High School Physics : Motion and Mechanics
Study concepts, example questions & explanations for High School Physics
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Example Questions
Example Question #1 : Newton's Law Of Universal Gravitation
Two satellites in space, each with a mass of 
To solve this problem, use Newton's law of universal gravitation:
We are given the constant, as well as the satellite masses and distance (radius). Using these values we can solve for the force.
Example Question #2 : Newton's Law Of Universal Gravitation
Which pulls harder gravitationally, the Earth on the Moon, or the Moon on the Earth? Which accelerates more?
The Earth on the Moon; the Moon
The moon on the Earth; the Moon
Both the same; the Moon
The Moon on the Earth; the Earth
The Earth on the Moon; the Earth
Both the same; the Moon
Newton’s 3rd law states that for every force there is an equal and opposite force. In other words, the force with which the moon pulls on the Earth is the same force that the Earth pulls on the moon.
Newton’s 2nd law states that the acceleration of an object is directly related to the force applied and inversely related to the mass of the object. Since both the earth and the moon have the same force acting on it, it is their masses that will determine who will accelerate more. Since there is an inverse relationship between the mass and acceleration, the object with the smaller mass will accelerate more. Therefore the moon will accelerate more.
Example Question #1 : Impulse And Momentum
A 
The fastest way to solve a problem like this is with momentum.
Remember that momentum is equal to mass times velocity: 
Using this transformation, we can see that momentum is also equal to force times time.
Expand this equation to include our given values.
Since the object is not moving at the end, its final velocity is zero. Plug in the given values and solve for the force.
We would expect the answer to be negative because the force of friction acts in the direction opposite to the initial velocity.
Example Question #2 : Impulse And Momentum
A 
The fastest way to solve a problem like this is with momentum.
Remember that momentum is equal to mass times velocity: 
Using this transformation, we can see that momentum is also equal to force times time.
Expand this equation to include our given values.
Since the box is not moving at the end, its final velocity is zero. Plug in the given values and solve for the time.
Example Question #3 : Impulse And Momentum
A man with a mass of m is painting a house. He stands on a tall ladder of height h. He leans over and falls straight down off the ladder. If he is in the air for s seconds, what will be his momentum right before he hits the ground?
The problem tells us he falls vertically off the ladder (straight down), so we don't need to worry about motion in the horizontal direction.
The equation for momentum is:
We can assume he falls from rest, which allows us to find the initial momentum.
From here, we can use the formula for impulse:
We know his initial momentum is zero, so we can remove this variable from the equation.
The problem tells us that his change in time is s seconds, so we can insert this in place of the time.
The only force acting upon man is the force due to gravity, which will always be given by the equation 
Example Question #4 : Impulse And Momentum
If an egg is dropped on concrete, it usually breaks. If an egg is dropped on grass, it may not break. What conclusion explains this result?
Grass is a softer texture
The egg is in contact with the grass for longer, so it absorbs less force
Grass has less mass than concrete
The density of grass is less than the density of concrete
The coefficient of friction of the grass on the egg is less than the coefficient of the concrete on the egg
The egg is in contact with the grass for longer, so it absorbs less force
In both cases the egg starts with the same velocity, so it has the same initial momentum. In both cases the egg stops moving at the end of its fall, so it has the same final velocity. The only thing that changes is the time and force of the impact. The force is produced by the deceleration resulting from the time that the egg is in contact with its point of impact.
As the time of contact increases, acceleration decreases and force decreases.
As the time of contact decreases, acceleration increases and force increases.
The grass will have less force on the egg, allowing for a lesser acceleration, due to a longer period of impact.
Example Question #5 : Impulse And Momentum
A 
The change in momentum is the final momentum minus the initial momentum, or 
Notice that the problem gives us the final SPEED of the ball but not the final VELOCITY. Since the ball "bounced back," it begins to move in the opposite direction, so its velocity at this point will be negative.
Plug in our values to solve:
Example Question #1 : Impulse And Momentum
The area under the curve on a Force versus time (F versus t) graph represents
Kinetic energy
Work
Momentum
Impulse
Impulse
If we were to examine the area under the curve of a constant force applied over a certain amount of time we would have a graph with a straight horizontal line. To find the area of that rectangle we would multiply the base times the height. The base would be the time (number of seconds the force was applied). The height would be the amount of force applied during this time. Force*time is equal to the impulse acting on the object which is equal to the change in momentum of the object.
Example Question #1 : Conservation Of Momentum
A 
We know that if the balls fell straight down after the crash, then the total momentum in the horizontal direction is zero. The only motion is due to gravity, rather than any remaining horizontal momentum. Based on conservation of momentum, the initial and final momentum values must be equal. If the final horizontal momentum is zero, then the initial horizontal momentum must also be zero.
In our situation, the final momentum is going to be zero.
Use the given values for the mass of each ball and initial velocity of the first ball to find the initial velocity of the second.
The negative sign tells us the second ball is traveling in the opposite direction as the first, meaning it must be moving east.
Example Question #1 : Conservation Of Momentum
Two equal mass balls (one green and the other yellow) are dropped from the same height and rebound off of the floor. The yellow ball rebounds to a higher position. Which ball is subjected to the greater magnitude of impulse during its collision with the floor?
The green ball
It’s impossible to tell since the time interval and the force have not been provided in the problem
Both balls were subjected to the same magnitude of impulse
The yellow ball
The yellow ball
The impulse is equal to the change in momentum. The change in momentum is equal to the mass times the change in velocity.
The yellow ball rebounds higher and therefore has a higher velocity after the rebound. Since it has a higher velocity after the collision, the overall change in momentum is greater. Therefore since the change in momentum is greater, the impulse is higher.
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