1. Newton’s First Law and Forces Using our Ideas to solve problems

2. Take our situation w. LaTasha and Anthony Our system? – Anthony and the sled F T = 120 N FNFN F g = -313.6 N F Ty = 60 N F Tx = 103.9 N Force: F x F y FNFN 0 F N FgFg 0 F g FTFT F Tx = F T cos(θ) = 103.9 N F Ty = F T sin(θ) = 60 N θ =30°

3. Do our Forces Balance? What’s happening? Once we know that, we’ll reason physically – Then we can finish our quantitative force diagram Object moves? – In the “+ x” direction – Why? – Unbalanced force – Why not the “y” direction? – Ice is holding the system up

4. So the ice is holding our system up, what then do we know? At rest in y-direction So... – Y-Forces balance Mathematically? ∑F y = 0 Use Force Table to get equation: ∑F y = F N + F g + F Ty Force: F x F y FNFN 0 F N FgFg 0 F g = 313.6 N FTFT F Tx = F T cos(θ) = 103.9 N F Ty = F T sin(θ) = 60 N

5. Now we can solve for F N ∑F y = F N + F g + F Ty Not moving in y-direction, so: 0 = F N - 313.6 N + 60 N +313.6 N = + 313.6 N 313.6 N = F N + 60 N -60 N = - 60 N 253.6 N = F N Last step: Does this answer make sense? Direction? Magnitude? Why?

6. Take our situation w. LaTasha and Anthony Our system? – Anthony and the sled accelerate in + x direction, due to net Force of 103.9 N right F T = 120 N F N = 253.6 N F g = -313.6 N F Ty = 60 N F Tx = 103.9 N θ =30°