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# Equations of motion ARGH! watch

1. Please please don't tell me the answers. I really want to do this but I'm trying so hard and I'm just not in the right frame of mind, i cant get my head around it please i just really need some tips and a way to get started on these

1) A ball is dropped from rest vertically from a height of 10m above the ground. Assume the acceleration due to gravity is 9.81ms(2) and that there is no air resistance.

What is the velocity of the ball before hitting the ground?
How long does it take to reach the floor?

Can i use the acceleration formula for deceleration aswell providing i put a minus infront of it?

2. A car brakes on a motorway and comes to rest in 120m. The deceleration is 4m/s(2)

how long does it take for it to come to rest?
What is the speed at the instant the breaks were applied?

Thankyou!
2. What have you done so far? Post your working.
3. Just use some suvat equations and plug in the numbers.
4. (Original post by Robpattinsonxxx)
Please please don't tell me the answers. I really want to do this but I'm trying so hard and I'm just not in the right frame of mind, i cant get my head around it please i just really need some tips and a way to get started on these

1) A ball is dropped from rest vertically from a height of 10m above the ground. Assume the acceleration due to gravity is 9.81ms(2) and that there is no air resistance.

What is the velocity of the ball before hitting the ground?
How long does it take to reach the floor?

Can i use the acceleration formula for deceleration aswell providing i put a minus infront of it?

2. A car brakes on a motorway and comes to rest in 120m. The deceleration is 4m/s(2)

how long does it take for it to come to rest?
What is the speed at the instant the breaks were applied?

Thankyou!
1) first put the directions...you can choose whatever you want, if for example, we take upwards as negative and downwards as positive. In which case, acceleration is downwards because gravity pulls everything downwards so it would be positive.
now, it says that the ball is dropped from rest..so the initial velocity is...?
v^2 = u^2 + 2as...now you have three quantities, so you can get the final velocity.

and now, you want to find the time, you can use either of s = ut + 0.5at^2 or v = u + at...you have all the quantities except t so it can be easily found. remember to take a as positive because that is what we have decided. if you had taken it negative, the answer would still come the same because then 'v' would have been negative as it's in the downward direction.

2) suppose the car is travelling to the right. then let's take the right direction as positive. because it is decelerating, the acceleration in THIS direction is negative. now can you subsitute and find out?

remember if it comes to rest, its final speed has to be...?

hope that helps, and I tried my best not to give away everything
5. (Original post by Robpattinsonxxx)
Please please don't tell me the answers. I really want to do this but I'm trying so hard and I'm just not in the right frame of mind, i cant get my head around it please i just really need some tips and a way to get started on these

1) A ball is dropped from rest vertically from a height of 10m above the ground. Assume the acceleration due to gravity is 9.81ms(2) and that there is no air resistance.

What is the velocity of the ball before hitting the ground?
How long does it take to reach the floor?

Can i use the acceleration formula for deceleration aswell providing i put a minus infront of it?

2. A car brakes on a motorway and comes to rest in 120m. The deceleration is 4m/s(2)

how long does it take for it to come to rest?
What is the speed at the instant the breaks were applied?

Thankyou!

For number 1 just use the Suvat equations. You know u =0 you know A= 9.81 and you know S or x =10 So we know touse V^2=U^2+2AS

so just plug in the numbers and take the square root.

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