All High School Physics Resources
Example Questions
Example Question #41 : Momentum
A hammer moving with a velocity of strikes a nail, after which the hammer has a velocity of . If the hammer strikes the nail with of force, how long were the two in contact?
The fastest way to solve a problem like this is with momentum.
Remember that momentum is equal to mass times velocity: . We can rewrite this equation in terms of force.
Using this transformation, we can see that momentum is also equal to force times time.
can also be thought of as .
Expand this equation to include our given values.
Since the hammer is not moving at the end, its final velocity is zero. We can plug in the given values on the left side of the equation.
The problem gave us the force of the hammer on the nail, but not the force of the nail on the hammer, which is what we need as we are looking purely at the momentum of the hammer.
Fortunately, Newton's third law can help us. It states that . This means that if the hammer exerts of force on the nail, then the nail must exert of force on the hammer.
We can plug that value in for the force, and solve for the time.
Example Question #22 : Calculating Momentum
Susan pushes a car with of force along a frictionless surface. How long does she need to push the car to get it to a velocity of , if it starts at rest?
The fastest way to solve a problem like this is with momentum.
Remember that momentum is equal to mass times velocity: . We can rewrite this equation in terms of force.
Using this transformation, we can see that momentum is also equal to force times time.
can also be thought of as .
Expand this equation to include our given values.
Since the object is not moving at the beginning, its initial velocity is zero. Plug in the given values and solve for the time.
Example Question #23 : Calculating Momentum
A tennis ball strikes a racket, moving at . After striking the racket, it bounces back at a speed of . What is the change in momentum?
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 #21 : Calculating Momentum
A tennis ball strikes a racket, moving at . After striking the racket, it bounces back at a speed of . If the ball is in contact with the racket for , how much force did the racket exert on the ball?
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The change in momentum is equal to the force exerted on the ball times time, or .
It is also equal to 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 "bounces back," it is moving in the opposite direction, so its velocity at this point will be negative.
Plug in our values to solve:
Plug that into our first equation to solve for the force the racket exerted on the ball:
Since , we can rewrite our answer as:
Example Question #41 : Momentum
A tennis ball moving at strikes a racket. After striking the racket, it bounces back at a speed of . If the racket exerts of force on the ball, how long were they in contact?
The change in the ball's momentum is equal to the force exerted on the ball times time, or .
It is also equal to 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 is moving in the opposite direction, and its velocity at this point will be negative.
Plug in our values to solve:
Plug that into our first equation:
Since our force is going in the same direction as our change in momentum, make both negative.
Since , we can rewrite our equation's units as:
Then, we can simplify the equation's units to get our final answer:
Example Question #381 : High School Physics
A tennis ball strikes a racket, moving at . After striking the racket, it bounces back at a speed of . If the ball is in contact with the racket for , what is the resultant acceleration on the ball?
First, we need to find the force applied to the ball since . We know the mass, but now we need the force.
The change in momentum is equal to the force exerted on the ball times time, or .
The change in momentum is also equal to 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 "bounces back," is moving in the opposite direction, and its velocity will be negative at this point.
Plug in our values to solve:
Plug that into our other equation:
Since , we can rewrite our answer as:
Plug that into our first equation:
Since , we can rewrite our answer as:
Then, we can reduce our answer's units to:
Example Question #21 : Calculating Momentum
A crate slides along the floor for before stopping. If it was initially moving with a velocity of , what is the force of friction?
The fastest way to solve a problem like this is with momentum.
Remember that momentum is equal to mass times velocity: . We can rewrite this equation in terms of force.
Using this transformation, we can see that momentum is also equal to force times time.
can also be thought of as .
Expand this equation to include our given values.
Since the crate is not moving at the end, its final velocity is zero. Plug in the given values and solve for the force.
Example Question #22 : Calculating Momentum
A crate slides along a floor with a starting velocity of . If the force due to friction is , how long will it take before stopping?
The fastest way to solve a problem like this is with momentum.
Remember that momentum is equal to mass times velocity: . We can rewrite this equation in terms of force.
Using this transformation, we can see that momentum is also equal to force times time.
can also be thought of as .
Expand this equation to include our given values.
Since the crate is not moving at the end, its final velocity is zero. Plug in the given values and solve for the time that it is in motion.
Example Question #321 : Motion And Mechanics
A ball moving at strikes a piece of paper. It breaks through the paper and continues moving in the same direction. If the paper exerted a force of on the ball and the two are in contact for , what is the final momentum of the ball?
Remember that momentum is equal to mass times velocity: . We can rewrite this equation in terms of force.
Using this transformation, we can see that momentum is also equal to force times time.
can also be thought of as .
Expand this equation to include our given values.
We are given the mass and initial velocity, allowing us to solve for the initial momentum, and the force and time, allowing us to evaluate the right side of the equation. Using these values we can solve for the final momentum of the ball.
Example Question #30 : Calculating Momentum
Susan pushes a car with of force on a frictionless surface. How long does she need to push the car to get it to a velocity of if it starts at rest?
The fastest way to solve a problem like this is with momentum.
Remember that momentum is equal to mass times velocity: . We can rewrite this equation in terms of force.
Using this transformation, we can see that momentum is also equal to force times time.
can also be thought of as .
Expand this equation to include our given values.
Since the car is not moving at the beginning, its initial velocity is zero. Plug in the given values and solve for the time required to change the velocity to the given value.