HESI A2 Exam: Forces and Motions
• Be familiar with the basics of each of Newton’s Laws
• What are the two primary characteristics of projectile motion?
• A Newton of force equals ____________
• What is the equation for net force?
• Gravity follows the inverse square law. What does this mean?
Intro
Inertia is the resistance to change in motion.
Velocity is the speed of an object in a specified direction.
Force is any influence that causes an object to change its shape or its state of motion.
Friction is a force acting in the opposite direction of motion when two objects come into contact with each other.
Newton’s Laws
Newton's First Law states that an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force. This sets a qualitative but not a quantitative parameter.
Newton’s second law does involve formulas for force, which you will not be tested on. However, you do need to know the second law states that the rate of change of momentum of a body is directly proportional to the force applied and this change in momentum takes place in the direction of the applied force.
Newton’s third law states that there is no such thing as a force that is not accompanied by an equal and opposite force.
Newton's law of universal gravitation states that a particle attracts every other particle in the universe using a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. However, it should be noted that gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915) which describes gravity not as a force but as a consequence of the curvature of spacetime caused by the uneven distribution of mass/energy.
Other Important Concepts
The characteristics of projectile motion are that 1. the horizontal component of velocity does not change and 2. the vertical acceleration due to gravity affects the vertical component of velocity. Typical examples of projectile motion are objects that are thrown, such as balls or rocks.
Balanced forces occur when an object moves at a constant speed.
The force of gravity between two objects is proportional to the product of the masses of those objects. The more massive the objects, the stronger the force of attraction; but, regardless of their mass, both objects will experience the same-sized force.
As an object falls, its speed increases because it’s being pulled on by gravity. The acceleration of gravity near the earth is g = -9.81 m/s^2. Velocity = -9.81 m/s^2 * time, or V = gt.
A Newton of force equals 1 kg * m/sec^2.
Force and Net Force Questions
A 10 kg object is moving to the east at 20 m/s^2. What is the net force acting on this object?
Net force = mass x acceleration. Therefore, we just multiply the two numbers together and get 10(20) = 200 kg * m/s^2. Remember that Newtons are calculated in kg * m/s^2. Therefore, the answer is 200 N.
If you increase the distance between two objects by a factor of two, what happens to the force between them?
Gravity follows an inverse-square law. Therefore, if you double the distance, you cut the force of gravity to a quarter ((1/2)^2).
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