Question

A small dog is trained to jump straight up a distance of 1.2 m. How much kinetic energy does the 7.2-kg dog need to jump this high? (The acceleration due to gravity is 9.8 m/s2.) Show your work.

Answer 1

@bahrom7893 help me please

Answer 2

What's the formula for KE?

Answer 3

i dont know bro

Answer 4

Look it up on Google and post it here. I'm here to guide you, not to give away the solution.

Answer 5

ok wait for me please

Answer 6

is this the formula KE= 1/2 (M X V^2),

Answer 7

@bahrom7893 wat does the formula stand for.

Answer 8

http://www.physicsclassroom.com/class/energy/u5l1c.cfm
That explains what the formula stands for.

Answer 9

1/2 * 7.2 * 9.8^2
345.744 is the correct answer

Answer 10

Why? You were given that acceleration is 9.8 m/s^2. The formula stated that you need to multiply by velocity squared.

Answer 11

how should i multiply

Answer 12

Find out velocity from the information they gave you.

Answer 13

or is it like this (1/2) * (7.2) *(9.8^2)^2
33205.25376

Answer 14

Nooo it is squared only once, but it should not be 9.8^2. V is not 9.8

Answer 15

is the velocity 1.2m

Answer 16

Do you know what velocity means?

Answer 17

no let me check wat it means

Answer 18

the rate at which an object changes its position is velocity. RIGHT

Answer 19

Okay, you found the definition somewhere. What exactly does that mean in your own words?

Answer 20

i dont exactly know

Answer 21

Okay, think about speed. If you cover 100km in 1hour, what's your speed?

Answer 22

it is 100km/h

Answer 23

Okay, so your velocity would also be 100km/h. Now imagine traveling backwards with the same speed. What is your velocity?

Answer 24

i think it is the same or 0km/h

Answer 25

it is the same 100km/h

Answer 26

Uhm close, actually since you're traveling backwards, the sign of velocity would be negative. So anything that moves forward, or to the right, or up would have a positive sign, and anything that moves backwards, left, or down would be negative.
So if you're traveling forward with 100km/h speed, your velocity is 100km/h. When you're traveling backwards with the same speed, then your velocity is -100km/h.

Answer 27

owk and for the question wat could be the velocity

Answer 28

How did you figure out the speed when I gave you distance and time?

Answer 29

i think u will divide distance/time

Answer 30

You have the distance, but not the time. But we also have acceleration. Can you figure out time from the acceleration?

Answer 31

no i cannot figure it out give me some ideas

Answer 32

or is it like this time=distance/speed

Answer 33

@dumbcow can u please help me

Answer 34

ok sorry was reading everything
you are given acceleration, im not sure how you determine inital velocity

i think you could also use
KE = Force*distance
KE = (mass*acc) * distance
KE = (7.2*9.8)*1.2

Answer 35

@dumbcow is it acceleration or gravity the 9.8

Answer 36

Note that energy is conserved, meaning that total energy = potential energy + kinetic energy. So you know kinetic energy (given above). When the dog starts jumping, his potential energy is zero, but his KE is maximum. That means when the dog reaches the peak of his jump, his KE will be zero. Since energy is conserved, the entire energy must be in the potential energy. Potential energy is mass * gravity * height. So equate KE with PE and that is how much KE is needed.

Answer 37

its acceleration of gravity

Answer 38

Acceleration is gravity in this problem.

Answer 39

like @ybarrap said....the Force needed to get off the ground equals the PE of m*g