# AP Physics 1 : Conservation of Energy

## Example Questions

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### Example Question #1 : Conservation Of Energy

A rollercoaster cart with a total mass of 1,000kg is at the top of a hill moving at . Assuming no friction and neglecting air resistance, what is the speed of the cart when it reaches the bottom of the hill, 30m below?

Explanation:

This question covers the conservation of energy:

From the problem statement, we know that the cart has both initial potential and kinetic energy. We can assume that there is no final potential energy. This means that there is final kinetic energy from the final velocity that we are trying to find.

After eliminating final potential energy, we can rewrite the equation as:

We can eliminate mass from the equation to get:

We are solving for final velocity, so rearranging, we get:

Values for each variable were given in the problem statement, so we can simply plug in these values to solve:

### Example Question #2 : Conservation Of Energy

A group of ten friends, each of mass 50kg, want to test the strength of a trampoline. At their highest jump, each friend is 2m above the trampoline. They all land on the trampoline at the same time and decelerate at a constant rate to a point of zero velocity over a period of 0.5s. The trampoline has a threshold of 5000N, above which it breaks. Do the friends succeed in breaking their trampoline?

Neglect air resistance and the distance the trampoline sags as a result of the friends landing.

Yes; they exert a force of

Yes; they exert a force of

No; they exert a force of

No; they exert a force of

Yes; they exert a force of

Explanation:

According to the problem statement, we can treat the friends as a single mass of 500kg. We can use the expression for conservation of energy to calculate the velocity as they hit the trampoline.

If we take the initial state to be when the friends are at their highest point and the final state to be when the friends hit the trampoline, we can rewrite:

Substituting in expressions:

Rearranging for velocity:

Plugging in our values:

The statement says that the friends decelerate from this velocity to zero over a period of 0.5 seconds. Therefore, we can write:

We can use this to calculate the force that the trampoline is exerting on the friends:

This is above the threshold of 5000 N, so the trampoline does break.

### Example Question #3 : Conservation Of Energy

For a baseball field, the distance between home plate and the center field wall is about  and the wall is  tall. If a player hits the ball at a level of  off the ground, an angle of  above the horizontal, and a velocity of , what is the total velocity of the ball as it passes over the center field wall at a height of ?

Neglect air resistance and assume

None of the other answers

Explanation:

There are two ways to solve this problem. The first uses the concept of conservation of energy, and the second uses kinematics. We'll go through both methods, as you should be comfortable using either one, as some problems won't have the ability to be easily solved using two different methods.

Method 1: Conservation of Energy

Note: The only reason we can use this method is because we know the height at which the ball crosses over the center field wall. Without that height, we would have to do method 2.

We will first split the initial velocity of the ball into its components:

Since we are neglecting air resistance, the x-component stays constant. Therefore we can say:

We can find the final y-componenet using our conservation of energy equation:

Substituting in our expressions:

Canceling out mass and rearranging for final velocity:

Note that this is one of the big five kineamtics equations with which you should be familiar. Plugging in our values, we get:

Now that we have the components, we can combine them to get the final velocity:

Method 2: Kinematics

We start off the same way by separating the velocities into their components:

Since we are neglecting air resistance, the x-component stays constant. Therefore we can say:

We also know how far the ball travels horizontally, so we can find out long it takes to cover that distance:

We can then use this time to find out the final vertical velocity:

The acceleration is subtracted because the acceleration is technically negative.

The velocity is negative because the ball is now traveling back downward. We can now combine the component velocities into the final velocity of the ball:

### Example Question #7 : Understanding Conservation Of Energy

A  rubber ball is released from a height of  and is allowed to bounce repeatedly. Each time the ball strikes the ground, ten percent of the ball's kinetic energy is lost in the collision. What is the maximum height the ball will reach after hitting the ground four times?

Explanation:

Each time the ball bounces, 10% of the kinetic energy is lost. This means that after colliding with the ground, the ball has 90% of its previous potential energy at the top of its flight. To find the total energy after four bounces, we multiply the initial potential energy by .

The initial potential energy is:

Use our exponential expression to find the remaining energy after four bounces.

Based on conservation of energy, we can find the height that the ball travels with this remaining kinetic energy

### Example Question #4 : Conservation Of Energy

A ball starts rolling up a  incline at speed of . Using conservation of energy, find out how much work gravity does on the ball when it travels from the bottom to the maximum height.

Explanation:

The energies involved in this problem are kinetic and potential energy. Conservation of energy shows that the initial energies will be equal to the final energies.

Choosing the bottom of the incline to be the zero height, the ball starts out with kinetic energy and zero potential energy. When the ball reaches maximum height, its velocity is zero (zero kinetic energy). This simplifies our energy equation.

Isolate the height variable and use the given values to solve for the maximum height.

This is the vertical height. The work done by gravity is calculated as the product for force and distance.

The minus sign indicates that the force of gravity acts downward (negative direction).

### Example Question #4 : Conservation Of Energy

A roller coaster of mass 500kg is at its highest point in a loop traveling at a velocity of . The loop is 15 meters tall. Assuming 5000J of energy is lost between the highest and lowest points of the loop, how fast is the roller coaster traveling when it reaches the lowest point of the loop?

Explanation:

This problem covers the conservation of energy (including friction):

The only term we can eliminate is final potential energy:

Expanding each of the terms, we get:

We can rearrange this to solve for final velocity:

Many students will look at this and feel that the equation has just become more complex or harder to follow. However, the idea behind this is to reduce calculation errors. By doing all of your subsitutions of variables and rearrangements of the equations before plugging in your values, you only have to do a single calculation. This greatly reduces the frequency of silly calculation errors. Furthermore, it tends to make it much easier to follow your units in case you need to troubleshoot the problem.

Now, plugging in all of our values, we get:

### Example Question #2 : Conservation Of Energy

Consider the following system:

The coefficient of friction between the block and the slope is . The block has a mass of  and a constant velocity. If the height of the slope is , how much work is done by friction over the distance ?

Explanation:

First, we'll write out the equation for conservation of energy:

The velocity of the block is constant, so kinetic energies will cancel out. Furthermore, we can remove final potential energy if we say that the final height is 0m. Therefore, we get:

Substituting in the expression for potential energy, we get:

Note that this problem cannot be solved by using the force of gravity and force of friction because we do not know the angle of the slope.

### Example Question #6 : Conservation Of Energy

A pebble is dropped from a height of . What is its speed  before it hits the ground? Disregard the effects of air resistance.

Explanation:

Energy in a system must be conserved, and the energy in the falling pebble is either potential or kinetic:

Potential energy is represented by the  terms.

Kinetic energy is represented by the  terms.

At the exact moment the pebble is released, its stationary, so

### Example Question #151 : Newtonian Mechanics

Terry believes he can throw a ball vertically and hit a target  above himself. If the ball weighs , how fast must it be traveling when it leaves his hand to just reach the target? Neglect air resistance.

Explanation:

This problem deals with both potential energy and kinetic energy.

Potential energy is expressed as:

Kinetic energy is expressed as:

Energy must be conserved, so set up the following equation:

The initial height can be treated as zero, as can the final velocity. Plug in these zero values into the above equation.

Solve for , the initial velocity the ball needs in order to reach a maximum height of .

### Example Question #3 : Conservation Of Energy

A 50kg block is released from its resting position at a height of 10m from the ground. If we neglect air resistance, what is the block's total energy at a height of 4m from the ground?

Explanation:

To solve this problem, we'll need to consider the initial energy of the block. Since it is starting from rest, it will not have any kinetic energy. Rather, all of its energy will be in the form of gravitational potential energy.

Once the block is released from its resting position, it will begin to fall towards the ground. As it falls, it will lose potential energy, but it will gain kinetic energy as it picks up speed. What's important to realize is that because we are neglecting air resistance, we have a situation in which total mechanical energy is conserved. Therefore, as the block loses potential energy during its fall, it will gain an exactly equal amount of kinetic energy. Therefore, no matter where the block is during its fall, its total mechanical energy will be the same! Keeping this in mind, we can equate the total mechanical energy of the block at any point during its fall with its initial potential energy, which we calculated above.

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