HELPPP! Y11 WORK AND ENERGY (1 Viewer)

[misssunshine]

Hi all,
I am having some minor troubles doing these questions. if you can help, that would be awesome!!
1. a) Calculate the change in PE when a student lifts a school bag of mass 10kg a vertical distance of 1.4 m above the floor. b) Calculate the PE when it is dropped.
2. A 30kg child slide from rest down a frictionless 3.5 m slide 2 m high. Calculate her speed when she lands on the ground.
3. A 1000kg car i moving at 25ms^-1. net force of 200N acts in a forward direction for a distance of 1km. What is the original KE of the car when it is moving at 25ms^-1 (is it just KE= 1/2 m v^2?, where v =25?) b. what is the final ke energy of the car?
Thanks in advance!!!

 

[jazz519]

You need to make use of the formula of potential energy = mgh

1) so m = 10kg, h = 1.4 m and g = 9.8 m/s^2 constant, so sub those in and you get your answers PE = 10 x 1.4 x 9.8

2) For this you need to understand that the potential energy from the object in the gravitational field is transferred to kinetic energy as it falls. So change in potential energy = change in kinetic energy

i.e. PE = KE
mgh = 1/2 m v^2

Apply that formula by subbing in relevant quantities

3) a) yes initial is 1/2 mv^2
b) you are getting more kinetic energy from work being done:
recall w = Force x distance = 200 x 1000 (need to put 1km into SI unit of m i.e. 1 km = 1000 m)
Then add that to initial kinetic energy calculated in part b, for final kinetic energy

 

[Drdusk]

Just on a technical note on question 2, jazz said , it's actually . Be careful because this has tripped up students in the past, and its a very common mistake

Also the final Ep is zero and the initial KE is zero. So you can just equate them. Like lets say you did do it the way your suggesting:


As you can see it gives you the same thing.

It's best if you write it the full way like you suggest and as I've done above because it removes the chance of silly mistakes.

 

[misssunshine]

Thank you sooo much!