1. Dynamics
Force Problems Day #1

2. Example #1 A 1000kg car is pushed with a force of 300N
What is the acceleration of the car?
How long would it take to accelerate to 108 km/h?
What force would be required to stop the vehicle in 30m from a speed of 108km/h?

3. G: R: F: a
Time (v2 = 30 m/s)
F (dist = 30 m)
m = 1000 kg
F = 300 N S:
𝑎= ∆𝑣 ∆𝑡
∆𝑡= ∆𝑣 𝑎
∆𝑡= 30−0 0.3
∆𝑡=100𝑠
𝑣 2 2 = 𝑣 𝑎𝑑
0= 𝑣 𝑎𝑑
− 𝑣 1 2 =2𝑎𝑑
− 𝑣 𝑑 =𝑎
− (30) =𝑎
−15𝑚/ 𝑠 2 =𝑎
𝐹=𝑚𝑎
𝐹=1000 −15
𝐹=−15000 𝑁 a)
𝐹=𝑚𝑎
𝐹 𝑚 =𝑎 =𝑎
0.3 𝑚/ 𝑠 2 =𝑎 b) c)

4. Example #2
A 70kg person is standing in an elevator. What is the normal force acting on the person if
The elevator is at rest
The elevator is moving up at 2 m/s
The elevator is accelerating downward at a rate of 2 m/s2

5. G: R: F: FN (at rest) FN (up at 2 m/s) FN (accel at 2 m/s2) m = 70 kgb)
Since there is no,
acceleration there
is no net force.
Therefore:
686𝑁[𝑈]= 𝐹 𝑁 c)
𝐹 = 𝐹 𝑁 + 𝐹 𝐺
−𝑚𝑎= 𝐹 𝑁 − 𝐹 𝐺
−𝑚𝑎+𝑚𝑔= 𝐹 𝑁
m(𝑔−𝑎)= 𝐹 𝑁
70 9.8−2 = 𝐹 𝑁
546𝑁 [𝑈]= 𝐹 𝑁
𝐹 = 𝐹 𝑁 + 𝐹 𝐺
0= 𝐹 𝑁 − 𝐹 𝐺
m𝑔= 𝐹 𝑁
(70)(9.8)= 𝐹 𝑁
686𝑁[𝑈]= 𝐹 𝑁
* At rest,
no net force