the earth.
The magnitude of the weight depends on the mass of the object.
The weight is distributed throughout the object. But we often think of it as collected and acting through a single point called the center of gravity.

5. Four Forces of Flight
Gateway To Technology
Unit 4 – Lesson 4.2 – Aeronautics
Mass
Mass (m) refers to the quantity of matter in an object. It is often confused with the concept of weight in the SI system. SI
gram (g)
U.S. Customary System
slug

6. Four Forces of Flight
Gateway To Technology
Unit 4 – Lesson 4.2 – Aeronautics
Mass vs. Weight
Contrary to popular practice, the terms mass (m) and weight (W) are not interchangeable and do not represent the same concept.
W = mg
weight = mass x acceleration due to gravity
(lb)
(slugs)
(ft/sec2)
(N)
(kg)
(m/sec2)
On Earth g = ft/sec2
= 9.8 m/sec2

7. Mass vs. Weight
An object, whether on the surface of the earth, in orbit, or on the surface of the moon, still has the same mass.
However, the weight of the same object will be different in all three instances, because the magnitude of gravity is different.

8. Mass vs. Weight
Both measurement systems have fallen prey to erroneous cultural practices.
In the SI system, a person’s weight is typically recorded in kilograms, when it should be recorded in Newtons.
In the U.S. Customary System, an object’s mass is typically recorded in pounds, when it should be recorded in slugs.

9. Forces and Motion
The motion of an object depends on the relative magnitude and direction of the forces acting on the object.
If the forces are balanced,
an object at rest remains at rest.
an object at constant velocity continues to move at that velocity.
If the forces are unbalanced, the object accelerates in the direction of the largest force.

10. Mini-rover Design Thrust
Frictional forces will increase with the weight of the mini-rover chassis.
Try to minimize the weight of the chassis.
Frictional forces will increase with the roughness of the materials in contact.
Try to make the materials in contact as smooth as possible.
Thrust
Friction
What do these facts mean with respect to your mini-rover design?

11. Mini-rover Design Thrust
The greater the Thrust, the greater the acceleration of the mini-rover.
Try to design the vehicle to have the greatest Thrust possible.
For as long as the Thrust is greater than the Friction, the vehicle will continue to accelerate.
Thrust
Friction
What do these facts mean with respect to your mini-rover design?